store.go

// Copyright 2014 The Cockroach Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.

package storage

import (
	"bytes"
	"context"
	"fmt"
	"math"
	"runtime"
	"strings"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"

	"github.com/coreos/etcd/raft"
	"github.com/coreos/etcd/raft/raftpb"
	"github.com/google/btree"
	opentracing "github.com/opentracing/opentracing-go"
	"github.com/pkg/errors"
	"golang.org/x/time/rate"

	"github.com/cockroachdb/cockroach/pkg/base"
	"github.com/cockroachdb/cockroach/pkg/config"
	"github.com/cockroachdb/cockroach/pkg/gossip"
	"github.com/cockroachdb/cockroach/pkg/internal/client"
	"github.com/cockroachdb/cockroach/pkg/keys"
	"github.com/cockroachdb/cockroach/pkg/roachpb"
	"github.com/cockroachdb/cockroach/pkg/rpc"
	"github.com/cockroachdb/cockroach/pkg/rpc/nodedialer"
	"github.com/cockroachdb/cockroach/pkg/settings"
	"github.com/cockroachdb/cockroach/pkg/settings/cluster"
	"github.com/cockroachdb/cockroach/pkg/sql/sqlutil"
	"github.com/cockroachdb/cockroach/pkg/storage/batcheval"
	"github.com/cockroachdb/cockroach/pkg/storage/closedts/container"
	"github.com/cockroachdb/cockroach/pkg/storage/compactor"
	"github.com/cockroachdb/cockroach/pkg/storage/engine"
	"github.com/cockroachdb/cockroach/pkg/storage/engine/enginepb"
	"github.com/cockroachdb/cockroach/pkg/storage/idalloc"
	"github.com/cockroachdb/cockroach/pkg/storage/spanset"
	"github.com/cockroachdb/cockroach/pkg/storage/stateloader"
	"github.com/cockroachdb/cockroach/pkg/storage/storagebase"
	"github.com/cockroachdb/cockroach/pkg/storage/tscache"
	"github.com/cockroachdb/cockroach/pkg/storage/txnwait"
	"github.com/cockroachdb/cockroach/pkg/util/envutil"
	"github.com/cockroachdb/cockroach/pkg/util/hlc"
	"github.com/cockroachdb/cockroach/pkg/util/humanizeutil"
	"github.com/cockroachdb/cockroach/pkg/util/limit"
	"github.com/cockroachdb/cockroach/pkg/util/log"
	"github.com/cockroachdb/cockroach/pkg/util/metric"
	"github.com/cockroachdb/cockroach/pkg/util/protoutil"
	"github.com/cockroachdb/cockroach/pkg/util/retry"
	"github.com/cockroachdb/cockroach/pkg/util/shuffle"
	"github.com/cockroachdb/cockroach/pkg/util/stop"
	"github.com/cockroachdb/cockroach/pkg/util/syncutil"
	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
	"github.com/cockroachdb/cockroach/pkg/util/tracing"
	"github.com/cockroachdb/cockroach/pkg/util/uuid"
)

const (
	// rangeIDAllocCount is the number of Range IDs to allocate per allocation.
	rangeIDAllocCount             = 10
	defaultHeartbeatIntervalTicks = 5

	// defaultRaftEntryCacheSize is the default size in bytes for a
	// store's Raft log entry cache.
	defaultRaftEntryCacheSize = 1 << 24 // 16M

	// replicaRequestQueueSize specifies the maximum number of requests to queue
	// for a replica.
	replicaRequestQueueSize = 100

	defaultGossipWhenCapacityDeltaExceedsFraction = 0.01

	// systemDataGossipInterval is the interval at which range lease
	// holders verify that the most recent system data is gossiped.
	// This ensures that system data is always eventually gossiped, even
	// if a range lease holder experiences a failure causing a missed
	// gossip update.
	systemDataGossipInterval = 1 * time.Minute
)

var changeTypeInternalToRaft = map[roachpb.ReplicaChangeType]raftpb.ConfChangeType{
	roachpb.ADD_REPLICA:    raftpb.ConfChangeAddNode,
	roachpb.REMOVE_REPLICA: raftpb.ConfChangeRemoveNode,
}

var storeSchedulerConcurrency = envutil.EnvOrDefaultInt(
	"COCKROACH_SCHEDULER_CONCURRENCY", 8*runtime.NumCPU())

// bulkIOWriteLimit is defined here because it is used by BulkIOWriteLimiter.
var bulkIOWriteLimit = settings.RegisterByteSizeSetting(
	"kv.bulk_io_write.max_rate",
	"the rate limit (bytes/sec) to use for writes to disk on behalf of bulk io ops",
	math.MaxInt64,
)

// importRequestsLimit limits concurrent import requests.
var importRequestsLimit = settings.RegisterIntSetting(
	"kv.bulk_io_write.concurrent_import_requests",
	"number of import requests a store will handle concurrently before queuing",
	1,
)

// ExportRequestsLimit is the number of Export requests that can run at once.
// Each extracts data from RocksDB to a temp file and then uploads it to cloud
// storage. In order to not exhaust the disk or memory, or saturate the network,
// limit the number of these that can be run in parallel. This number was chosen
// by a guessing - it could be improved by more measured heuristics. Exported
// here since we check it in in the caller to limit generated requests as well
// to prevent excessive queuing.
var ExportRequestsLimit = settings.RegisterIntSetting(
	"kv.bulk_io_write.concurrent_export_requests",
	"number of export requests a store will handle concurrently before queuing",
	5,
)

// TestStoreConfig has some fields initialized with values relevant in tests.
func TestStoreConfig(clock *hlc.Clock) StoreConfig {
	if clock == nil {
		clock = hlc.NewClock(hlc.UnixNano, time.Nanosecond)
	}
	st := cluster.MakeTestingClusterSettings()
	sc := StoreConfig{
		Settings:   st,
		AmbientCtx: log.AmbientContext{Tracer: st.Tracer},
		Clock:      clock,
		CoalescedHeartbeatsInterval: 50 * time.Millisecond,
		RaftHeartbeatIntervalTicks:  1,
		ScanInterval:                10 * time.Minute,
		TimestampCachePageSize:      tscache.TestSklPageSize,
		HistogramWindowInterval:     metric.TestSampleInterval,
		EnableEpochRangeLeases:      true,
		ClosedTimestamp:             container.NoopContainer(),
	}

	// Tests should never send unsequenced transactional writes,
	// so we set this testing knob in all tests.
	sc.TestingKnobs.EvalKnobs.DisallowUnsequencedTransactionalWrites = true

	// Use shorter Raft tick settings in order to minimize start up and failover
	// time in tests.
	sc.RaftElectionTimeoutTicks = 3
	sc.RaftTickInterval = 100 * time.Millisecond
	sc.SetDefaults()
	return sc
}

var (
	raftMaxSizePerMsg   = envutil.EnvOrDefaultInt("COCKROACH_RAFT_MAX_SIZE_PER_MSG", 16*1024)
	raftMaxInflightMsgs = envutil.EnvOrDefaultInt("COCKROACH_RAFT_MAX_INFLIGHT_MSGS", 64)
)

func newRaftConfig(
	strg raft.Storage, id uint64, appliedIndex uint64, storeCfg StoreConfig, logger raft.Logger,
) *raft.Config {
	return &raft.Config{
		ID:            id,
		Applied:       appliedIndex,
		ElectionTick:  storeCfg.RaftElectionTimeoutTicks,
		HeartbeatTick: storeCfg.RaftHeartbeatIntervalTicks,
		Storage:       strg,
		Logger:        logger,

		PreVote: true,

		// MaxSizePerMsg controls how many Raft log entries the leader will send to
		// followers in a single MsgApp.
		MaxSizePerMsg: uint64(raftMaxSizePerMsg),
		// MaxInflightMsgs controls how many "inflight" messages Raft will send to
		// a follower without hearing a response. The total number of Raft log
		// entries is a combination of this setting and MaxSizePerMsg. The current
		// settings provide for up to 1 MB of raft log to be sent without
		// acknowledgement. With an average entry size of 1 KB that translates to
		// ~1024 commands that might be executed in the handling of a single
		// raft.Ready operation.
		MaxInflightMsgs: raftMaxInflightMsgs,
	}
}

// verifyKeys verifies keys. If checkEndKey is true, then the end key
// is verified to be non-nil and greater than start key. If
// checkEndKey is false, end key is verified to be nil. Additionally,
// verifies that start key is less than KeyMax and end key is less
// than or equal to KeyMax. It also verifies that a key range that
// contains range-local keys is completely range-local.
func verifyKeys(start, end roachpb.Key, checkEndKey bool) error {
	if bytes.Compare(start, roachpb.KeyMax) >= 0 {
		return errors.Errorf("start key %q must be less than KeyMax", start)
	}
	if !checkEndKey {
		if len(end) != 0 {
			return errors.Errorf("end key %q should not be specified for this operation", end)
		}
		return nil
	}
	if end == nil {
		return errors.Errorf("end key must be specified")
	}
	if bytes.Compare(roachpb.KeyMax, end) < 0 {
		return errors.Errorf("end key %q must be less than or equal to KeyMax", end)
	}
	{
		sAddr, err := keys.Addr(start)
		if err != nil {
			return err
		}
		eAddr, err := keys.Addr(end)
		if err != nil {
			return err
		}
		if !sAddr.Less(eAddr) {
			return errors.Errorf("end key %q must be greater than start %q", end, start)
		}
		if !bytes.Equal(sAddr, start) {
			if bytes.Equal(eAddr, end) {
				return errors.Errorf("start key is range-local, but end key is not")
			}
		} else if bytes.Compare(start, keys.LocalMax) < 0 {
			// It's a range op, not local but somehow plows through local data -
			// not cool.
			return errors.Errorf("start key in [%q,%q) must be greater than LocalMax", start, end)
		}
	}

	return nil
}

// rangeKeyItem is a common interface for roachpb.Key and Range.
type rangeKeyItem interface {
	endKey() roachpb.RKey
}

// rangeBTreeKey is a type alias of roachpb.RKey that implements the
// rangeKeyItem interface and the btree.Item interface.
type rangeBTreeKey roachpb.RKey

var _ rangeKeyItem = rangeBTreeKey{}

func (k rangeBTreeKey) endKey() roachpb.RKey {
	return (roachpb.RKey)(k)
}

var _ btree.Item = rangeBTreeKey{}

func (k rangeBTreeKey) Less(i btree.Item) bool {
	return k.endKey().Less(i.(rangeKeyItem).endKey())
}

// A NotBootstrappedError indicates that an engine has not yet been
// bootstrapped due to a store identifier not being present.
type NotBootstrappedError struct{}

// Error formats error.
func (e *NotBootstrappedError) Error() string {
	return "store has not been bootstrapped"
}

// A storeReplicaVisitor calls a visitor function for each of a store's
// initialized Replicas (in unspecified order).
type storeReplicaVisitor struct {
	store   *Store
	repls   []*Replica // Replicas to be visited.
	visited int        // Number of visited ranges, -1 before first call to Visit()
}

// Len implements shuffle.Interface.
func (rs storeReplicaVisitor) Len() int { return len(rs.repls) }

// Swap implements shuffle.Interface.
func (rs storeReplicaVisitor) Swap(i, j int) { rs.repls[i], rs.repls[j] = rs.repls[j], rs.repls[i] }

// newStoreReplicaVisitor constructs a storeReplicaVisitor.
func newStoreReplicaVisitor(store *Store) *storeReplicaVisitor {
	return &storeReplicaVisitor{
		store:   store,
		visited: -1,
	}
}

// Visit calls the visitor with each Replica until false is returned.
func (rs *storeReplicaVisitor) Visit(visitor func(*Replica) bool) {
	// Copy the range IDs to a slice so that we iterate over some (possibly
	// stale) view of all Replicas without holding the Store lock. In particular,
	// no locks are acquired during the copy process.
	rs.repls = nil
	rs.store.mu.replicas.Range(func(k int64, v unsafe.Pointer) bool {
		rs.repls = append(rs.repls, (*Replica)(v))
		return true
	})

	// The Replicas are already in "unspecified order" due to map iteration,
	// but we want to make sure it's completely random to prevent issues in
	// tests where stores are scanning replicas in lock-step and one store is
	// winning the race and getting a first crack at processing the replicas on
	// its queues.
	//
	// TODO(peter): Re-evaluate whether this is necessary after we allow
	// rebalancing away from the leaseholder. See TestRebalance_3To5Small.
	shuffle.Shuffle(rs)

	rs.visited = 0
	for _, repl := range rs.repls {
		// TODO(tschottdorf): let the visitor figure out if something's been
		// destroyed once we return errors from mutexes (#9190). After all, it
		// can still happen with this code.
		rs.visited++
		repl.mu.RLock()
		destroyed := repl.mu.destroyStatus
		initialized := repl.isInitializedRLocked()
		repl.mu.RUnlock()
		if initialized && (destroyed.IsAlive() || destroyed.reason == destroyReasonRemovalPending) && !visitor(repl) {
			break
		}
	}
	rs.visited = 0
}

// EstimatedCount returns an estimated count of the underlying store's
// replicas.
//
// TODO(tschottdorf): this method has highly doubtful semantics.
func (rs *storeReplicaVisitor) EstimatedCount() int {
	if rs.visited <= 0 {
		return rs.store.ReplicaCount()
	}
	return len(rs.repls) - rs.visited
}

type raftRequestInfo struct {
	req        *RaftMessageRequest
	respStream RaftMessageResponseStream
}

type raftRequestQueue struct {
	syncutil.Mutex
	infos []raftRequestInfo
}

// A Store maintains a map of ranges by start key. A Store corresponds
// to one physical device.
type Store struct {
	Ident              *roachpb.StoreIdent // pointer to catch access before Start() is called
	cfg                StoreConfig
	db                 *client.DB
	engine             engine.Engine               // The underlying key-value store
	compactor          *compactor.Compactor        // Schedules compaction of the engine
	tsCache            tscache.Cache               // Most recent timestamps for keys / key ranges
	allocator          Allocator                   // Makes allocation decisions
	rangeIDAlloc       *idalloc.Allocator          // Range ID allocator
	gcQueue            *gcQueue                    // Garbage collection queue
	mergeQueue         *mergeQueue                 // Range merging queue
	splitQueue         *splitQueue                 // Range splitting queue
	replicateQueue     *replicateQueue             // Replication queue
	replicaGCQueue     *replicaGCQueue             // Replica GC queue
	raftLogQueue       *raftLogQueue               // Raft log truncation queue
	raftSnapshotQueue  *raftSnapshotQueue          // Raft repair queue
	tsMaintenanceQueue *timeSeriesMaintenanceQueue // Time series maintenance queue
	scanner            *replicaScanner             // Replica scanner
	consistencyQueue   *consistencyQueue           // Replica consistency check queue
	metrics            *StoreMetrics
	intentResolver     *intentResolver
	raftEntryCache     *raftEntryCache
	limiters           batcheval.Limiters

	// gossipRangeCountdown and leaseRangeCountdown are countdowns of
	// changes to range and leaseholder counts, after which the store
	// descriptor will be re-gossiped earlier than the normal periodic
	// gossip interval. Updated atomically.
	gossipRangeCountdown int32
	gossipLeaseCountdown int32
	// gossipQueriesPerSecondVal and gossipWritesPerSecond serve similar
	// purposes, but simply record the most recently gossiped value so that we
	// can tell if a newly measured value differs by enough to justify
	// re-gossiping the store.
	gossipQueriesPerSecondVal syncutil.AtomicFloat64
	gossipWritesPerSecondVal  syncutil.AtomicFloat64

	coalescedMu struct {
		syncutil.Mutex
		heartbeats         map[roachpb.StoreIdent][]RaftHeartbeat
		heartbeatResponses map[roachpb.StoreIdent][]RaftHeartbeat
	}
	// 1 if the store was started, 0 if it wasn't. To be accessed using atomic
	// ops.
	started int32
	stopper *stop.Stopper
	// The time when the store was Start()ed, in nanos.
	startedAt    int64
	nodeDesc     *roachpb.NodeDescriptor
	initComplete sync.WaitGroup // Signaled by async init tasks

	// Semaphore to limit concurrent non-empty snapshot application.
	snapshotApplySem chan struct{}

	// Channel of newly-acquired expiration-based leases that we want to
	// proactively renew.
	expirationBasedLeaseChan chan *Replica

	// draining holds a bool which indicates whether this store is draining. See
	// SetDraining() for a more detailed explanation of behavior changes.
	//
	// TODO(bdarnell,tschottdorf): Would look better inside of `mu`, which at
	// the time of its creation was riddled with deadlock (but that situation
	// has likely improved).
	draining atomic.Value

	// Locking notes: To avoid deadlocks, the following lock order must be
	// obeyed: Replica.raftMu < Replica.readOnlyCmdMu < Store.mu < Replica.mu
	// < Replica.unreachablesMu < Store.coalescedMu < Store.scheduler.mu.
	// (It is not required to acquire every lock in sequence, but when multiple
	// locks are held at the same time, it is incorrect to acquire a lock with
	// "lesser" value in this sequence after one with "greater" value).
	//
	// Methods of Store with a "Locked" suffix require that
	// Store.mu.Mutex be held. Other locking requirements are indicated
	// in comments.
	//
	// The locking structure here is complex because A) Store is a
	// container of Replicas, so it must generally be consulted before
	// doing anything with any Replica, B) some Replica operations
	// (including splits) modify the Store. Therefore we generally lock
	// Store.mu to find a Replica, release it, then call a method on the
	// Replica. These short-lived locks of Store.mu and Replica.mu are
	// often surrounded by a long-lived lock of Replica.raftMu as
	// described below.
	//
	// There are two major entry points to this stack of locks:
	// Store.Send (which handles incoming RPCs) and raft-related message
	// processing (including handleRaftReady on the processRaft
	// goroutine and HandleRaftRequest on GRPC goroutines). Reads are
	// processed solely through Store.Send; writes start out on
	// Store.Send until they propose their raft command and then they
	// finish on the raft goroutines.
	//
	// TODO(bdarnell): a Replica could be destroyed immediately after
	// Store.Send finds the Replica and releases the lock. We need
	// another RWMutex to be held by anything using a Replica to ensure
	// that everything is finished before releasing it. #7169
	//
	// Detailed description of the locks:
	//
	// * Replica.raftMu: Held while any raft messages are being processed
	//   (including handleRaftReady and HandleRaftRequest) or while the set of
	//   Replicas in the Store is being changed (which may happen outside of raft
	//   via the replica GC queue).
	//
	//   If holding raftMus for multiple different replicas simultaneously,
	//   acquire the locks in the order that the replicas appear in replicasByKey.
	//
	// * Replica.readOnlyCmdMu (RWMutex): Held in read mode while any
	//   read-only command is in progress on the replica; held in write
	//   mode while executing a commit trigger. This is necessary
	//   because read-only commands mutate the Replica's timestamp cache
	//   (while holding Replica.mu in addition to readOnlyCmdMu). The
	//   RWMutex ensures that no reads are being executed during a split
	//   (which copies the timestamp cache) while still allowing
	//   multiple reads in parallel (#3148). TODO(bdarnell): this lock
	//   only needs to be held during splitTrigger, not all triggers.
	//
	// * Store.mu: Protects the Store's map of its Replicas. Acquired and
	//   released briefly at the start of each request; metadata operations like
	//   splits acquire it again to update the map. Even though these lock
	//   acquisitions do not make up a single critical section, it is safe thanks
	//   to Replica.raftMu which prevents any concurrent modifications.
	//
	// * Replica.mu: Protects the Replica's in-memory state. Acquired
	//   and released briefly as needed (note that while the lock is
	//   held "briefly" in that it is not held for an entire request, we
	//   do sometimes do I/O while holding the lock, as in
	//   Replica.Entries). This lock should be held when calling any
	//   methods on the raft group. Raft may call back into the Replica
	//   via the methods of the raft.Storage interface, which assume the
	//   lock is held even though they do not follow our convention of
	//   the "Locked" suffix.
	//
	// * Store.scheduler.mu: Protects the Raft scheduler internal
	//   state. Callbacks from the scheduler are performed while not holding this
	//   mutex in order to observe the above ordering constraints.
	//
	// Splits and merges deserve special consideration: they operate on two
	// ranges. For splits, this might seem fine because the right-hand range is
	// brand new, but an uninitialized version may have been created by a raft
	// message before we process the split (see commentary on
	// Replica.splitTrigger). We make this safe, for both splits and merges, by
	// locking the right-hand range for the duration of the Raft command
	// containing the split/merge trigger.
	//
	// Note that because we acquire and release Store.mu and Replica.mu
	// repeatedly rather than holding a lock for an entire request, we are
	// actually relying on higher-level locks to ensure that things don't change
	// out from under us. In particular, handleRaftReady accesses the replicaID
	// more than once, and we rely on Replica.raftMu to ensure that this is not
	// modified by a concurrent HandleRaftRequest. (#4476)

	mu struct {
		syncutil.RWMutex
		// Map of replicas by Range ID (map[roachpb.RangeID]*Replica). This
		// includes `uninitReplicas`. May be read without holding Store.mu.
		replicas syncutil.IntMap
		// A btree key containing objects of type *Replica or
		// *ReplicaPlaceholder (both of which have an associated key range, on
		// the EndKey of which the btree is keyed)
		replicasByKey  *btree.BTree
		uninitReplicas map[roachpb.RangeID]*Replica // Map of uninitialized replicas by Range ID
		// replicaPlaceholders is a map to access all placeholders, so they can
		// be directly accessed and cleared after stepping all raft groups.
		replicaPlaceholders map[roachpb.RangeID]*ReplicaPlaceholder
	}

	// The unquiesced subset of replicas.
	unquiescedReplicas struct {
		syncutil.Mutex
		m map[roachpb.RangeID]struct{}
	}

	// replicaQueues is a map of per-Replica incoming request queues. These
	// queues might more naturally belong in Replica, but are kept separate to
	// avoid reworking the locking in getOrCreateReplica which requires
	// Replica.raftMu to be held while a replica is being inserted into
	// Store.mu.replicas.
	replicaQueues syncutil.IntMap // map[roachpb.RangeID]*raftRequestQueue

	scheduler *raftScheduler

	// livenessMap is a map from nodeID to a bool indicating
	// liveness. It is updated periodically in raftTickLoop().
	livenessMap atomic.Value

	// cachedCapacity caches information on store capacity to prevent
	// expensive recomputations in case leases or replicas are rapidly
	// rebalancing.
	cachedCapacity struct {
		syncutil.Mutex
		roachpb.StoreCapacity
	}

	counts struct {
		// Number of placeholders removed due to error.
		removedPlaceholders int32
		// Number of placeholders successfully filled by a snapshot.
		filledPlaceholders int32
		// Number of placeholders removed due to a snapshot that was dropped by
		// raft.
		droppedPlaceholders int32
	}
}

var _ client.Sender = &Store{}

// A StoreConfig encompasses the auxiliary objects and configuration
// required to create a store.
// All fields holding a pointer or an interface are required to create
// a store; the rest will have sane defaults set if omitted.
type StoreConfig struct {
	AmbientCtx log.AmbientContext
	base.RaftConfig

	Settings     *cluster.Settings
	Clock        *hlc.Clock
	DB           *client.DB
	Gossip       *gossip.Gossip
	NodeLiveness *NodeLiveness
	StorePool    *StorePool
	Transport    *RaftTransport
	NodeDialer   *nodedialer.Dialer
	RPCContext   *rpc.Context

	ClosedTimestamp *container.Container

	// SQLExecutor is used by the store to execute SQL statements.
	SQLExecutor sqlutil.InternalExecutor

	// TimeSeriesDataStore is an interface used by the store's time series
	// maintenance queue to dispatch individual maintenance tasks.
	TimeSeriesDataStore TimeSeriesDataStore

	// DontRetryPushTxnFailures will propagate a push txn failure immediately
	// instead of utilizing the txn wait queue to wait for the transaction to
	// finish or be pushed by a higher priority contender.
	DontRetryPushTxnFailures bool

	// CoalescedHeartbeatsInterval is the interval for which heartbeat messages
	// are queued and then sent as a single coalesced heartbeat; it is a
	// fraction of the RaftTickInterval so that heartbeats don't get delayed by
	// an entire tick. Delaying coalescing heartbeat responses has a bad
	// interaction with quiescence because the coalesced (delayed) heartbeat
	// response can unquiesce the leader. Consider:
	//
	// T+0: leader queues MsgHeartbeat
	// T+1: leader sends MsgHeartbeat
	//                                        follower receives MsgHeartbeat
	//                                        follower queues MsgHeartbeatResp
	// T+2: leader queues quiesce message
	//                                        follower sends MsgHeartbeatResp
	//      leader receives MsgHeartbeatResp
	// T+3: leader sends quiesce message
	//
	// Thus we want to make sure that heartbeats are responded to faster than
	// the quiesce cadence.
	CoalescedHeartbeatsInterval time.Duration

	// RaftHeartbeatIntervalTicks is the number of ticks that pass between heartbeats.
	RaftHeartbeatIntervalTicks int

	// ScanInterval is the default value for the scan interval
	ScanInterval time.Duration

	// ScanMinIdleTime is the minimum time the scanner will be idle between ranges.
	// If enabled (> 0), the scanner may complete in more than ScanInterval for
	// stores with many ranges.
	ScanMinIdleTime time.Duration

	// ScanMaxIdleTime is the maximum time the scanner will be idle between ranges.
	// If enabled (> 0), the scanner may complete in less than ScanInterval for small
	// stores.
	ScanMaxIdleTime time.Duration

	// If LogRangeEvents is true, major changes to ranges will be logged into
	// the range event log.
	LogRangeEvents bool

	// RaftEntryCacheSize is the size in bytes of the Raft log entry cache
	// shared by all Raft groups managed by the store.
	RaftEntryCacheSize uint64

	// IntentResolverTaskLimit is the maximum number of asynchronous tasks that
	// may be started by the intent resolver. -1 indicates no asynchronous tasks
	// are allowed. 0 uses the default value (defaultIntentResolverTaskLimit)
	// which is non-zero.
	IntentResolverTaskLimit int

	TestingKnobs StoreTestingKnobs

	// concurrentSnapshotApplyLimit specifies the maximum number of empty
	// snapshots and the maximum number of non-empty snapshots that are permitted
	// to be applied concurrently.
	concurrentSnapshotApplyLimit int

	// TimestampCachePageSize is (server.Config).TimestampCachePageSize
	TimestampCachePageSize uint32

	// HistogramWindowInterval is (server.Config).HistogramWindowInterval
	HistogramWindowInterval time.Duration

	// EnableEpochRangeLeases controls whether epoch-based range leases are used.
	EnableEpochRangeLeases bool

	// GossipWhenCapacityDeltaExceedsFraction specifies the fraction from the last
	// gossiped store capacity values which need be exceeded before the store will
	// gossip immediately without waiting for the periodic gossip interval.
	GossipWhenCapacityDeltaExceedsFraction float64
}

// StoreTestingKnobs is a part of the context used to control parts of
// the system. The Testing*Filter functions are called at various
// points in the request pipeline if they are non-nil. These can be
// used either for synchronization (e.g. to write to a channel when a
// particular point is reached) or to change the behavior by returning
// an error (which aborts all further processing for the command).
type StoreTestingKnobs struct {
	EvalKnobs batcheval.TestingKnobs

	// TestingRequestFilter is called before evaluating each command on a
	// replica. The filter is run before the request is added to the
	// CommandQueue, so blocking in the filter will not block interfering
	// requests. If it returns an error, the command will not be evaluated.
	TestingRequestFilter storagebase.ReplicaRequestFilter

	// TestingProposalFilter is called before proposing each command.
	TestingProposalFilter storagebase.ReplicaProposalFilter

	// TestingApplyFilter is called before applying the results of a
	// command on each replica. If it returns an error, the command will
	// not be applied. If it returns an error on some replicas but not
	// others, the behavior is poorly defined unless that error is a
	// ReplicaCorruptionError.
	TestingApplyFilter storagebase.ReplicaApplyFilter

	// TestingPostApplyFilter is called after a command is applied to
	// rocksdb but before in-memory side effects have been processed.
	TestingPostApplyFilter storagebase.ReplicaApplyFilter

	// TestingResponseFilter is called after the replica processes a
	// command in order for unittests to modify the batch response,
	// error returned to the client, or to simulate network failures.
	TestingResponseFilter storagebase.ReplicaResponseFilter

	// If non-nil, BadChecksumPanic is called by CheckConsistency() instead of
	// panicking on a checksum mismatch.
	BadChecksumPanic func(roachpb.StoreIdent)
	// If non-nil, BadChecksumReportDiff is called by CheckConsistency() on a
	// checksum mismatch to report the diff between snapshots.
	BadChecksumReportDiff func(roachpb.StoreIdent, []ReplicaSnapshotDiff)
	// Disables the use of optional one phase commits. Even when enabled, requests
	// that set the Require1PC flag are permitted to use one phase commits. This
	// prevents wedging node liveness, which requires one phase commits during
	// liveness updates.
	DisableOptional1PC bool
	// A hack to manipulate the clock before sending a batch request to a replica.
	// TODO(kaneda): This hook is not encouraged to use. Get rid of it once
	// we make TestServer take a ManualClock.
	ClockBeforeSend func(*hlc.Clock, roachpb.BatchRequest)
	// OnCampaign is called if the replica campaigns for Raft leadership
	// when initializing the Raft group. Note that this method is invoked
	// with both Replica.raftMu and Replica.mu locked.
	OnCampaign func(*Replica)
	// OnCommandQueueAction is called when the BatchRequest performs an action
	// on the CommandQueue.
	OnCommandQueueAction func(*roachpb.BatchRequest, storagebase.CommandQueueAction)
	// MaxOffset, if set, overrides the server clock's MaxOffset at server
	// creation time.
	// See also DisableMaxOffsetCheck.
	MaxOffset time.Duration
	// DisableMaxOffsetCheck disables the rejection (in Store.Send) of requests
	// with the timestamp too much in the future. Normally, this rejection is a
	// good sanity check, but certain tests unfortunately insert a "message from
	// the future" into the system to advance the clock of a TestServer. We
	// should get rid of such practices once we make TestServer take a
	// ManualClock.
	DisableMaxOffsetCheck bool
	// DontPreventUseOfOldLeaseOnStart disables the initialization of
	// replica.mu.minLeaseProposedTS on replica.Init(). This has the effect of
	// allowing the replica to use the lease that it had in a previous life (in
	// case the tests persisted the engine used in said previous life).
	DontPreventUseOfOldLeaseOnStart bool
	// DisableAutomaticLeaseRenewal enables turning off the background worker
	// that attempts to automatically renew expiration-based leases.
	DisableAutomaticLeaseRenewal bool
	// LeaseRequestEvent, if set, is called when replica.requestLeaseLocked() is
	// called to acquire a new lease. This can be used to assert that a request
	// triggers a lease acquisition.
	LeaseRequestEvent func(ts hlc.Timestamp)
	// LeaseTransferBlockedOnExtensionEvent, if set, is called when
	// replica.TransferLease() encounters an in-progress lease extension.
	// nextLeader is the replica that we're trying to transfer the lease to.
	LeaseTransferBlockedOnExtensionEvent func(nextLeader roachpb.ReplicaDescriptor)
	// DisableGCQueue disables the GC queue.
	DisableGCQueue bool
	// DisableMergeQueue disables the merge queue.
	DisableMergeQueue bool
	// DisableReplicaGCQueue disables the replica GC queue.
	DisableReplicaGCQueue bool
	// DisableReplicateQueue disables the replication queue.
	DisableReplicateQueue bool
	// DisableReplicaRebalancing disables rebalancing of replicas but otherwise
	// leaves the replicate queue operational.
	DisableReplicaRebalancing bool
	// DisableSplitQueue disables the split queue.
	DisableSplitQueue bool
	// DisableTimeSeriesMaintenanceQueue disables the time series maintenance
	// queue.
	DisableTimeSeriesMaintenanceQueue bool
	// DisableRaftSnapshotQueue disables the raft snapshot queue.
	DisableRaftSnapshotQueue bool
	// DisableConsistencyQueue disables the consistency checker.
	DisableConsistencyQueue bool
	// DisableScanner disables the replica scanner.
	DisableScanner bool
	// DisablePeriodicGossips disables periodic gossiping.
	DisablePeriodicGossips bool
	// DisableLeaderFollowsLeaseholder disables attempts to transfer raft
	// leadership when it diverges from the range's leaseholder.
	DisableLeaderFollowsLeaseholder bool
	// DisableRefreshReasonNewLeader disables refreshing pending commands when a new
	// leader is discovered.
	DisableRefreshReasonNewLeader bool
	// DisableRefreshReasonNewLeaderOrConfigChange disables refreshing pending
	// commands when a new leader is discovered or when a config change is
	// dropped.
	DisableRefreshReasonNewLeaderOrConfigChange bool
	// DisableRefreshReasonTicks disables refreshing pending commands when a
	// snapshot is applied.
	DisableRefreshReasonSnapshotApplied bool
	// DisableRefreshReasonTicks disables refreshing pending commands
	// periodically.
	DisableRefreshReasonTicks bool
	// RefreshReasonTicksPeriod overrides the default period over which
	// pending commands are refreshed. The period is specified as a multiple
	// of Raft group ticks.
	RefreshReasonTicksPeriod int
	// DisableProcessRaft disables the process raft loop.
	DisableProcessRaft bool
	// DisableLastProcessedCheck disables checking on replica queue last processed times.
	DisableLastProcessedCheck bool
	// ReplicateQueueAcceptsUnsplit allows the replication queue to
	// process ranges that need to be split, for use in tests that use
	// the replication queue but disable the split queue.
	ReplicateQueueAcceptsUnsplit bool
	// SplitQueuePurgatoryChan allows a test to control the channel used to
	// trigger split queue purgatory processing.
	SplitQueuePurgatoryChan <-chan time.Time
	// SkipMinSizeCheck, if set, makes the store creation process skip the check
	// for a minimum size.
	SkipMinSizeCheck bool
	// DisableAsyncIntentResolution disables the async intent resolution
	// path (but leaves synchronous resolution). This can avoid some
	// edge cases in tests that start and stop servers.
	DisableAsyncIntentResolution bool
	// ForceSyncIntentResolution forces all asynchronous intent resolution to be
	// performed synchronously. It is equivalent to setting IntentResolverTaskLimit
	// to -1.
	ForceSyncIntentResolution bool
	// DisableLeaseCapacityGossip disables the ability of a changing number of
	// leases to trigger the store to gossip its capacity. With this enabled,
	// only changes in the number of replicas can cause the store to gossip its
	// capacity.
	DisableLeaseCapacityGossip bool
	// BootstrapVersion overrides the version the stores will be bootstrapped with.
	BootstrapVersion *cluster.ClusterVersion
}

var _ base.ModuleTestingKnobs = &StoreTestingKnobs{}

// ModuleTestingKnobs is part of the base.ModuleTestingKnobs interface.
func (*StoreTestingKnobs) ModuleTestingKnobs() {}

// Valid returns true if the StoreConfig is populated correctly.
// We don't check for Gossip and DB since some of our tests pass
// that as nil.
func (sc *StoreConfig) Valid() bool {
	return sc.Clock != nil && sc.Transport != nil &&
		sc.RaftTickInterval != 0 && sc.RaftHeartbeatIntervalTicks > 0 &&
		sc.RaftElectionTimeoutTicks > 0 && sc.ScanInterval >= 0 &&
		sc.AmbientCtx.Tracer != nil
}

// SetDefaults initializes unset fields in StoreConfig to values
// suitable for use on a local network.
// TODO(tschottdorf): see if this ought to be configurable via flags.
func (sc *StoreConfig) SetDefaults() {
	sc.RaftConfig.SetDefaults()

	if sc.CoalescedHeartbeatsInterval == 0 {
		sc.CoalescedHeartbeatsInterval = sc.RaftTickInterval / 2
	}
	if sc.RaftHeartbeatIntervalTicks == 0 {
		sc.RaftHeartbeatIntervalTicks = defaultHeartbeatIntervalTicks
	}
	if sc.RaftEntryCacheSize == 0 {
		sc.RaftEntryCacheSize = defaultRaftEntryCacheSize
	}
	if sc.IntentResolverTaskLimit == -1 || sc.TestingKnobs.ForceSyncIntentResolution {
		sc.IntentResolverTaskLimit = 0
	} else if sc.IntentResolverTaskLimit == 0 {
		sc.IntentResolverTaskLimit = defaultIntentResolverTaskLimit
	}
	if sc.concurrentSnapshotApplyLimit == 0 {
		// NB: setting this value higher than 1 is likely to degrade client
		// throughput.
		sc.concurrentSnapshotApplyLimit =
			envutil.EnvOrDefaultInt("COCKROACH_CONCURRENT_SNAPSHOT_APPLY_LIMIT", 1)
	}

	if sc.GossipWhenCapacityDeltaExceedsFraction == 0 {
		sc.GossipWhenCapacityDeltaExceedsFraction = defaultGossipWhenCapacityDeltaExceedsFraction
	}
}

// LeaseExpiration returns an int64 to increment a manual clock with to
// make sure that all active range leases expire.
func (sc *StoreConfig) LeaseExpiration() int64 {
	// Due to lease extensions, the remaining interval can be longer than just
	// the sum of the offset (=length of stasis period) and the active
	// duration, but definitely not by 2x.
	maxOffset := sc.Clock.MaxOffset()
	if maxOffset == timeutil.ClocklessMaxOffset {
		// Don't do shady math on clockless reads.
		maxOffset = 0
	}
	return 2 * (sc.RangeLeaseActiveDuration() + maxOffset).Nanoseconds()
}

// NewStore returns a new instance of a store.
func NewStore(cfg StoreConfig, eng engine.Engine, nodeDesc *roachpb.NodeDescriptor) *Store {
	// TODO(tschottdorf): find better place to set these defaults.
	cfg.SetDefaults()

	if !cfg.Valid() {
		log.Fatalf(context.Background(), "invalid store configuration: %+v", &cfg)
	}
	s := &Store{
		cfg:      cfg,
		db:       cfg.DB, // TODO(tschottdorf): remove redundancy.
		engine:   eng,
		nodeDesc: nodeDesc,
		metrics:  newStoreMetrics(cfg.HistogramWindowInterval),
	}
	if cfg.RPCContext != nil {
		s.allocator = MakeAllocator(cfg.StorePool, cfg.RPCContext.RemoteClocks.Latency)
	} else {
		s.allocator = MakeAllocator(cfg.StorePool, func(string) (time.Duration, bool) {
			return 0, false
		})
	}
	s.intentResolver = newIntentResolver(s, cfg.IntentResolverTaskLimit)
	s.raftEntryCache = newRaftEntryCache(cfg.RaftEntryCacheSize)
	s.draining.Store(false)
	s.scheduler = newRaftScheduler(s.cfg.AmbientCtx, s.metrics, s, storeSchedulerConcurrency)

	s.coalescedMu.Lock()
	s.coalescedMu.heartbeats = map[roachpb.StoreIdent][]RaftHeartbeat{}
	s.coalescedMu.heartbeatResponses = map[roachpb.StoreIdent][]RaftHeartbeat{}
	s.coalescedMu.Unlock()

	s.mu.Lock()
	s.mu.replicaPlaceholders = map[roachpb.RangeID]*ReplicaPlaceholder{}
	s.mu.replicasByKey = btree.New(64 /* degree */)
	s.mu.uninitReplicas = map[roachpb.RangeID]*Replica{}
	s.mu.Unlock()

	s.unquiescedReplicas.Lock()
	s.unquiescedReplicas.m = map[roachpb.RangeID]struct{}{}
	s.unquiescedReplicas.Unlock()

	tsCacheMetrics := tscache.MakeMetrics()
	s.tsCache = tscache.New(cfg.Clock, cfg.TimestampCachePageSize, tsCacheMetrics)
	s.metrics.registry.AddMetricStruct(tsCacheMetrics)

	s.compactor = compactor.NewCompactor(
		s.cfg.Settings,
		s.engine.(engine.WithSSTables),
		func() (roachpb.StoreCapacity, error) {
			return s.Capacity(false /* useCached */)
		},
		func(ctx context.Context) {
			s.asyncGossipStore(ctx, "compactor-initiated rocksdb compaction", false /* useCached */)
		},
	)
	s.metrics.registry.AddMetricStruct(s.compactor.Metrics)

	s.snapshotApplySem = make(chan struct{}, cfg.concurrentSnapshotApplyLimit)

	// The channel size here is arbitrary. We don't want it to fill up, but it
	// isn't a disaster if it does and it shouldn't unless a huge number of meta2
	// range leases are acquired at once.
	s.expirationBasedLeaseChan = make(chan *Replica, 64)

	s.limiters.BulkIOWriteRate = rate.NewLimiter(rate.Limit(bulkIOWriteLimit.Get(&cfg.Settings.SV)), bulkIOWriteBurst)
	bulkIOWriteLimit.SetOnChange(&cfg.Settings.SV, func() {
		s.limiters.BulkIOWriteRate.SetLimit(rate.Limit(bulkIOWriteLimit.Get(&cfg.Settings.SV)))
	})
	s.limiters.ConcurrentImports = limit.MakeConcurrentRequestLimiter(
		"importRequestLimiter", int(importRequestsLimit.Get(&cfg.Settings.SV)),
	)
	importRequestsLimit.SetOnChange(&cfg.Settings.SV, func() {
		s.limiters.ConcurrentImports.SetLimit(int(importRequestsLimit.Get(&cfg.Settings.SV)))
	})
	s.limiters.ConcurrentExports = limit.MakeConcurrentRequestLimiter(
		"exportRequestLimiter", int(ExportRequestsLimit.Get(&cfg.Settings.SV)),
	)
	ExportRequestsLimit.SetOnChange(&cfg.Settings.SV, func() {
		s.limiters.ConcurrentExports.SetLimit(int(ExportRequestsLimit.Get(&cfg.Settings.SV)))
	})

	if s.cfg.Gossip != nil {
		// Add range scanner and configure with queues.
		s.scanner = newReplicaScanner(
			s.cfg.AmbientCtx, s.cfg.Clock, cfg.ScanInterval,
			cfg.ScanMinIdleTime, cfg.ScanMaxIdleTime, newStoreReplicaVisitor(s),
		)
		s.gcQueue = newGCQueue(s, s.cfg.Gossip)
		s.mergeQueue = newMergeQueue(s, s.db, s.cfg.Gossip)
		s.splitQueue = newSplitQueue(s, s.db, s.cfg.Gossip)
		s.replicateQueue = newReplicateQueue(s, s.cfg.Gossip, s.allocator)
		s.replicaGCQueue = newReplicaGCQueue(s, s.db, s.cfg.Gossip)
		s.raftLogQueue = newRaftLogQueue(s, s.db, s.cfg.Gossip)
		s.raftSnapshotQueue = newRaftSnapshotQueue(s, s.cfg.Gossip)
		s.consistencyQueue = newConsistencyQueue(s, s.cfg.Gossip)
		s.scanner.AddQueues(
			s.gcQueue, s.mergeQueue, s.splitQueue, s.replicateQueue, s.replicaGCQueue,
			s.raftLogQueue, s.raftSnapshotQueue, s.consistencyQueue)

		if s.cfg.TimeSeriesDataStore != nil {
			s.tsMaintenanceQueue = newTimeSeriesMaintenanceQueue(
				s, s.db, s.cfg.Gossip, s.cfg.TimeSeriesDataStore,
			)
			s.scanner.AddQueues(s.tsMaintenanceQueue)
		}
	}

	if cfg.TestingKnobs.DisableGCQueue {
		s.setGCQueueActive(false)
	}
	if cfg.TestingKnobs.DisableMergeQueue {
		s.setMergeQueueActive(false)
	}
	if cfg.TestingKnobs.DisableReplicaGCQueue {
		s.setReplicaGCQueueActive(false)
	}
	if cfg.TestingKnobs.DisableReplicateQueue {
		s.setReplicateQueueActive(false)
	}
	if cfg.TestingKnobs.DisableSplitQueue {
		s.setSplitQueueActive(false)
	}
	if cfg.TestingKnobs.DisableTimeSeriesMaintenanceQueue {
		s.setTimeSeriesMaintenanceQueueActive(false)
	}
	if cfg.TestingKnobs.DisableRaftSnapshotQueue {
		s.setRaftSnapshotQueueActive(false)
	}
	if cfg.TestingKnobs.DisableConsistencyQueue {
		s.setConsistencyQueueActive(false)
	}
	if cfg.TestingKnobs.DisableScanner {
		s.setScannerActive(false)
	}

	return s
}

// String formats a store for debug output.
func (s *Store) String() string {
	return fmt.Sprintf("[n%d,s%d]", s.Ident.NodeID, s.Ident.StoreID)
}

// ClusterSettings returns the node's ClusterSettings.
func (s *Store) ClusterSettings() *cluster.Settings {
	return s.cfg.Settings
}

// AnnotateCtx is a convenience wrapper; see AmbientContext.
func (s *Store) AnnotateCtx(ctx context.Context) context.Context {
	return s.cfg.AmbientCtx.AnnotateCtx(ctx)
}

// The maximum amount of time waited for leadership shedding before commencing
// to drain a store.
const raftLeadershipTransferWait = 5 * time.Second

// SetDraining (when called with 'true') causes incoming lease transfers to be
// rejected, prevents all of the Store's Replicas from acquiring or extending
// range leases, and attempts to transfer away any leases owned.
// When called with 'false', returns to the normal mode of operation.
func (s *Store) SetDraining(drain bool) {
	s.draining.Store(drain)
	if !drain {
		newStoreReplicaVisitor(s).Visit(func(r *Replica) bool {
			r.mu.Lock()
			r.mu.draining = false
			r.mu.Unlock()
			return true
		})
		return
	}

	var wg sync.WaitGroup

	ctx := log.WithLogTag(context.Background(), "drain", nil)
	transferAllAway := func() int {
		// Limit the number of concurrent lease transfers.
		sem := make(chan struct{}, 100)
		sysCfg, sysCfgSet := s.cfg.Gossip.GetSystemConfig()
		// Incremented for every lease or Raft leadership transfer attempted. We try
		// to send both the lease and the Raft leaders away, but this may not
		// reliably work. Instead, we run the surrounding retry loop until there are
		// no leaders/leases left (ignoring single-replica or uninitialized Raft
		// groups).
		var numTransfersAttempted int32
		newStoreReplicaVisitor(s).Visit(func(r *Replica) bool {
			wg.Add(1)
			if err := s.stopper.RunLimitedAsyncTask(
				r.AnnotateCtx(ctx), "storage.Store: draining replica", sem, true, /* wait */
				func(ctx context.Context) {
					defer wg.Done()

					r.mu.Lock()
					r.mu.draining = true
					status := r.raftStatusRLocked()
					// needsRaftTransfer is true when we can reasonably hope to transfer
					// this replica's lease and/or Raft leadership away.
					needsRaftTransfer := status != nil &&
						len(status.Progress) > 1 &&
						!(status.RaftState == raft.StateFollower && status.Lead != 0)
					r.mu.Unlock()

					var drainingLease roachpb.Lease
					for {
						var llHandle *leaseRequestHandle
						r.mu.Lock()
						lease, nextLease := r.getLeaseRLocked()
						if nextLease != nil && nextLease.OwnedBy(s.StoreID()) {
							llHandle = r.mu.pendingLeaseRequest.JoinRequest()
						}
						r.mu.Unlock()

						if llHandle != nil {
							<-llHandle.C()
							continue
						}
						drainingLease = lease
						break
					}

					needsLeaseTransfer := len(r.Desc().Replicas) > 1 &&
						drainingLease.OwnedBy(s.StoreID()) &&
						r.IsLeaseValid(drainingLease, s.Clock().Now())

					if needsLeaseTransfer || needsRaftTransfer {
						atomic.AddInt32(&numTransfersAttempted, 1)
					}

					if needsLeaseTransfer {
						desc := r.Desc()
						zone := config.DefaultZoneConfig()
						if sysCfgSet {
							var err error
							zone, err = sysCfg.GetZoneConfigForKey(desc.StartKey)
							if log.V(1) && err != nil {
								log.Errorf(ctx, "could not get zone config for key %s when draining: %s", desc.StartKey, err)
							}
						}
						leaseTransferred, err := s.replicateQueue.transferLease(
							ctx,
							r,
							desc,
							zone,
							transferLeaseOptions{},
						)
						if log.V(1) && !leaseTransferred {
							// Note that a nil error means that there were no suitable
							// candidates.
							log.Errorf(
								ctx,
								"did not transfer lease %s for replica %s when draining: %v",
								drainingLease,
								desc,
								err,
							)
						}
						if err == nil && leaseTransferred {
							// If we just transferred the lease away, Raft leadership will
							// usually transfer with it. Invoking a separate Raft leadership
							// transfer would only obstruct this.
							needsRaftTransfer = false
						}
					}

					if needsRaftTransfer {
						r.raftMu.Lock()
						r.maybeTransferRaftLeadership(ctx, drainingLease.Replica.ReplicaID)
						r.raftMu.Unlock()
					}
				}); err != nil {
				if log.V(1) {
					log.Errorf(ctx, "error running draining task: %s", err)
				}
				wg.Done()
				return false
			}
			return true
		})
		wg.Wait()
		return int(numTransfersAttempted)
	}

	transferAllAway()

	var cancel func()
	ctx, cancel = context.WithTimeout(ctx, raftLeadershipTransferWait)
	defer cancel()

	opts := retry.Options{
		InitialBackoff: 10 * time.Millisecond,
		MaxBackoff:     time.Second,
		Multiplier:     2,
	}
	// Avoid retry.ForDuration because of https://github.com/cockroachdb/cockroach/issues/25091.
	everySecond := log.Every(time.Second)
	if err := retry.WithMaxAttempts(ctx, opts, 10000, func() error {
		if numRemaining := transferAllAway(); numRemaining > 0 {
			err := errors.Errorf("waiting for %d replicas to transfer their lease away", numRemaining)
			if everySecond.ShouldLog() {
				log.Info(ctx, err)
			}
			return err
		}
		return nil
	}); err != nil {
		// You expect this message when shutting down a server in an unhealthy
		// cluster. If we see it on healthy ones, there's likely something to fix.
		log.Warningf(ctx, "unable to drain cleanly within %s, service might briefly deteriorate: %s", raftLeadershipTransferWait, err)
	}
}

// IsStarted returns true if the Store has been started.
func (s *Store) IsStarted() bool {
	return atomic.LoadInt32(&s.started) == 1
}

// IterateIDPrefixKeys helps visit system keys that use RangeID prefixing ( such as
// RaftHardStateKey, RaftTombstoneKey, and many others). Such keys could in principle exist at any
// RangeID, and this helper efficiently discovers all the keys of the desired type (as specified by
// the supplied `keyFn`) and, for each key-value pair discovered, unmarshals it into `msg` and then
// invokes `f`.
//
// Iteration stops on the first error (and will pass through that error).
func IterateIDPrefixKeys(
	ctx context.Context,
	eng engine.Reader,
	keyFn func(roachpb.RangeID) roachpb.Key,
	msg protoutil.Message,
	f func(_ roachpb.RangeID) (more bool, _ error),
) error {
	rangeID := roachpb.RangeID(1)
	iter := eng.NewIterator(engine.IterOptions{
		UpperBound: keys.LocalRangeIDPrefix.PrefixEnd().AsRawKey(),
	})
	defer iter.Close()

	for {
		bumped := false
		mvccKey := engine.MakeMVCCMetadataKey(keyFn(rangeID))
		iter.Seek(mvccKey)

		if ok, err := iter.Valid(); !ok {
			return err
		}

		unsafeKey := iter.UnsafeKey()

		if !bytes.HasPrefix(unsafeKey.Key, keys.LocalRangeIDPrefix) {
			// Left the local keyspace, so we're done.
			return nil
		}

		curRangeID, _, _, _, err := keys.DecodeRangeIDKey(unsafeKey.Key)
		if err != nil {
			return err
		}

		if curRangeID > rangeID {
			// `bumped` is always `false` here, but let's be explicit.
			if !bumped {
				rangeID = curRangeID
				bumped = true
			}
			mvccKey = engine.MakeMVCCMetadataKey(keyFn(rangeID))
		}

		if !unsafeKey.Key.Equal(mvccKey.Key) {
			if !bumped {
				// Don't increment the rangeID if it has already been incremented
				// above, or we could skip past a value we ought to see.
				rangeID++
				bumped = true // for completeness' sake; continuing below anyway
			}
			continue
		}

		ok, err := engine.MVCCGetProto(
			ctx, eng, unsafeKey.Key, hlc.Timestamp{}, true /* consistent */, nil /* txn */, msg,
		)
		if err != nil {
			return err
		}
		if !ok {
			return errors.Errorf("unable to unmarshal %s into %T", unsafeKey.Key, msg)
		}

		more, err := f(rangeID)
		if !more || err != nil {
			return err
		}
		rangeID++
	}
}

// IterateRangeDescriptors calls the provided function with each descriptor
// from the provided Engine. The return values of this method and fn have
// semantics similar to engine.MVCCIterate.
func IterateRangeDescriptors(
	ctx context.Context, eng engine.Reader, fn func(desc roachpb.RangeDescriptor) (bool, error),
) error {
	log.Event(ctx, "beginning range descriptor iteration")
	// Iterator over all range-local key-based data.
	start := keys.RangeDescriptorKey(roachpb.RKeyMin)
	end := keys.RangeDescriptorKey(roachpb.RKeyMax)

	allCount := 0
	matchCount := 0
	bySuffix := make(map[string]int)
	kvToDesc := func(kv roachpb.KeyValue) (bool, error) {
		allCount++
		// Only consider range metadata entries; ignore others.
		_, suffix, _, err := keys.DecodeRangeKey(kv.Key)
		if err != nil {
			return false, err
		}
		bySuffix[string(suffix)]++
		if !bytes.Equal(suffix, keys.LocalRangeDescriptorSuffix) {
			return false, nil
		}
		var desc roachpb.RangeDescriptor
		if err := kv.Value.GetProto(&desc); err != nil {
			return false, err
		}
		matchCount++
		return fn(desc)
	}

	_, err := engine.MVCCIterate(ctx, eng, start, end, hlc.MaxTimestamp, false /* consistent */, false, /* tombstones */
		nil /* txn */, false /* reverse */, kvToDesc)
	log.Eventf(ctx, "iterated over %d keys to find %d range descriptors (by suffix: %v)",
		allCount, matchCount, bySuffix)
	return err
}

// ReadStoreIdent reads the StoreIdent from the store.
// It returns *NotBootstrappedError if the ident is missing (meaning that the
// store needs to be bootstrapped).
func ReadStoreIdent(ctx context.Context, eng engine.Engine) (roachpb.StoreIdent, error) {
	var ident roachpb.StoreIdent
	ok, err := engine.MVCCGetProto(
		ctx, eng, keys.StoreIdentKey(), hlc.Timestamp{}, true, nil, &ident)
	if err != nil {
		return roachpb.StoreIdent{}, err
	} else if !ok {
		return roachpb.StoreIdent{}, &NotBootstrappedError{}
	}
	return ident, err
}

// Start the engine, set the GC and read the StoreIdent.
func (s *Store) Start(ctx context.Context, stopper *stop.Stopper) error {
	s.stopper = stopper

	// Populate the store ident. If not bootstrapped, ReadStoreIntent will
	// return an error.
	ident, err := ReadStoreIdent(ctx, s.engine)
	if err != nil {
		return err
	}
	s.Ident = &ident

	// Set the store ID for logging.
	s.cfg.AmbientCtx.AddLogTagInt("s", int(s.StoreID()))
	ctx = s.AnnotateCtx(ctx)
	log.Event(ctx, "read store identity")

	// If the nodeID is 0, it has not be assigned yet.
	if s.nodeDesc.NodeID != 0 && s.Ident.NodeID != s.nodeDesc.NodeID {
		return errors.Errorf("node id:%d does not equal the one in node descriptor:%d", s.Ident.NodeID, s.nodeDesc.NodeID)
	}
	// Always set gossip NodeID before gossiping any info.
	if s.cfg.Gossip != nil {
		s.cfg.Gossip.NodeID.Set(ctx, s.Ident.NodeID)
	}

	// Create ID allocators.
	idAlloc, err := idalloc.NewAllocator(
		s.cfg.AmbientCtx, keys.RangeIDGenerator, s.db, 2 /* minID */, rangeIDAllocCount, s.stopper,
	)
	if err != nil {
		return err
	}
	s.rangeIDAlloc = idAlloc

	now := s.cfg.Clock.Now()
	s.startedAt = now.WallTime

	// Migrate legacy tombstones away. This is safe to do unconditionally: this is
	// a post-v2.0 binary, so we're guaranteed that every node in the cluster is
	// running a version that understands the non-legacy tombstones (v2.0 or
	// later).
	//
	// We want to run this migration the first time the node boots with this
	// binary version so that we can assume local data never contains legacy range
	// tombstones. For simplicity, we do it on *every* boot. Should this be found
	// to impact startup times too much, we can make it only run the first time
	// this binary version is booted.
	{
		tBegin := timeutil.Now()
		if err := migrateLegacyTombstones(ctx, s.engine); err != nil {
			return errors.Wrapf(err, "migrating legacy tombstones for %v", s.engine)
		}
		f := log.Eventf

		dur := timeutil.Since(tBegin)
		if dur > 10*time.Second {
			f = log.Infof
		}
		f(ctx, "ran legacy tombstone migration in %s", dur)
	}

	// Iterate over all range descriptors, ignoring uncommitted versions
	// (consistent=false). Uncommitted intents which have been abandoned
	// due to a split crashing halfway will simply be resolved on the
	// next split attempt. They can otherwise be ignored.

	// TODO(peter): While we have to iterate to find the replica descriptors
	// serially, we can perform the migrations and replica creation
	// concurrently. Note that while we can perform this initialization
	// concurrently, all of the initialization must be performed before we start
	// listening for Raft messages and starting the process Raft loop.
	err = IterateRangeDescriptors(ctx, s.engine,
		func(desc roachpb.RangeDescriptor) (bool, error) {
			if !desc.IsInitialized() {
				return false, errors.Errorf("found uninitialized RangeDescriptor: %+v", desc)
			}

			rep, err := NewReplica(&desc, s, 0)
			if err != nil {
				return false, err
			}

			// We can't lock s.mu across NewReplica due to the lock ordering
			// constraint (*Replica).raftMu < (*Store).mu. See the comment on
			// (Store).mu.
			s.mu.Lock()
			err = s.addReplicaInternalLocked(rep)
			s.mu.Unlock()
			if err != nil {
				return false, err
			}

			// Add this range and its stats to our counter.
			s.metrics.ReplicaCount.Inc(1)
			s.metrics.addMVCCStats(rep.GetMVCCStats())

			if _, ok := desc.GetReplicaDescriptor(s.StoreID()); !ok {
				// We are no longer a member of the range, but we didn't GC the replica
				// before shutting down. Add the replica to the GC queue.
				if added, err := s.replicaGCQueue.Add(rep, replicaGCPriorityRemoved); err != nil {
					log.Errorf(ctx, "%s: unable to add replica to GC queue: %s", rep, err)
				} else if added {
					log.Infof(ctx, "%s: added to replica GC queue", rep)
				}
			}

			// Note that we do not create raft groups at this time; they will be created
			// on-demand the first time they are needed. This helps reduce the amount of
			// election-related traffic in a cold start.
			// Raft initialization occurs when we propose a command on this range or
			// receive a raft message addressed to it.
			// TODO(bdarnell): Also initialize raft groups when read leases are needed.
			// TODO(bdarnell): Scan all ranges at startup for unapplied log entries
			// and initialize those groups.
			return false, nil
		})
	if err != nil {
		return err
	}

	// Start Raft processing goroutines.
	s.cfg.Transport.Listen(s.StoreID(), s)
	s.processRaft(ctx)

	// Register a callback to unquiesce any ranges with replicas on a
	// node transitioning from non-live to live.
	if s.cfg.NodeLiveness != nil {
		s.cfg.NodeLiveness.RegisterCallback(s.nodeIsLiveCallback)
	}

	// Gossip is only ever nil while bootstrapping a cluster and
	// in unittests.
	if s.cfg.Gossip != nil {
		// Register update channel for any changes to the system config.
		// This may trigger splits along structured boundaries,
		// and update max range bytes.
		gossipUpdateC := s.cfg.Gossip.RegisterSystemConfigChannel()
		s.stopper.RunWorker(ctx, func(context.Context) {
			for {
				select {
				case <-gossipUpdateC:
					cfg, _ := s.cfg.Gossip.GetSystemConfig()
					s.systemGossipUpdate(cfg)
				case <-s.stopper.ShouldStop():
					return
				}
			}
		})

		// Start a single goroutine in charge of periodically gossiping the
		// sentinel and first range metadata if we have a first range.
		// This may wake up ranges and requires everything to be set up and
		// running.
		s.startGossip()

		// Start the scanner. The construction here makes sure that the scanner
		// only starts after Gossip has connected, and that it does not block Start
		// from returning (as doing so might prevent Gossip from ever connecting).
		s.stopper.RunWorker(ctx, func(context.Context) {
			select {
			case <-s.cfg.Gossip.Connected:
				s.scanner.Start(s.stopper)
			case <-s.stopper.ShouldStop():
				return
			}
		})

		// Run metrics computation up front to populate initial statistics.
		if err = s.ComputeMetrics(ctx, -1); err != nil {
			log.Infof(ctx, "%s: failed initial metrics computation: %s", s, err)
		}
		log.Event(ctx, "computed initial metrics")
	}

	if !s.cfg.TestingKnobs.DisableAutomaticLeaseRenewal {
		s.startLeaseRenewer(ctx)
	}

	// Start the storage engine compactor.
	if envutil.EnvOrDefaultBool("COCKROACH_ENABLE_COMPACTOR", true) {
		s.compactor.Start(s.AnnotateCtx(context.Background()), s.stopper)
	}

	// Set the started flag (for unittests).
	atomic.StoreInt32(&s.started, 1)

	return nil
}

// WaitForInit waits for any asynchronous processes begun in Start()
// to complete their initialization. In particular, this includes
// gossiping. In some cases this may block until the range GC queue
// has completed its scan. Only for testing.
func (s *Store) WaitForInit() {
	s.initComplete.Wait()
}

var errPeriodicGossipsDisabled = errors.New("periodic gossip is disabled")

// startGossip runs an infinite loop in a goroutine which regularly checks
// whether the store has a first range or config replica and asks those ranges
// to gossip accordingly.
func (s *Store) startGossip() {
	wakeReplica := func(ctx context.Context, repl *Replica) error {
		// Acquire the range lease, which in turn triggers system data gossip
		// functions (e.g. MaybeGossipSystemConfig or MaybeGossipNodeLiveness).
		_, pErr := repl.getLeaseForGossip(ctx)
		return pErr.GoError()
	}

	if s.cfg.TestingKnobs.DisablePeriodicGossips {
		wakeReplica = func(context.Context, *Replica) error {
			return errPeriodicGossipsDisabled
		}
	}

	gossipFns := []struct {
		key         roachpb.Key
		fn          func(context.Context, *Replica) error
		description string
		interval    time.Duration
	}{
		{
			key: roachpb.KeyMin,
			fn: func(ctx context.Context, repl *Replica) error {
				// The first range is gossiped by all replicas, not just the lease
				// holder, so wakeReplica is not used here.
				return repl.maybeGossipFirstRange(ctx).GoError()
			},
			description: "first range descriptor",
			interval:    sentinelGossipInterval,
		},
		{
			key:         keys.SystemConfigSpan.Key,
			fn:          wakeReplica,
			description: "system config",
			interval:    systemDataGossipInterval,
		},
		{
			key:         keys.NodeLivenessSpan.Key,
			fn:          wakeReplica,
			description: "node liveness",
			interval:    systemDataGossipInterval,
		},
	}

	// Periodic updates run in a goroutine and signal a WaitGroup upon completion
	// of their first iteration.
	s.initComplete.Add(len(gossipFns))
	for _, gossipFn := range gossipFns {
		gossipFn := gossipFn // per-iteration copy
		s.stopper.RunWorker(context.Background(), func(ctx context.Context) {
			ticker := time.NewTicker(gossipFn.interval)
			defer ticker.Stop()
			for first := true; ; {
				// Retry in a backoff loop until gossipFn succeeds. The gossipFn might
				// temporarily fail (e.g. because node liveness hasn't initialized yet
				// making it impossible to get an epoch-based range lease), in which
				// case we want to retry quickly.
				retryOptions := base.DefaultRetryOptions()
				retryOptions.Closer = s.stopper.ShouldStop()
				for r := retry.Start(retryOptions); r.Next(); {
					if repl := s.LookupReplica(roachpb.RKey(gossipFn.key)); repl != nil {
						annotatedCtx := repl.AnnotateCtx(ctx)
						if err := gossipFn.fn(annotatedCtx, repl); err != nil {
							log.Warningf(annotatedCtx, "could not gossip %s: %s", gossipFn.description, err)
							if err != errPeriodicGossipsDisabled {
								continue
							}
						}
					}
					break
				}
				if first {
					first = false
					s.initComplete.Done()
				}
				select {
				case <-ticker.C:
				case <-s.stopper.ShouldStop():
					return
				}
			}
		})
	}
}

// startLeaseRenewer runs an infinite loop in a goroutine which regularly
// checks whether the store has any expiration-based leases that should be
// proactively renewed and attempts to continue renewing them.
//
// This reduces user-visible latency when range lookups are needed to serve a
// request and reduces ping-ponging of r1's lease to different replicas as
// maybeGossipFirstRange is called on each (e.g.  #24753).
func (s *Store) startLeaseRenewer(ctx context.Context) {
	// Start a goroutine that watches and proactively renews certain
	// expiration-based leases.
	s.stopper.RunWorker(ctx, func(ctx context.Context) {
		repls := make(map[*Replica]struct{})
		timer := timeutil.NewTimer()
		defer timer.Stop()

		// Determine how frequently to attempt to ensure that we have each lease.
		// The divisor used here is somewhat arbitrary, but needs to be large
		// enough to ensure we'll attempt to renew the lease reasonably early
		// within the RangeLeaseRenewalDuration time window. This means we'll wake
		// up more often that strictly necessary, but it's more maintainable than
		// attempting to accurately determine exactly when each iteration of a
		// lease expires and when we should attempt to renew it as a result.
		renewalDuration := s.cfg.RangeLeaseActiveDuration() / 5
		for {
			for repl := range repls {
				annotatedCtx := repl.AnnotateCtx(ctx)
				_, pErr := repl.redirectOnOrAcquireLease(annotatedCtx)
				if pErr != nil {
					if _, ok := pErr.GetDetail().(*roachpb.NotLeaseHolderError); !ok {
						log.Warningf(annotatedCtx, "failed to proactively renew lease: %s", pErr)
					}
					delete(repls, repl)
					continue
				}
			}

			if len(repls) > 0 {
				timer.Reset(renewalDuration)
			}
			select {
			case repl := <-s.expirationBasedLeaseChan:
				repls[repl] = struct{}{}
				// If we got one entry off the channel, there may be more (e.g. a node
				// holding a bunch of meta2 ranges just failed), so keep pulling until
				// we can't get any more.
			continuepulling:
				for {
					select {
					case repl := <-s.expirationBasedLeaseChan:
						repls[repl] = struct{}{}
					default:
						break continuepulling
					}
				}
			case <-timer.C:
				timer.Read = true
			case <-s.stopper.ShouldStop():
				return
			}
		}
	})
}

// systemGossipUpdate is a callback for gossip updates to
// the system config which affect range split boundaries.
func (s *Store) systemGossipUpdate(cfg config.SystemConfig) {
	// For every range, update its MaxBytes and check if it needs to be split or
	// merged.
	newStoreReplicaVisitor(s).Visit(func(repl *Replica) bool {
		if zone, err := cfg.GetZoneConfigForKey(repl.Desc().StartKey); err == nil {
			repl.SetByteThresholds(zone.RangeMinBytes, zone.RangeMaxBytes)
		}
		s.mergeQueue.MaybeAdd(repl, s.cfg.Clock.Now())
		s.splitQueue.MaybeAdd(repl, s.cfg.Clock.Now())
		return true // more
	})
}

func (s *Store) asyncGossipStore(ctx context.Context, reason string, useCached bool) {
	if err := s.stopper.RunAsyncTask(
		ctx, fmt.Sprintf("storage.Store: gossip on %s", reason),
		func(ctx context.Context) {
			if err := s.GossipStore(ctx, useCached); err != nil {
				log.Warningf(ctx, "error gossiping on %s: %s", reason, err)
			}
		}); err != nil {
		log.Warningf(ctx, "unable to gossip on %s: %s", reason, err)
	}
}

// GossipStore broadcasts the store on the gossip network.
func (s *Store) GossipStore(ctx context.Context, useCached bool) error {
	select {
	case <-s.cfg.Gossip.Connected:
	default:
		// Nothing to do if gossip is not connected.
		return nil
	}

	// Temporarily indicate that we're gossiping the store capacity to avoid
	// recursively triggering a gossip of the store capacity.
	syncutil.StoreFloat64(&s.gossipQueriesPerSecondVal, -1)
	syncutil.StoreFloat64(&s.gossipWritesPerSecondVal, -1)

	storeDesc, err := s.Descriptor(useCached)
	if err != nil {
		return errors.Wrapf(err, "problem getting store descriptor for store %+v", s.Ident)
	}

	// Set countdown target for re-gossiping capacity earlier than
	// the usual periodic interval. Re-gossip more rapidly for RangeCount
	// changes because allocators with stale information are much more
	// likely to make bad decisions.
	rangeCountdown := float64(storeDesc.Capacity.RangeCount) * s.cfg.GossipWhenCapacityDeltaExceedsFraction
	atomic.StoreInt32(&s.gossipRangeCountdown, int32(math.Ceil(math.Min(rangeCountdown, 3))))
	leaseCountdown := float64(storeDesc.Capacity.LeaseCount) * s.cfg.GossipWhenCapacityDeltaExceedsFraction
	atomic.StoreInt32(&s.gossipLeaseCountdown, int32(math.Ceil(math.Max(leaseCountdown, 1))))
	syncutil.StoreFloat64(&s.gossipQueriesPerSecondVal, storeDesc.Capacity.QueriesPerSecond)
	syncutil.StoreFloat64(&s.gossipWritesPerSecondVal, storeDesc.Capacity.WritesPerSecond)

	// Unique gossip key per store.
	gossipStoreKey := gossip.MakeStoreKey(storeDesc.StoreID)
	// Gossip store descriptor.
	return s.cfg.Gossip.AddInfoProto(gossipStoreKey, storeDesc, gossip.StoreTTL)
}

type capacityChangeEvent int

const (
	rangeAddEvent capacityChangeEvent = iota
	rangeRemoveEvent
	leaseAddEvent
	leaseRemoveEvent
)

// maybeGossipOnCapacityChange decrements the countdown on range
// and leaseholder counts. If it reaches 0, then we trigger an
// immediate gossip of this store's descriptor, to include updated
// capacity information.
func (s *Store) maybeGossipOnCapacityChange(ctx context.Context, cce capacityChangeEvent) {
	if s.cfg.TestingKnobs.DisableLeaseCapacityGossip && (cce == leaseAddEvent || cce == leaseRemoveEvent) {
		return
	}

	// Incrementally adjust stats to keep them up to date even if the
	// capacity is gossiped, but isn't due yet to be recomputed from scratch.
	s.cachedCapacity.Lock()
	switch cce {
	case rangeAddEvent:
		s.cachedCapacity.RangeCount++
	case rangeRemoveEvent:
		s.cachedCapacity.RangeCount--
	case leaseAddEvent:
		s.cachedCapacity.LeaseCount++
	case leaseRemoveEvent:
		s.cachedCapacity.LeaseCount--
	}
	s.cachedCapacity.Unlock()

	if ((cce == rangeAddEvent || cce == rangeRemoveEvent) && atomic.AddInt32(&s.gossipRangeCountdown, -1) == 0) ||
		((cce == leaseAddEvent || cce == leaseRemoveEvent) && atomic.AddInt32(&s.gossipLeaseCountdown, -1) == 0) {
		// Reset countdowns to avoid unnecessary gossiping.
		atomic.StoreInt32(&s.gossipRangeCountdown, 0)
		atomic.StoreInt32(&s.gossipLeaseCountdown, 0)
		s.asyncGossipStore(ctx, "capacity change", true /* useCached */)
	}
}

// recordNewPerSecondStats takes recently calculated values for the number of
// queries and key writes the store is handling and decides whether either has
// changed enough to justify re-gossiping the store's capacity.
func (s *Store) recordNewPerSecondStats(newQPS, newWPS float64) {
	oldQPS := syncutil.LoadFloat64(&s.gossipQueriesPerSecondVal)
	oldWPS := syncutil.LoadFloat64(&s.gossipWritesPerSecondVal)
	if oldQPS == -1 || oldWPS == -1 {
		// Gossiping of store capacity is already ongoing.
		return
	}

	const minAbsoluteChange = 100
	updateForQPS := (newQPS < oldQPS*.5 || newQPS > oldQPS*1.5) && math.Abs(newQPS-oldQPS) > minAbsoluteChange
	updateForWPS := (newWPS < oldWPS*.5 || newWPS > oldWPS*1.5) && math.Abs(newWPS-oldWPS) > minAbsoluteChange

	if !updateForQPS && !updateForWPS {
		return
	}

	var message string
	if updateForQPS && updateForWPS {
		message = "queries-per-second and writes-per-second change"
	} else if updateForQPS {
		message = "queries-per-second change"
	} else {
		message = "writes-per-second change"
	}
	// TODO(a-robinson): Use the provided values to avoid having to recalculate
	// them in GossipStore.
	s.asyncGossipStore(context.TODO(), message, false /* useCached */)
}

// GossipDeadReplicas broadcasts the store's dead replicas on the gossip
// network.
func (s *Store) GossipDeadReplicas(ctx context.Context) error {
	deadReplicas := s.deadReplicas()
	// Don't gossip if there's nothing to gossip.
	if len(deadReplicas.Replicas) == 0 {
		return nil
	}
	// Unique gossip key per store.
	key := gossip.MakeDeadReplicasKey(s.StoreID())
	// Gossip dead replicas.
	return s.cfg.Gossip.AddInfoProto(key, &deadReplicas, gossip.StoreTTL)
}

// Bootstrap writes a new store ident to the underlying engine. To
// ensure that no crufty data already exists in the engine, it scans
// the engine contents before writing the new store ident. The engine
// should be completely empty. It returns an error if called on a
// non-empty engine.
func Bootstrap(
	ctx context.Context, eng engine.Engine, ident roachpb.StoreIdent, cv cluster.ClusterVersion,
) error {
	exIdent, err := ReadStoreIdent(ctx, eng)
	if err == nil {
		return errors.Errorf("engine %s is already bootstrapped with ident %s", eng, exIdent)
	}
	if _, ok := err.(*NotBootstrappedError); !ok {
		return err
	}

	if err := checkEngineEmpty(ctx, eng); err != nil {
		return errors.Wrap(err, "cannot verify empty engine for bootstrap")
	}

	batch := eng.NewBatch()
	if err := engine.MVCCPutProto(
		ctx,
		batch,
		nil,
		keys.StoreIdentKey(),
		hlc.Timestamp{},
		nil,
		&ident,
	); err != nil {
		batch.Close()
		return err
	}
	if err := WriteClusterVersion(ctx, batch, cv); err != nil {
		batch.Close()
		return errors.Wrap(err, "cannot write cluster version")
	}
	if err := batch.Commit(true /* sync */); err != nil {
		return errors.Wrap(err, "persisting bootstrap data")
	}

	return nil
}

// WriteLastUpTimestamp records the supplied timestamp into the "last up" key
// on this store. This value should be refreshed whenever this store's node
// updates its own liveness record; it is used by a restarting store to
// determine the approximate time that it stopped.
func (s *Store) WriteLastUpTimestamp(ctx context.Context, time hlc.Timestamp) error {
	ctx = s.AnnotateCtx(ctx)
	return engine.MVCCPutProto(
		ctx,
		s.engine,
		nil,
		keys.StoreLastUpKey(),
		hlc.Timestamp{},
		nil,
		&time,
	)
}

// ReadLastUpTimestamp returns the "last up" timestamp recorded in this store.
// This value can be used to approximate the last time the engine was was being
// served as a store by a running node. If the store does not contain a "last
// up" timestamp (for example, on a newly bootstrapped store), the zero
// timestamp is returned instead.
func (s *Store) ReadLastUpTimestamp(ctx context.Context) (hlc.Timestamp, error) {
	var timestamp hlc.Timestamp
	ok, err := engine.MVCCGetProto(
		ctx, s.Engine(), keys.StoreLastUpKey(), hlc.Timestamp{}, true, nil, &timestamp)
	if err != nil {
		return hlc.Timestamp{}, err
	} else if !ok {
		return hlc.Timestamp{}, nil
	}
	return timestamp, nil
}

// WriteHLCUpperBound records an upper bound to the wall time of the HLC
func (s *Store) WriteHLCUpperBound(ctx context.Context, time int64) error {
	ctx = s.AnnotateCtx(ctx)
	ts := hlc.Timestamp{WallTime: time}
	batch := s.Engine().NewBatch()
	// Write has to sync to disk to ensure HLC monotonicity across restarts
	defer batch.Close()
	if err := engine.MVCCPutProto(
		ctx,
		batch,
		nil,
		keys.StoreHLCUpperBoundKey(),
		hlc.Timestamp{},
		nil,
		&ts,
	); err != nil {
		return err
	}

	if err := batch.Commit(true /* sync */); err != nil {
		return err
	}
	return nil
}

// ReadHLCUpperBound returns the upper bound to the wall time of the HLC
// If this value does not exist 0 is returned
func ReadHLCUpperBound(ctx context.Context, e engine.Engine) (int64, error) {
	var timestamp hlc.Timestamp
	ok, err := engine.MVCCGetProto(
		ctx, e, keys.StoreHLCUpperBoundKey(), hlc.Timestamp{}, true, nil, &timestamp)
	if err != nil {
		return 0, err
	} else if !ok {
		return 0, nil
	}
	return timestamp.WallTime, nil
}

// ReadMaxHLCUpperBound returns the maximum of the stored hlc upper bounds
// among all the engines. This value is optionally persisted by the server and
// it is guaranteed to be higher than any wall time used by the HLC. If this
// value is persisted, HLC wall clock monotonicity is guaranteed across server
// restarts
func ReadMaxHLCUpperBound(ctx context.Context, engines []engine.Engine) (int64, error) {
	var hlcUpperBound int64
	for _, e := range engines {
		engineHLCUpperBound, err := ReadHLCUpperBound(ctx, e)
		if err != nil {
			return 0, err
		}
		if engineHLCUpperBound > hlcUpperBound {
			hlcUpperBound = engineHLCUpperBound
		}
	}
	return hlcUpperBound, nil
}

func checkEngineEmpty(ctx context.Context, eng engine.Engine) error {
	kvs, err := engine.Scan(
		eng,
		engine.MakeMVCCMetadataKey(roachpb.Key(roachpb.RKeyMin)),
		engine.MakeMVCCMetadataKey(roachpb.Key(roachpb.RKeyMax)),
		10,
	)
	if err != nil {
		return err
	}
	if len(kvs) > 0 {
		// See if this is an already-bootstrapped store.
		ident, err := ReadStoreIdent(ctx, eng)
		if err != nil {
			return errors.Wrap(err, "unable to read store ident")
		}
		keyVals := make([]string, len(kvs))
		for i, kv := range kvs {
			keyVals[i] = fmt.Sprintf("%s: %q", kv.Key, kv.Value)
		}
		return errors.Errorf("engine belongs to store %s, contains %s", ident, keyVals)
	}
	return nil
}

// GetReplica fetches a replica by Range ID. Returns an error if no replica is found.
func (s *Store) GetReplica(rangeID roachpb.RangeID) (*Replica, error) {
	if value, ok := s.mu.replicas.Load(int64(rangeID)); ok {
		return (*Replica)(value), nil
	}
	return nil, roachpb.NewRangeNotFoundError(rangeID)
}

// LookupReplica looks up the replica that contains the specified key. It
// returns nil if no such replica exists.
func (s *Store) LookupReplica(key roachpb.RKey) *Replica {
	s.mu.RLock()
	defer s.mu.RUnlock()
	var repl *Replica
	// Simulate AscendGreater by calling key.Next. We want to skip the replica
	// whose end key equals key, as end keys are exclusive.
	s.mu.replicasByKey.AscendGreaterOrEqual(rangeBTreeKey(key.Next()), func(item btree.Item) bool {
		repl, _ = item.(*Replica)
		// Stop iterating immediately. The first item we see is the only one that
		// can possibly contain key.
		return false
	})
	if repl == nil || !repl.Desc().ContainsKey(key) {
		return nil
	}
	return repl
}

// lookupPrecedingReplica finds the replica in this store that immediately
// precedes the specified key without containing it. It returns nil if no such
// replica exists. It ignores replica placeholders.
//
// Concretely, when key represents a key within replica R,
// lookupPrecedingReplica returns the replica that immediately precedes R in
// replicasByKey.
func (s *Store) lookupPrecedingReplica(key roachpb.RKey) *Replica {
	s.mu.RLock()
	defer s.mu.RUnlock()
	var repl *Replica
	s.mu.replicasByKey.DescendLessOrEqual(rangeBTreeKey(key), func(item btree.Item) bool {
		var ok bool
		repl, ok = item.(*Replica)
		return !ok // keep iterating if not a *Replica
	})
	return repl
}

// getOverlappingKeyRangeLocked returns a KeyRange from the Store overlapping the given
// descriptor (or nil if no such KeyRange exists).
func (s *Store) getOverlappingKeyRangeLocked(rngDesc *roachpb.RangeDescriptor) KeyRange {
	var kr KeyRange

	s.mu.replicasByKey.AscendGreaterOrEqual(rangeBTreeKey(rngDesc.StartKey.Next()),
		func(item btree.Item) bool {
			kr = item.(KeyRange)
			return false
		})

	if kr != nil && kr.Desc().StartKey.Less(rngDesc.EndKey) {
		return kr
	}

	return nil
}

// RaftStatus returns the current raft status of the local replica of
// the given range.
func (s *Store) RaftStatus(rangeID roachpb.RangeID) *raft.Status {
	if value, ok := s.mu.replicas.Load(int64(rangeID)); ok {
		return (*Replica)(value).RaftStatus()
	}
	return nil
}

// BootstrapRange creates the first range in the cluster and manually
// writes it to the store. Default range addressing records are
// created for meta1 and meta2. Default configurations for
// zones are created. All configs are specified
// for the empty key prefix, meaning they apply to the entire
// database. The zone requires three replicas with no other specifications.
// It also adds the range tree and the root node, the first range, to it.
// The 'initialValues' are written as well after each value's checksum
// is initialized.
func (s *Store) BootstrapRange(
	initialValues []roachpb.KeyValue, bootstrapVersion roachpb.Version,
) error {
	desc := &roachpb.RangeDescriptor{
		RangeID:       1,
		StartKey:      roachpb.RKeyMin,
		EndKey:        roachpb.RKeyMax,
		NextReplicaID: 2,
		Replicas: []roachpb.ReplicaDescriptor{
			{
				NodeID:    1,
				StoreID:   1,
				ReplicaID: 1,
			},
		},
	}
	if err := desc.Validate(); err != nil {
		return err
	}
	batch := s.engine.NewBatch()
	defer batch.Close()
	ms := &enginepb.MVCCStats{}
	now := s.cfg.Clock.Now()
	ctx := context.Background()

	// Bootstrap version information. We don't do this if this is v1.0, which is
	// never going to be true in versions that have this code in production, but
	// can be true in tests.
	if bootstrapVersion != cluster.VersionByKey(cluster.VersionBase) {
		if err := engine.MVCCPutProto(ctx, batch, ms /* ms */, keys.BootstrapVersionKey, hlc.Timestamp{}, nil, &bootstrapVersion); err != nil {
			return err
		}
	}
	// Range descriptor.
	if err := engine.MVCCPutProto(ctx, batch, ms, keys.RangeDescriptorKey(desc.StartKey), now, nil, desc); err != nil {
		return err
	}
	// Replica GC timestamp.
	if err := engine.MVCCPutProto(ctx, batch, nil /* ms */, keys.RangeLastReplicaGCTimestampKey(desc.RangeID), hlc.Timestamp{}, nil, &now); err != nil {
		return err
	}
	// Range addressing for meta2.
	meta2Key := keys.RangeMetaKey(roachpb.RKeyMax)
	if err := engine.MVCCPutProto(ctx, batch, ms, meta2Key.AsRawKey(), now, nil, desc); err != nil {
		return err
	}
	// Range addressing for meta1.
	meta1Key := keys.RangeMetaKey(meta2Key)
	if err := engine.MVCCPutProto(ctx, batch, ms, meta1Key.AsRawKey(), now, nil, desc); err != nil {
		return err
	}

	// Now add all passed-in default entries.
	for _, kv := range initialValues {
		// Initialize the checksums.
		kv.Value.InitChecksum(kv.Key)
		if err := engine.MVCCPut(ctx, batch, ms, kv.Key, now, kv.Value, nil); err != nil {
			return err
		}
	}

	lease := roachpb.BootstrapLease()
	updatedMS, err := stateloader.WriteInitialState(ctx, s.cfg.Settings, batch, *ms, *desc,
		lease, hlc.Timestamp{}, hlc.Timestamp{})
	if err != nil {
		return err
	}
	*ms = updatedMS

	return batch.Commit(true /* sync */)
}

// ClusterID accessor.
func (s *Store) ClusterID() uuid.UUID { return s.Ident.ClusterID }

// StoreID accessor.
func (s *Store) StoreID() roachpb.StoreID { return s.Ident.StoreID }

// Clock accessor.
func (s *Store) Clock() *hlc.Clock { return s.cfg.Clock }

// Engine accessor.
func (s *Store) Engine() engine.Engine { return s.engine }

// DB accessor.
func (s *Store) DB() *client.DB { return s.cfg.DB }

// Gossip accessor.
func (s *Store) Gossip() *gossip.Gossip { return s.cfg.Gossip }

// Compactor accessor.
func (s *Store) Compactor() *compactor.Compactor { return s.compactor }

// Stopper accessor.
func (s *Store) Stopper() *stop.Stopper { return s.stopper }

// Tracer accessor.
func (s *Store) Tracer() opentracing.Tracer { return s.cfg.AmbientCtx.Tracer }

// TestingKnobs accessor.
func (s *Store) TestingKnobs() *StoreTestingKnobs { return &s.cfg.TestingKnobs }

// IsDraining accessor.
func (s *Store) IsDraining() bool {
	return s.draining.Load().(bool)
}

// NewRangeDescriptor creates a new descriptor based on start and end
// keys and the supplied roachpb.Replicas slice. It allocates a new
// range ID and returns a RangeDescriptor whose Replicas are a copy
// of the supplied replicas slice, with appropriate ReplicaIDs assigned.
func (s *Store) NewRangeDescriptor(
	ctx context.Context, start, end roachpb.RKey, replicas []roachpb.ReplicaDescriptor,
) (*roachpb.RangeDescriptor, error) {
	id, err := s.rangeIDAlloc.Allocate(ctx)
	if err != nil {
		return nil, err
	}
	desc := &roachpb.RangeDescriptor{
		RangeID:       roachpb.RangeID(id),
		StartKey:      start,
		EndKey:        end,
		Replicas:      append([]roachpb.ReplicaDescriptor(nil), replicas...),
		NextReplicaID: roachpb.ReplicaID(len(replicas) + 1),
	}
	for i := range desc.Replicas {
		desc.Replicas[i].ReplicaID = roachpb.ReplicaID(i + 1)
	}
	return desc, nil
}

// splitPreApply is called when the raft command is applied. Any
// changes to the given ReadWriter will be written atomically with the
// split commit.
func splitPreApply(
	ctx context.Context, st *cluster.Settings, eng engine.ReadWriter, split roachpb.SplitTrigger,
) {
	// Update the raft HardState with the new Commit value now that the
	// replica is initialized (combining it with existing or default
	// Term and Vote).
	rsl := stateloader.Make(st, split.RightDesc.RangeID)
	if err := rsl.SynthesizeRaftState(ctx, eng); err != nil {
		log.Fatal(ctx, err)
	}
}

// splitPostApply is the part of the split trigger which coordinates the actual
// split with the Store. Requires that Replica.raftMu is held.
//
// TODO(tschottdorf): Want to merge this with SplitRange, but some legacy
// testing code calls SplitRange directly.
func splitPostApply(
	ctx context.Context, deltaMS enginepb.MVCCStats, split *roachpb.SplitTrigger, r *Replica,
) {
	// The right hand side of the split was already created (and its raftMu
	// acquired) in Replica.acquireSplitLock. It must be present here.
	rightRng, err := r.store.GetReplica(split.RightDesc.RangeID)
	if err != nil {
		log.Fatalf(ctx, "unable to find RHS replica: %s", err)
	}
	{
		rightRng.mu.Lock()
		// Already holding raftMu, see above.
		err := rightRng.initRaftMuLockedReplicaMuLocked(&split.RightDesc, r.store.Clock(), 0)
		rightRng.mu.Unlock()
		if err != nil {
			log.Fatal(ctx, err)
		}
	}

	// Finish initialization of the RHS.
	r.mu.Lock()
	rightRng.mu.Lock()
	// Copy the minLeaseProposedTS from the LHS.
	rightRng.mu.minLeaseProposedTS = r.mu.minLeaseProposedTS
	rightLease := *rightRng.mu.state.Lease
	rightRng.mu.Unlock()
	r.mu.Unlock()
	log.Event(ctx, "copied timestamp cache")

	// Invoke the leasePostApply method to ensure we properly initialize
	// the replica according to whether it holds the lease. This enables
	// the txnWaitQueue.
	rightRng.leasePostApply(ctx, rightLease)

	// Add the RHS replica to the store. This step atomically updates
	// the EndKey of the LHS replica and also adds the RHS replica
	// to the store's replica map.
	if err := r.store.SplitRange(ctx, r, rightRng); err != nil {
		// Our in-memory state has diverged from the on-disk state.
		log.Fatalf(ctx, "%s: failed to update Store after split: %s", r, err)
	}

	// Update store stats with difference in stats before and after split.
	r.store.metrics.addMVCCStats(deltaMS)

	now := r.store.Clock().Now()

	// While performing the split, zone config changes or a newly created table
	// might require the range to be split again. Enqueue both the left and right
	// ranges to speed up such splits. See #10160.
	r.store.splitQueue.MaybeAdd(r, now)
	r.store.splitQueue.MaybeAdd(rightRng, now)

	// If the range was not properly replicated before the split, the replicate
	// queue may not have picked it up (due to the need for a split). Enqueue
	// both the left and right ranges to speed up a potentially necessary
	// replication. See #7022 and #7800.
	r.store.replicateQueue.MaybeAdd(r, now)
	r.store.replicateQueue.MaybeAdd(rightRng, now)

	if len(split.RightDesc.Replicas) == 1 {
		// TODO(peter): In single-node clusters, we enqueue the right-hand side of
		// the split (the new range) for Raft processing so that the corresponding
		// Raft group is created. This shouldn't be necessary for correctness, but
		// some tests rely on this (e.g. server.TestNodeStatusWritten).
		r.store.enqueueRaftUpdateCheck(rightRng.RangeID)
	}
}

// SplitRange shortens the original range to accommodate the new range. The new
// range is added to the ranges map and the replicasByKey btree. origRng.raftMu
// and newRng.raftMu must be held.
//
// This is only called from the split trigger in the context of the execution
// of a Raft command.
func (s *Store) SplitRange(ctx context.Context, origRng, newRng *Replica) error {
	origDesc := origRng.Desc()
	newDesc := newRng.Desc()

	if !bytes.Equal(origDesc.EndKey, newDesc.EndKey) ||
		bytes.Compare(origDesc.StartKey, newDesc.StartKey) >= 0 {
		return errors.Errorf("orig range is not splittable by new range: %+v, %+v", origDesc, newDesc)
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	if exRng, ok := s.mu.uninitReplicas[newDesc.RangeID]; ok {
		// If we have an uninitialized replica of the new range we require pointer
		// equivalence with newRng. See Store.splitTriggerPostApply().
		if exRng != newRng {
			log.Fatalf(ctx, "found unexpected uninitialized replica: %s vs %s", exRng, newRng)
		}
		s.unlinkReplicaByRangeIDLocked(newDesc.RangeID)
	}

	// Replace the end key of the original range with the start key of
	// the new range. Reinsert the range since the btree is keyed by range end keys.
	if kr := s.mu.replicasByKey.Delete(origRng); kr != origRng {
		return errors.Errorf("replicasByKey unexpectedly contains %v instead of replica %s", kr, origRng)
	}

	copyDesc := *origDesc
	copyDesc.IncrementGeneration()
	copyDesc.EndKey = append([]byte(nil), newDesc.StartKey...)
	origRng.setDescWithoutProcessUpdate(&copyDesc)

	// Clear the LHS txn wait queue, to redirect to the RHS if
	// appropriate. We do this after setDescWithoutProcessUpdate
	// to ensure that no pre-split commands are inserted into the
	// txnWaitQueue after we clear it.
	origRng.txnWaitQueue.Clear(false /* disable */)

	// The rangefeed processor will no longer be provided logical ops for
	// its entire range, so it needs to be shut down and all registrations
	// need to retry.
	// TODO(nvanbenschoten): It should be possible to only reject registrations
	// that overlap with the new range of the split and keep registrations that
	// are only interested in keys that are still on the original range running.
	origRng.disconnectRangefeedWithReasonRaftMuLocked(
		roachpb.RangeFeedRetryError_REASON_RANGE_SPLIT,
	)

	// Clear the original range's request stats, since they include requests for
	// spans that are now owned by the new range.
	origRng.leaseholderStats.resetRequestCounts()
	origRng.writeStats.splitRequestCounts(newRng.writeStats)

	if kr := s.mu.replicasByKey.ReplaceOrInsert(origRng); kr != nil {
		return errors.Errorf("replicasByKey unexpectedly contains %s when inserting replica %s", kr, origRng)
	}

	if err := s.addReplicaInternalLocked(newRng); err != nil {
		return errors.Errorf("couldn't insert range %v in replicasByKey btree: %s", newRng, err)
	}

	// Update the max bytes and other information of the new range.
	// This may not happen if the system config has not yet been loaded.
	// Since this is done under the store lock, system config update will
	// properly set these fields.
	if err := newRng.updateRangeInfo(newRng.Desc()); err != nil {
		return err
	}

	// Add the range to metrics and maybe gossip on capacity change.
	s.metrics.ReplicaCount.Inc(1)
	s.maybeGossipOnCapacityChange(ctx, rangeAddEvent)

	return s.processRangeDescriptorUpdateLocked(ctx, origRng)
}

// MergeRange expands the left-hand replica, leftRepl, to absorb the right-hand
// replica, identified by rightDesc. The right-hand replica must exist on this
// store and the raftMus for both the left-hand and right-hand replicas must be
// held.
func (s *Store) MergeRange(
	ctx context.Context,
	leftRepl *Replica,
	updatedEndKey roachpb.RKey,
	rightDesc roachpb.RangeDescriptor,
) error {
	leftDesc := leftRepl.Desc()

	if !leftDesc.EndKey.Less(updatedEndKey) {
		return errors.Errorf("the new end key is not greater than the current one: %+v <= %+v",
			updatedEndKey, leftDesc.EndKey)
	}

	rightRepl, err := s.GetReplica(rightDesc.RangeID)
	if err != nil {
		return err
	}

	leftRepl.raftMu.AssertHeld()
	rightRepl.raftMu.AssertHeld()

	// Shut down rangefeed processors on either side of the merge.
	//
	// It isn't strictly necessary to shut-down a rangefeed processor on the
	// surviving replica in a merge, but we choose to in order to avoid clients
	// who were monitoring both sides of the merge from establishing multiple
	// partial rangefeeds to the surviving range.
	// TODO(nvanbenschoten): does this make sense? We could just adjust the
	// bounds of the leftRepl.Processor.
	//
	// NB: removeReplicaImpl also disconnects any initialized rangefeeds with
	// REASON_REPLICA_REMOVED. That's ok because we will have already
	// disconnected the rangefeed here.
	leftRepl.disconnectRangefeedWithReasonRaftMuLocked(
		roachpb.RangeFeedRetryError_REASON_RANGE_MERGED,
	)
	rightRepl.disconnectRangefeedWithReasonRaftMuLocked(
		roachpb.RangeFeedRetryError_REASON_RANGE_MERGED,
	)

	if err := rightRepl.postDestroyRaftMuLocked(ctx, rightRepl.GetMVCCStats()); err != nil {
		return err
	}

	// Note that we were called (indirectly) from raft processing so we must
	// call removeReplicaImpl directly to avoid deadlocking on the right-hand
	// replica's raftMu.
	if err := s.removeReplicaImpl(ctx, rightRepl, rightDesc.NextReplicaID, RemoveOptions{
		DestroyData: false, // the replica was destroyed when the merge commit applied
	}); err != nil {
		return errors.Errorf("cannot remove range: %s", err)
	}

	if leftRepl.leaseholderStats != nil {
		leftRepl.leaseholderStats.resetRequestCounts()
	}
	if leftRepl.writeStats != nil {
		// Note: this could be drastically improved by adding a replicaStats method
		// that merges stats. Resetting stats is typically bad for the rebalancing
		// logic that depends on them.
		leftRepl.writeStats.resetRequestCounts()
	}

	// Clear the wait queue to redirect the queued transactions to the
	// left-hand replica, if necessary.
	rightRepl.txnWaitQueue.Clear(true /* disable */)

	leftLease, _ := leftRepl.GetLease()
	rightLease, _ := rightRepl.GetLease()
	if leftLease.OwnedBy(s.Ident.StoreID) && !rightLease.OwnedBy(s.Ident.StoreID) {
		// We hold the lease for the LHS, but do not hold the lease for the RHS.
		// That means we don't have up-to-date timestamp cache entries for the
		// keyspace previously owned by the RHS. Bump the low water mark for the RHS
		// keyspace to the current time. (Our current time is guaranteed to be later
		// than any timestamp cache entries on the RHS thanks to the chain of
		// causality established by the GetSnapshotForMerge request we sent to the
		// RHS during the merge transaction.)
		//
		// TODO(benesch): perhaps it would be less disruptive to bump to the time
		// that the RHS executed the GetSnapshotForMerge request instead of the
		// current time? Plumbing that timestamp into the merge trigger would not be
		// difficult.
		s.tsCache.SetLowWater(rightDesc.StartKey.AsRawKey(), rightDesc.EndKey.AsRawKey(), s.Clock().Now())
	}

	// Update the end key of the subsuming range.
	copy := *leftDesc
	copy.IncrementGeneration()
	copy.EndKey = updatedEndKey
	return leftRepl.setDesc(&copy)
}

// addReplicaInternalLocked adds the replica to the replicas map and the
// replicasByKey btree. Returns an error if a replica with
// the same Range ID or a KeyRange that overlaps has already been added to
// this store. addReplicaInternalLocked requires that the store lock is held.
func (s *Store) addReplicaInternalLocked(repl *Replica) error {
	if !repl.IsInitialized() {
		return errors.Errorf("attempted to add uninitialized replica %s", repl)
	}

	if err := s.addReplicaToRangeMapLocked(repl); err != nil {
		return err
	}

	if exRange := s.getOverlappingKeyRangeLocked(repl.Desc()); exRange != nil {
		return errors.Errorf("%s: cannot addReplicaInternalLocked; range %s has overlapping range %s", s, repl, exRange.Desc())
	}

	if exRngItem := s.mu.replicasByKey.ReplaceOrInsert(repl); exRngItem != nil {
		return errors.Errorf("%s: cannot addReplicaInternalLocked; range for key %v already exists in replicasByKey btree", s,
			exRngItem.(KeyRange).endKey())
	}

	return nil
}

// addPlaceholderLocked adds the specified placeholder. Requires that the
// raftMu of the replica whose place is being held is locked.
func (s *Store) addPlaceholder(placeholder *ReplicaPlaceholder) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.addPlaceholderLocked(placeholder)
}

// addPlaceholderLocked adds the specified placeholder. Requires that Store.mu
// and the raftMu of the replica whose place is being held are locked.
func (s *Store) addPlaceholderLocked(placeholder *ReplicaPlaceholder) error {
	rangeID := placeholder.Desc().RangeID
	if exRng := s.mu.replicasByKey.ReplaceOrInsert(placeholder); exRng != nil {
		return errors.Errorf("%s overlaps with existing KeyRange %s in replicasByKey btree", placeholder, exRng)
	}
	if exRng, ok := s.mu.replicaPlaceholders[rangeID]; ok {
		return errors.Errorf("%s has ID collision with existing KeyRange %s", placeholder, exRng)
	}
	s.mu.replicaPlaceholders[rangeID] = placeholder
	return nil
}

// removePlaceholder removes a placeholder for the specified range if it
// exists, returning true if a placeholder was present and removed and false
// otherwise. Requires that the raftMu of the replica whose place is being held
// is locked.
func (s *Store) removePlaceholder(ctx context.Context, rngID roachpb.RangeID) bool {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.removePlaceholderLocked(ctx, rngID)
}

// removePlaceholderLocked removes the specified placeholder. Requires that
// Store.mu and the raftMu of the replica whose place is being held are locked.
func (s *Store) removePlaceholderLocked(ctx context.Context, rngID roachpb.RangeID) bool {
	placeholder, ok := s.mu.replicaPlaceholders[rngID]
	if !ok {
		return false
	}
	switch exRng := s.mu.replicasByKey.Delete(placeholder).(type) {
	case *ReplicaPlaceholder:
		delete(s.mu.replicaPlaceholders, rngID)
		return true
	case nil:
		log.Fatalf(ctx, "r%d: placeholder not found", rngID)
	default:
		log.Fatalf(ctx, "r%d: expected placeholder, got %T", rngID, exRng)
	}
	return false // appease the compiler
}

// addReplicaToRangeMapLocked adds the replica to the replicas map.
func (s *Store) addReplicaToRangeMapLocked(repl *Replica) error {
	if _, loaded := s.mu.replicas.LoadOrStore(int64(repl.RangeID), unsafe.Pointer(repl)); loaded {
		return errors.Errorf("%s: replica already exists", repl)
	}
	// Check whether the replica is unquiesced but not in the map. This
	// can happen during splits and merges, where the uninitialized (but
	// also unquiesced) replica is removed from the unquiesced replica
	// map in advance of this method being called.
	s.unquiescedReplicas.Lock()
	if _, ok := s.unquiescedReplicas.m[repl.RangeID]; !repl.mu.quiescent && !ok {
		s.unquiescedReplicas.m[repl.RangeID] = struct{}{}
	}
	s.unquiescedReplicas.Unlock()
	return nil
}

// RemoveOptions bundles boolean parameters for Store.RemoveReplica.
type RemoveOptions struct {
	DestroyData bool
}

// RemoveReplica removes the replica from the store's replica map and from the
// sorted replicasByKey btree.
//
// The NextReplicaID from the replica descriptor that was used to make the
// removal decision is passed in. Removal is aborted if the replica ID has
// advanced to or beyond the NextReplicaID since the removal decision was made.
//
// If opts.DestroyReplica is false, replica.destroyRaftMuLocked is not called.
func (s *Store) RemoveReplica(
	ctx context.Context, rep *Replica, nextReplicaID roachpb.ReplicaID, opts RemoveOptions,
) error {
	rep.raftMu.Lock()
	defer rep.raftMu.Unlock()
	return s.removeReplicaImpl(ctx, rep, nextReplicaID, opts)
}

// removeReplicaImpl is the implementation of RemoveReplica, which is sometimes
// called directly when the necessary lock is already held. It requires that
// Replica.raftMu is held and that s.mu is not held.
func (s *Store) removeReplicaImpl(
	ctx context.Context, rep *Replica, nextReplicaID roachpb.ReplicaID, opts RemoveOptions,
) error {
	log.Infof(ctx, "removing replica")

	// We check both rep.mu.ReplicaID and rep.mu.state.Desc's replica ID because
	// they can differ in cases when a replica's ID is increased due to an
	// incoming raft message (see #14231 for background).
	rep.mu.Lock()
	if rep.mu.replicaID >= nextReplicaID {
		rep.mu.Unlock()
		return errors.Errorf("cannot remove replica %s; replica ID has changed (%s >= %s)",
			rep, rep.mu.replicaID, nextReplicaID)
	}
	desc := rep.mu.state.Desc
	if repDesc, ok := desc.GetReplicaDescriptor(s.StoreID()); ok && repDesc.ReplicaID >= nextReplicaID {
		rep.mu.Unlock()
		return errors.Errorf("cannot remove replica %s; replica descriptor's ID has changed (%s >= %s)",
			rep, repDesc.ReplicaID, nextReplicaID)
	}
	rep.mu.Unlock()

	if _, err := s.GetReplica(rep.RangeID); err != nil {
		return err
	}

	s.mu.Lock()
	if placeholder := s.getOverlappingKeyRangeLocked(desc); placeholder != rep {
		// This is a fatal error because uninitialized replicas shouldn't make it
		// this far. This method will need some changes when we introduce GC of
		// uninitialized replicas.
		s.mu.Unlock()
		log.Fatalf(ctx, "replica %+v unexpectedly overlapped by %+v", rep, placeholder)
	}
	// Adjust stats before calling Destroy. This can be called before or after
	// Destroy, but this configuration helps avoid races in stat verification
	// tests.
	s.metrics.subtractMVCCStats(rep.GetMVCCStats())
	s.metrics.ReplicaCount.Dec(1)
	s.mu.Unlock()

	// The replica will no longer exist, so cancel any rangefeed registrations.
	rep.disconnectRangefeedWithReasonRaftMuLocked(
		roachpb.RangeFeedRetryError_REASON_REPLICA_REMOVED,
	)

	// Mark the replica as destroyed and (optionally) destroy the on-disk data
	// while not holding Store.mu. This is safe because we're holding
	// Replica.raftMu and the replica is present in Store.mu.replicasByKey
	// (preventing any concurrent access to the replica's key range).

	rep.readOnlyCmdMu.Lock()
	rep.mu.Lock()
	rep.cancelPendingCommandsLocked()
	rep.mu.internalRaftGroup = nil
	rep.mu.destroyStatus.Set(roachpb.NewRangeNotFoundError(rep.RangeID), destroyReasonRemoved)
	rep.mu.Unlock()
	rep.readOnlyCmdMu.Unlock()

	if opts.DestroyData {
		if err := rep.destroyRaftMuLocked(ctx, nextReplicaID); err != nil {
			return err
		}
	}

	s.mu.Lock()
	defer s.mu.Unlock()
	s.unlinkReplicaByRangeIDLocked(rep.RangeID)
	if placeholder := s.mu.replicasByKey.Delete(rep); placeholder != rep {
		// We already checked that our replica was present in replicasByKey
		// above. Nothing should have been able to change that.
		log.Fatalf(ctx, "replica %+v unexpectedly overlapped by %+v", rep, placeholder)
	}
	delete(s.mu.replicaPlaceholders, rep.RangeID)
	// TODO(peter): Could release s.mu.Lock() here.
	s.maybeGossipOnCapacityChange(ctx, rangeRemoveEvent)
	s.scanner.RemoveReplica(rep)

	return nil
}

// unlinkReplicaByRangeIDLocked removes all of the store's references to the
// provided replica that are keyed by its range ID. The replica may also need
// to be removed from the replicasByKey map.
//
// store.mu must be held.
func (s *Store) unlinkReplicaByRangeIDLocked(rangeID roachpb.RangeID) {
	s.mu.AssertHeld()
	s.unquiescedReplicas.Lock()
	delete(s.unquiescedReplicas.m, rangeID)
	s.unquiescedReplicas.Unlock()
	delete(s.mu.uninitReplicas, rangeID)
	s.replicaQueues.Delete(int64(rangeID))
	s.mu.replicas.Delete(int64(rangeID))
}

// processRangeDescriptorUpdate should be called whenever a replica's range
// descriptor is updated, to update the store's maps of its ranges to match
// the updated descriptor. Since the latter update requires acquiring the store
// lock (which cannot always safely be done by replicas), this function call
// should be deferred until it is safe to acquire the store lock.
func (s *Store) processRangeDescriptorUpdate(ctx context.Context, repl *Replica) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.processRangeDescriptorUpdateLocked(ctx, repl)
}

// processRangeDescriptorUpdateLocked requires that Store.mu and Replica.raftMu
// are locked.
func (s *Store) processRangeDescriptorUpdateLocked(ctx context.Context, repl *Replica) error {
	if !repl.IsInitialized() {
		return errors.Errorf("attempted to process uninitialized range %s", repl)
	}

	rangeID := repl.RangeID

	if _, ok := s.mu.uninitReplicas[rangeID]; !ok {
		// Do nothing if the range has already been initialized.
		return nil
	}
	delete(s.mu.uninitReplicas, rangeID)

	if exRange := s.getOverlappingKeyRangeLocked(repl.Desc()); exRange != nil {
		return errors.Errorf("%s: cannot processRangeDescriptorUpdate; range %s has overlapping range %s", s, repl, exRange.Desc())
	}
	if exRngItem := s.mu.replicasByKey.ReplaceOrInsert(repl); exRngItem != nil {
		return errors.Errorf("range for key %v already exists in replicasByKey btree",
			(exRngItem.(*Replica)).endKey())
	}

	// Add the range to metrics and maybe gossip on capacity change.
	s.metrics.ReplicaCount.Inc(1)
	s.maybeGossipOnCapacityChange(ctx, rangeAddEvent)

	return nil
}

// Attrs returns the attributes of the underlying store.
func (s *Store) Attrs() roachpb.Attributes {
	return s.engine.Attrs()
}

// Capacity returns the capacity of the underlying storage engine. Note that
// this does not include reservations.
// Note that Capacity() has the side effect of updating some of the store's
// internal statistics about its replicas.
func (s *Store) Capacity(useCached bool) (roachpb.StoreCapacity, error) {
	if useCached {
		s.cachedCapacity.Lock()
		capacity := s.cachedCapacity.StoreCapacity
		s.cachedCapacity.Unlock()
		if capacity != (roachpb.StoreCapacity{}) {
			return capacity, nil
		}
	}

	capacity, err := s.engine.Capacity()
	if err != nil {
		return capacity, err
	}

	now := s.cfg.Clock.Now()
	var leaseCount int32
	var rangeCount int32
	var logicalBytes int64
	var totalQueriesPerSecond float64
	var totalWritesPerSecond float64
	replicaCount := s.metrics.ReplicaCount.Value()
	bytesPerReplica := make([]float64, 0, replicaCount)
	writesPerReplica := make([]float64, 0, replicaCount)
	newStoreReplicaVisitor(s).Visit(func(r *Replica) bool {
		rangeCount++
		if r.OwnsValidLease(now) {
			leaseCount++
		}
		mvccStats := r.GetMVCCStats()
		logicalBytes += mvccStats.Total()
		bytesPerReplica = append(bytesPerReplica, float64(mvccStats.Total()))
		// TODO(a-robinson): How dangerous is it that these numbers will be
		// incorrectly low the first time or two it gets gossiped when a store
		// starts? We can't easily have a countdown as its value changes like for
		// leases/replicas.
		if qps, dur := r.leaseholderStats.avgQPS(); dur >= MinStatsDuration {
			totalQueriesPerSecond += qps
			// TODO(a-robinson): Calculate percentiles for qps? Get rid of other percentiles?
		}
		if wps, dur := r.writeStats.avgQPS(); dur >= MinStatsDuration {
			totalWritesPerSecond += wps
			writesPerReplica = append(writesPerReplica, wps)
		}
		return true
	})
	capacity.RangeCount = rangeCount
	capacity.LeaseCount = leaseCount
	capacity.LogicalBytes = logicalBytes
	capacity.QueriesPerSecond = totalQueriesPerSecond
	capacity.WritesPerSecond = totalWritesPerSecond
	capacity.BytesPerReplica = roachpb.PercentilesFromData(bytesPerReplica)
	capacity.WritesPerReplica = roachpb.PercentilesFromData(writesPerReplica)
	s.recordNewPerSecondStats(totalQueriesPerSecond, totalWritesPerSecond)

	s.cachedCapacity.Lock()
	s.cachedCapacity.StoreCapacity = capacity
	s.cachedCapacity.Unlock()

	return capacity, nil
}

// ReplicaCount returns the number of replicas contained by this store. This
// method is O(n) in the number of replicas and should not be called from
// performance critical code.
func (s *Store) ReplicaCount() int {
	var count int
	s.mu.replicas.Range(func(_ int64, _ unsafe.Pointer) bool {
		count++
		return true
	})
	return count
}

// Registry returns the store registry.
func (s *Store) Registry() *metric.Registry {
	return s.metrics.registry
}

// Metrics returns the store's metric struct.
func (s *Store) Metrics() *StoreMetrics {
	return s.metrics
}

// MVCCStats returns the current MVCCStats accumulated for this store.
// TODO(mrtracy): This should be removed as part of #4465, this is only needed
// to support the current NodeStatus structures which will be changing.
func (s *Store) MVCCStats() enginepb.MVCCStats {
	s.metrics.mu.Lock()
	defer s.metrics.mu.Unlock()
	return s.metrics.mu.stats
}

// Descriptor returns a StoreDescriptor including current store
// capacity information.
func (s *Store) Descriptor(useCached bool) (*roachpb.StoreDescriptor, error) {
	capacity, err := s.Capacity(useCached)
	if err != nil {
		return nil, err
	}

	// Initialize the store descriptor.
	return &roachpb.StoreDescriptor{
		StoreID:  s.Ident.StoreID,
		Attrs:    s.Attrs(),
		Node:     *s.nodeDesc,
		Capacity: capacity,
	}, nil
}

// deadReplicas returns a list of all the corrupt replicas on the store.
func (s *Store) deadReplicas() roachpb.StoreDeadReplicas {
	// We can't use a storeReplicaVisitor here as it skips destroyed replicas.
	//
	// TODO(bram): does this need to visit all the replicas? Could we just use the
	// store pool to locate any dead replicas on this store directly?
	var deadReplicas []roachpb.ReplicaIdent
	s.mu.replicas.Range(func(k int64, v unsafe.Pointer) bool {
		r := (*Replica)(v)
		r.mu.RLock()
		corrupted := r.mu.destroyStatus.reason == destroyReasonCorrupted
		desc := r.mu.state.Desc
		r.mu.RUnlock()
		replicaDesc, ok := desc.GetReplicaDescriptor(s.Ident.StoreID)
		if ok && corrupted {
			deadReplicas = append(deadReplicas, roachpb.ReplicaIdent{
				RangeID: desc.RangeID,
				Replica: replicaDesc,
			})
		}
		return true
	})
	return roachpb.StoreDeadReplicas{
		StoreID:  s.Ident.StoreID,
		Replicas: deadReplicas,
	}
}

// Send fetches a range based on the header's replica, assembles method, args &
// reply into a Raft Cmd struct and executes the command using the fetched
// range.
// An incoming request may be transactional or not. If it is not transactional,
// the timestamp at which it executes may be higher than that optionally
// specified through the incoming BatchRequest, and it is not guaranteed that
// all operations are written at the same timestamp. If it is transactional, a
// timestamp must not be set - it is deduced automatically from the
// transaction. In particular, the read (original) timestamp will be used for
// all reads _and writes_ (see the TxnMeta.OrigTimestamp for details).
//
// Should a transactional operation be forced to a higher timestamp (for
// instance due to the timestamp cache or finding a committed value in the path
// of one of its writes), the response will have a transaction set which should
// be used to update the client transaction.
func (s *Store) Send(
	ctx context.Context, ba roachpb.BatchRequest,
) (br *roachpb.BatchResponse, pErr *roachpb.Error) {
	// Attach any log tags from the store to the context (which normally
	// comes from gRPC).
	ctx = s.AnnotateCtx(ctx)
	for _, union := range ba.Requests {
		arg := union.GetInner()
		header := arg.Header()
		if err := verifyKeys(header.Key, header.EndKey, roachpb.IsRange(arg)); err != nil {
			return nil, roachpb.NewError(err)
		}
	}

	if err := ba.SetActiveTimestamp(s.Clock().Now); err != nil {
		return nil, roachpb.NewError(err)
	}

	if s.cfg.TestingKnobs.ClockBeforeSend != nil {
		s.cfg.TestingKnobs.ClockBeforeSend(s.cfg.Clock, ba)
	}

	// Update our clock with the incoming request timestamp. This advances the
	// local node's clock to a high water mark from all nodes with which it has
	// interacted. We hold on to the resulting timestamp - we know that any
	// write with a higher timestamp we run into later must have started after
	// this point in (absolute) time.
	var now hlc.Timestamp
	if s.cfg.TestingKnobs.DisableMaxOffsetCheck {
		now = s.cfg.Clock.Update(ba.Timestamp)
	} else {
		// If the command appears to come from a node with a bad clock,
		// reject it now before we reach that point.
		var err error
		if now, err = s.cfg.Clock.UpdateAndCheckMaxOffset(ba.Timestamp); err != nil {
			return nil, roachpb.NewError(err)
		}
	}

	defer func() {
		if r := recover(); r != nil {
			// On panic, don't run the defer. It's probably just going to panic
			// again due to undefined state.
			panic(r)
		}
		if ba.Txn != nil {
			// We're in a Txn, so we can reduce uncertainty restarts by attaching
			// the above timestamp to the returned response or error. The caller
			// can use it to shorten its uncertainty interval when it comes back to
			// this node.
			if pErr != nil {
				pErr.OriginNode = ba.Replica.NodeID
				if txn := pErr.GetTxn(); txn != nil {
					// Clone the txn, as we'll modify it.
					pErr.SetTxn(txn)
				} else {
					pErr.SetTxn(ba.Txn)
				}
				pErr.GetTxn().UpdateObservedTimestamp(ba.Replica.NodeID, now)
			} else {
				if br.Txn == nil {
					br.Txn = ba.Txn
				}
				br.Txn.UpdateObservedTimestamp(ba.Replica.NodeID, now)
				// Update our clock with the outgoing response txn timestamp
				// (if timestamp has been forwarded).
				if ba.Timestamp.Less(br.Txn.Timestamp) {
					s.cfg.Clock.Update(br.Txn.Timestamp)
				}
			}
		} else {
			if pErr == nil {
				// Update our clock with the outgoing response timestamp.
				// (if timestamp has been forwarded).
				if ba.Timestamp.Less(br.Timestamp) {
					s.cfg.Clock.Update(br.Timestamp)
				}
			}
		}

		if pErr != nil {
			pErr.Now = now
		} else {
			br.Now = now
		}
	}()

	if ba.Txn != nil {
		// We make our transaction aware that no other operation that causally
		// precedes it could have started after `now`. This is important: If we
		// wind up pushing a value, it will be in our immediate future, and not
		// updating the top end of our uncertainty timestamp would lead to a
		// restart (at least in the absence of a prior observed timestamp from
		// this node, in which case the following is a no-op).
		if _, ok := ba.Txn.GetObservedTimestamp(ba.Replica.NodeID); !ok {
			shallowTxn := *ba.Txn
			shallowTxn.UpdateObservedTimestamp(ba.Replica.NodeID, now)
			ba.Txn = &shallowTxn
		}
	}

	if log.V(1) {
		log.Eventf(ctx, "executing %s", ba)
	} else if log.HasSpanOrEvent(ctx) {
		log.Eventf(ctx, "executing %d requests", len(ba.Requests))
	}

	var cleanupAfterWriteIntentError func(newWIErr *roachpb.WriteIntentError, newIntentTxn *enginepb.TxnMeta)
	defer func() {
		if cleanupAfterWriteIntentError != nil {
			// This request wrote an intent only if there was no error, the request
			// is transactional, the transaction is still pending, and the request
			// wasn't read-only.
			if pErr == nil && ba.Txn != nil && br.Txn.Status == roachpb.PENDING && !ba.IsReadOnly() {
				cleanupAfterWriteIntentError(nil, &br.Txn.TxnMeta)
			} else {
				cleanupAfterWriteIntentError(nil, nil)
			}
		}
	}()

	// Add the command to the range for execution; exit retry loop on success.
	for {
		// Exit loop if context has been canceled or timed out.
		if err := ctx.Err(); err != nil {
			return nil, roachpb.NewError(err)
		}

		// Get range and add command to the range for execution.
		repl, err := s.GetReplica(ba.RangeID)
		if err != nil {
			return nil, roachpb.NewError(err)
		}
		if !repl.IsInitialized() {
			repl.mu.RLock()
			replicaID := repl.mu.replicaID
			repl.mu.RUnlock()

			// If we have an uninitialized copy of the range, then we are
			// probably a valid member of the range, we're just in the
			// process of getting our snapshot. If we returned
			// RangeNotFoundError, the client would invalidate its cache,
			// but we can be smarter: the replica that caused our
			// uninitialized replica to be created is most likely the
			// leader.
			return nil, roachpb.NewError(&roachpb.NotLeaseHolderError{
				RangeID:     ba.RangeID,
				LeaseHolder: repl.creatingReplica,
				// The replica doesn't have a range descriptor yet, so we have to build
				// a ReplicaDescriptor manually.
				Replica: roachpb.ReplicaDescriptor{
					NodeID:    repl.store.nodeDesc.NodeID,
					StoreID:   repl.store.StoreID(),
					ReplicaID: replicaID,
				},
			})
		}

		// If necessary, the request may need to wait in the txn wait queue,
		// pending updates to the target transaction for either PushTxn or
		// QueryTxn requests.
		if br, pErr = s.maybeWaitForPushee(ctx, &ba, repl); br != nil || pErr != nil {
			return br, pErr
		}
		br, pErr = repl.Send(ctx, ba)
		if pErr == nil {
			return br, nil
		}

		// Handle push txn failures and write intent conflicts locally and
		// retry. Other errors are returned to caller.
		switch t := pErr.GetDetail().(type) {
		case *roachpb.TransactionPushError:
			// On a transaction push error, retry immediately if doing so will
			// enqueue into the txnWaitQueue in order to await further updates to
			// the unpushed txn's status. We check ShouldPushImmediately to avoid
			// retrying non-queueable PushTxnRequests (see #18191).
			dontRetry := s.cfg.DontRetryPushTxnFailures
			if !dontRetry && ba.IsSinglePushTxnRequest() {
				pushReq := ba.Requests[0].GetInner().(*roachpb.PushTxnRequest)
				dontRetry = txnwait.ShouldPushImmediately(pushReq)
			}
			if dontRetry {
				// If we're not retrying on push txn failures return a txn retry error
				// after the first failure to guarantee a retry.
				if ba.Txn != nil {
					err := roachpb.NewTransactionRetryError(roachpb.RETRY_REASON_UNKNOWN)
					return nil, roachpb.NewErrorWithTxn(err, ba.Txn)
				}
				return nil, pErr
			}

			// Enqueue unsuccessfully pushed transaction on the txnWaitQueue and
			// retry the command.
			repl.txnWaitQueue.Enqueue(&t.PusheeTxn)
			pErr = nil

		case *roachpb.WriteIntentError:
			// Process and resolve write intent error. We do this here because
			// this is the code path with the requesting client waiting.
			if pErr.Index != nil {
				var pushType roachpb.PushTxnType
				if ba.IsWrite() {
					pushType = roachpb.PUSH_ABORT
				} else {
					pushType = roachpb.PUSH_TIMESTAMP
				}

				index := pErr.Index
				args := ba.Requests[index.Index].GetInner()
				// Make a copy of the header for the upcoming push; we will update
				// the timestamp.
				h := ba.Header
				// We must push at least to h.Timestamp, but in fact we want to
				// go all the way up to a timestamp which was taken off the HLC
				// after our operation started. This allows us to not have to
				// restart for uncertainty as we come back and read.
				h.Timestamp.Forward(now)
				// We are going to hand the header (and thus the transaction proto)
				// to the RPC framework, after which it must not be changed (since
				// that could race). Since the subsequent execution of the original
				// request might mutate the transaction, make a copy here.
				//
				// See #9130.
				if h.Txn != nil {
					clonedTxn := h.Txn.Clone()
					h.Txn = &clonedTxn
				}
				// Handle the case where we get more than one write intent error;
				// we need to cleanup the previous attempt to handle it to allow
				// any other pusher queued up behind this RPC to proceed.
				if cleanupAfterWriteIntentError != nil {
					cleanupAfterWriteIntentError(t, nil)
				}
				if cleanupAfterWriteIntentError, pErr =
					s.intentResolver.processWriteIntentError(ctx, pErr, args, h, pushType); pErr != nil {
					// Do not propagate ambiguous results; assume success and retry original op.
					if _, ok := pErr.GetDetail().(*roachpb.AmbiguousResultError); !ok {
						// Preserve the error index.
						pErr.Index = index
						return nil, pErr
					}
					pErr = nil
				}
				// We've resolved the write intent; retry command.
			}

		case *roachpb.MergeInProgressError:
			// A merge was in progress. We need to retry the command after the merge
			// completes, as signaled by the closing of the replica's mergeComplete
			// channel. Note that the merge may have already completed, in which case
			// its mergeComplete channel will be nil.
			mergeCompleteCh := repl.getMergeCompleteCh()
			if mergeCompleteCh != nil {
				select {
				case <-mergeCompleteCh:
					// Merge complete. Retry the command.
				case <-ctx.Done():
					return nil, roachpb.NewError(ctx.Err())
				case <-s.stopper.ShouldQuiesce():
					return nil, roachpb.NewError(&roachpb.NodeUnavailableError{})
				}
			}
			pErr = nil
		}

		if pErr != nil {
			return nil, pErr
		}
	}
}

// RangeFeed registers a rangefeed over the specified span. It sends updates to
// the provided stream and returns with an optional error when the rangefeed is
// complete.
func (s *Store) RangeFeed(
	ctx context.Context, args *roachpb.RangeFeedRequest, stream roachpb.Internal_RangeFeedServer,
) *roachpb.Error {
	ctx = s.AnnotateCtx(ctx)
	if err := verifyKeys(args.Span.Key, args.Span.EndKey, true); err != nil {
		return roachpb.NewError(err)
	}

	// Get range and add command to the range for execution.
	repl, err := s.GetReplica(args.RangeID)
	if err != nil {
		return roachpb.NewError(err)
	}
	if !repl.IsInitialized() {
		repl.mu.RLock()
		replicaID := repl.mu.replicaID
		repl.mu.RUnlock()

		// If we have an uninitialized copy of the range, then we are
		// probably a valid member of the range, we're just in the
		// process of getting our snapshot. If we returned
		// RangeNotFoundError, the client would invalidate its cache,
		// but we can be smarter: the replica that caused our
		// uninitialized replica to be created is most likely the
		// leader.
		return roachpb.NewError(&roachpb.NotLeaseHolderError{
			RangeID:     args.RangeID,
			LeaseHolder: repl.creatingReplica,
			// The replica doesn't have a range descriptor yet, so we have to build
			// a ReplicaDescriptor manually.
			Replica: roachpb.ReplicaDescriptor{
				NodeID:    repl.store.nodeDesc.NodeID,
				StoreID:   repl.store.StoreID(),
				ReplicaID: replicaID,
			},
		})
	}
	return repl.RangeFeed(ctx, args, stream)
}

// maybeWaitForPushee potentially diverts the incoming request to
// the txnwait.Queue, where it will wait for updates to the target
// transaction.
func (s *Store) maybeWaitForPushee(
	ctx context.Context, ba *roachpb.BatchRequest, repl *Replica,
) (*roachpb.BatchResponse, *roachpb.Error) {
	// If this is a push txn request, check the push queue first, which
	// may cause this request to wait and either return a successful push
	// txn response or else allow this request to proceed.
	if ba.IsSinglePushTxnRequest() {
		pushReq := ba.Requests[0].GetInner().(*roachpb.PushTxnRequest)
		pushResp, pErr := repl.txnWaitQueue.MaybeWaitForPush(repl.AnnotateCtx(ctx), repl, pushReq)
		// Copy the request in anticipation of setting the force arg and
		// updating the Now timestamp (see below).
		pushReqCopy := *pushReq
		if pErr == txnwait.ErrDeadlock {
			// We've experienced a deadlock; set Force=true on push request,
			// and set the push type to ABORT.
			pushReqCopy.Force = true
			pushReqCopy.PushType = roachpb.PUSH_ABORT
		} else if pErr != nil {
			return nil, pErr
		} else if pushResp != nil {
			br := &roachpb.BatchResponse{}
			br.Add(pushResp)
			return br, nil
		}
		// Move the push timestamp forward to the current time, as this
		// request may have been waiting to push the txn. If we don't
		// move the timestamp forward to the current time, we may fail
		// to push a txn which has expired.
		pushReqCopy.Now.Forward(s.Clock().Now())
		ba.Requests = nil
		ba.Add(&pushReqCopy)
	} else if ba.IsSingleQueryTxnRequest() {
		// For query txn requests, wait in the txn wait queue either for
		// transaction update or for dependent transactions to change.
		queryReq := ba.Requests[0].GetInner().(*roachpb.QueryTxnRequest)
		pErr := repl.txnWaitQueue.MaybeWaitForQuery(repl.AnnotateCtx(ctx), repl, queryReq)
		if pErr != nil {
			return nil, pErr
		}
	}

	return nil, nil
}

// HandleSnapshot reads an incoming streaming snapshot and applies it if
// possible.
func (s *Store) HandleSnapshot(
	header *SnapshotRequest_Header, stream SnapshotResponseStream,
) error {
	s.metrics.raftRcvdMessages[raftpb.MsgSnap].Inc(1)

	if s.IsDraining() {
		return stream.Send(&SnapshotResponse{
			Status:  SnapshotResponse_DECLINED,
			Message: storeDrainingMsg,
		})
	}

	ctx := s.AnnotateCtx(stream.Context())
	return s.receiveSnapshot(ctx, header, stream)
}

func (s *Store) uncoalesceBeats(
	ctx context.Context,
	beats []RaftHeartbeat,
	fromReplica, toReplica roachpb.ReplicaDescriptor,
	msgT raftpb.MessageType,
	respStream RaftMessageResponseStream,
) {
	if len(beats) == 0 {
		return
	}
	if log.V(4) {
		log.Infof(ctx, "uncoalescing %d beats of type %v: %+v", len(beats), msgT, beats)
	}
	beatReqs := make([]RaftMessageRequest, len(beats))
	for i, beat := range beats {
		msg := raftpb.Message{
			Type:   msgT,
			From:   uint64(beat.FromReplicaID),
			To:     uint64(beat.ToReplicaID),
			Term:   beat.Term,
			Commit: beat.Commit,
		}
		beatReqs[i] = RaftMessageRequest{
			RangeID: beat.RangeID,
			FromReplica: roachpb.ReplicaDescriptor{
				NodeID:    fromReplica.NodeID,
				StoreID:   fromReplica.StoreID,
				ReplicaID: beat.FromReplicaID,
			},
			ToReplica: roachpb.ReplicaDescriptor{
				NodeID:    toReplica.NodeID,
				StoreID:   toReplica.StoreID,
				ReplicaID: beat.ToReplicaID,
			},
			Message: msg,
			Quiesce: beat.Quiesce,
		}
		if log.V(4) {
			log.Infof(ctx, "uncoalesced beat: %+v", beatReqs[i])
		}

		if err := s.HandleRaftUncoalescedRequest(ctx, &beatReqs[i], respStream); err != nil {
			log.Errorf(ctx, "could not handle uncoalesced heartbeat %s", err)
		}
	}
}

// HandleRaftRequest dispatches a raft message to the appropriate Replica. It
// requires that s.mu is not held.
func (s *Store) HandleRaftRequest(
	ctx context.Context, req *RaftMessageRequest, respStream RaftMessageResponseStream,
) *roachpb.Error {
	if len(req.Heartbeats)+len(req.HeartbeatResps) > 0 {
		if req.RangeID != 0 {
			log.Fatalf(ctx, "coalesced heartbeats must have rangeID == 0")
		}
		s.uncoalesceBeats(ctx, req.Heartbeats, req.FromReplica, req.ToReplica, raftpb.MsgHeartbeat, respStream)
		s.uncoalesceBeats(ctx, req.HeartbeatResps, req.FromReplica, req.ToReplica, raftpb.MsgHeartbeatResp, respStream)
		return nil
	}
	return s.HandleRaftUncoalescedRequest(ctx, req, respStream)
}

// HandleRaftUncoalescedRequest dispatches a raft message to the appropriate
// Replica. It requires that s.mu is not held.
func (s *Store) HandleRaftUncoalescedRequest(
	ctx context.Context, req *RaftMessageRequest, respStream RaftMessageResponseStream,
) *roachpb.Error {

	if len(req.Heartbeats)+len(req.HeartbeatResps) > 0 {
		log.Fatalf(ctx, "HandleRaftUncoalescedRequest cannot be given coalesced heartbeats or heartbeat responses, received %s", req)
	}
	// HandleRaftRequest is called on locally uncoalesced heartbeats (which are
	// not sent over the network if the environment variable is set) so do not
	// count them.
	s.metrics.raftRcvdMessages[req.Message.Type].Inc(1)

	value, ok := s.replicaQueues.Load(int64(req.RangeID))
	if !ok {
		value, _ = s.replicaQueues.LoadOrStore(int64(req.RangeID), unsafe.Pointer(&raftRequestQueue{}))
	}
	q := (*raftRequestQueue)(value)
	q.Lock()
	if len(q.infos) >= replicaRequestQueueSize {
		q.Unlock()
		// TODO(peter): Return an error indicating the request was dropped. Note
		// that dropping the request is safe. Raft will retry.
		s.metrics.RaftRcvdMsgDropped.Inc(1)
		return nil
	}
	q.infos = append(q.infos, raftRequestInfo{
		req:        req,
		respStream: respStream,
	})
	q.Unlock()

	s.scheduler.EnqueueRaftRequest(req.RangeID)
	return nil
}

// withReplicaForRequest calls the supplied function with the (lazily
// initialized) Replica specified in the request. The replica passed to
// the function will have its Replica.raftMu locked.
func (s *Store) withReplicaForRequest(
	ctx context.Context, req *RaftMessageRequest, f func(context.Context, *Replica) *roachpb.Error,
) *roachpb.Error {
	// Lazily create the replica.
	r, _, err := s.getOrCreateReplica(
		ctx,
		req.RangeID,
		req.ToReplica.ReplicaID,
		&req.FromReplica,
	)
	if err != nil {
		return roachpb.NewError(err)
	}
	defer r.raftMu.Unlock()
	ctx = r.AnnotateCtx(ctx)
	r.setLastReplicaDescriptors(req)
	return f(ctx, r)
}

// processRaftRequestWithReplica processes the (non-snapshot) Raft request on
// the specified replica. Notably, it does not handle updates to the Raft Ready
// state; callers will probably want to handle this themselves at some point.
func (s *Store) processRaftRequestWithReplica(
	ctx context.Context, r *Replica, req *RaftMessageRequest,
) *roachpb.Error {
	if req.Message.Type == raftpb.MsgSnap {
		log.Fatalf(ctx, "unexpected snapshot: %+v", req)
	}

	if req.Quiesce {
		if req.Message.Type != raftpb.MsgHeartbeat {
			log.Fatalf(ctx, "unexpected quiesce: %+v", req)
		}
		status := r.RaftStatus()
		if status != nil && status.Term == req.Message.Term && status.Commit == req.Message.Commit {
			if r.quiesce() {
				return nil
			}
		}
		if log.V(4) {
			log.Infof(ctx, "not quiescing: local raft status is %+v, incoming quiesce message is %+v", status, req.Message)
		}
	}

	if req.ToReplica.ReplicaID == 0 {
		log.VEventf(ctx, 1, "refusing incoming Raft message %s from %+v to %+v",
			req.Message.Type, req.FromReplica, req.ToReplica)
		return roachpb.NewErrorf(
			"cannot recreate replica that is not a member of its range (StoreID %s not found in r%d)",
			r.store.StoreID(), req.RangeID,
		)
	}

	if err := r.stepRaftGroup(req); err != nil {
		return roachpb.NewError(err)
	}
	return nil
}

// processRaftSnapshotRequest processes the incoming snapshot Raft request on
// the request's specified replica. This snapshot can be preemptive or not. If
// not, the function makes sure to handle any updated Raft Ready state.
func (s *Store) processRaftSnapshotRequest(
	ctx context.Context, snapHeader *SnapshotRequest_Header, inSnap IncomingSnapshot,
) *roachpb.Error {
	return s.withReplicaForRequest(ctx, &snapHeader.RaftMessageRequest, func(
		ctx context.Context, r *Replica,
	) (pErr *roachpb.Error) {
		if snapHeader.RaftMessageRequest.Message.Type != raftpb.MsgSnap {
			log.Fatalf(ctx, "expected snapshot: %+v", snapHeader.RaftMessageRequest)
		}

		// Check to see if a snapshot can be applied. Snapshots can always be applied
		// to initialized replicas. Note that if we add a placeholder we need to
		// already be holding Replica.raftMu in order to prevent concurrent
		// raft-ready processing of uninitialized replicas.
		var addedPlaceholder bool
		var removePlaceholder bool
		if err := func() error {
			s.mu.Lock()
			defer s.mu.Unlock()
			placeholder, err := s.canApplySnapshotLocked(ctx, snapHeader)
			if err != nil {
				// If the storage cannot accept the snapshot, return an
				// error before passing it to RawNode.Step, since our
				// error handling options past that point are limited.
				log.Infof(ctx, "cannot apply snapshot: %s", err)
				return err
			}

			if placeholder != nil {
				// NB: The placeholder added here is either removed below after a
				// preemptive snapshot is applied or after the next call to
				// Replica.handleRaftReady. Note that we can only get here if the
				// replica doesn't exist or is uninitialized.
				if err := s.addPlaceholderLocked(placeholder); err != nil {
					log.Fatalf(ctx, "could not add vetted placeholder %s: %s", placeholder, err)
				}
				addedPlaceholder = true
			}
			return nil
		}(); err != nil {
			return roachpb.NewError(err)
		}

		if addedPlaceholder {
			// If we added a placeholder remove it before we return unless some other
			// part of the code takes ownership of the removal (indicated by setting
			// removePlaceholder to false).
			removePlaceholder = true
			defer func() {
				if removePlaceholder {
					if s.removePlaceholder(ctx, snapHeader.RaftMessageRequest.RangeID) {
						atomic.AddInt32(&s.counts.removedPlaceholders, 1)
					}
				}
			}()
		}

		if snapHeader.IsPreemptive() {
			defer func() {
				s.mu.Lock()
				defer s.mu.Unlock()

				// We need to remove the placeholder regardless of whether the snapshot
				// applied successfully or not.
				if addedPlaceholder {
					// Clear the replica placeholder; we are about to swap it with a real replica.
					if !s.removePlaceholderLocked(ctx, snapHeader.RaftMessageRequest.RangeID) {
						log.Fatalf(ctx, "could not remove placeholder after preemptive snapshot")
					}
					if pErr == nil {
						atomic.AddInt32(&s.counts.filledPlaceholders, 1)
					} else {
						atomic.AddInt32(&s.counts.removedPlaceholders, 1)
					}
					removePlaceholder = false
				}

				if pErr == nil {
					// If the snapshot succeeded, process the range descriptor update.
					if err := s.processRangeDescriptorUpdateLocked(ctx, r); err != nil {
						pErr = roachpb.NewError(err)
					}
				}
			}()

			// Requiring that the Term is set in a message makes sure that we
			// get all of Raft's internal safety checks (it confuses messages
			// at term zero for internal messages). The sending side uses the
			// term from the snapshot itself, but we'll just check nonzero.
			if snapHeader.RaftMessageRequest.Message.Term == 0 {
				return roachpb.NewErrorf(
					"preemptive snapshot from term %d received with zero term",
					snapHeader.RaftMessageRequest.Message.Snapshot.Metadata.Term,
				)
			}
			// TODO(tschottdorf): A lot of locking of the individual Replica
			// going on below as well. I think that's more easily refactored
			// away; what really matters is that the Store doesn't do anything
			// else with that same Replica (or one that might conflict with us
			// while we still run). In effect, we'd want something like:
			//
			// 1. look up the snapshot's key range
			// 2. get an exclusive lock for operations on that key range from
			//    the store (or discard the snapshot)
			//    (at the time of writing, we have checked the key range in
			//    canApplySnapshotLocked above, but there are concerns about two
			//    conflicting operations passing that check simultaneously,
			//    see #7830)
			// 3. do everything below (apply the snapshot through temp Raft group)
			// 4. release the exclusive lock on the snapshot's key range
			//
			// There are two future outcomes: Either we begin receiving
			// legitimate Raft traffic for this Range (hence learning the
			// ReplicaID and becoming a real Replica), or the Replica GC queue
			// decides that the ChangeReplicas as a part of which the
			// preemptive snapshot was sent has likely failed and removes both
			// in-memory and on-disk state.
			r.mu.Lock()
			// We are paranoid about applying preemptive snapshots (which
			// were constructed via our code rather than raft) to the "real"
			// raft group. Instead, we destroy the "real" raft group if one
			// exists (this is rare in production, although it occurs in
			// tests), apply the preemptive snapshot to a temporary raft
			// group, then discard that one as well to be replaced by a real
			// raft group when we get a new replica ID.
			//
			// It might be OK instead to apply preemptive snapshots just
			// like normal ones (essentially switching between regular and
			// preemptive mode based on whether or not we have a raft group,
			// instead of based on the replica ID of the snapshot message).
			// However, this is a risk that we're not yet willing to take.
			// Additionally, without some additional plumbing work, doing so
			// would limit the effectiveness of RaftTransport.SendSync for
			// preemptive snapshots.
			r.mu.internalRaftGroup = nil
			needTombstone := r.mu.state.Desc.NextReplicaID != 0
			r.mu.Unlock()

			appliedIndex, _, err := r.raftMu.stateLoader.LoadAppliedIndex(ctx, r.store.Engine())
			if err != nil {
				return roachpb.NewError(err)
			}
			raftGroup, err := raft.NewRawNode(
				newRaftConfig(
					raft.Storage((*replicaRaftStorage)(r)),
					preemptiveSnapshotRaftGroupID,
					// We pass the "real" applied index here due to subtleties
					// arising in the case in which Raft discards the snapshot:
					// It would instruct us to apply entries, which would have
					// crashing potential for any choice of dummy value below.
					appliedIndex,
					r.store.cfg,
					&raftLogger{ctx: ctx},
				), nil)
			if err != nil {
				return roachpb.NewError(err)
			}
			// We have a Raft group; feed it the message.
			if err := raftGroup.Step(snapHeader.RaftMessageRequest.Message); err != nil {
				return roachpb.NewError(errors.Wrap(err, "unable to process preemptive snapshot"))
			}
			// In the normal case, the group should ask us to apply a snapshot.
			// If it doesn't, our snapshot was probably stale. In that case we
			// still go ahead and apply a noop because we want that case to be
			// counted by stats as a successful application.
			var ready raft.Ready
			if raftGroup.HasReady() {
				ready = raftGroup.Ready()
			}

			if needTombstone {
				// Bump the min replica ID, but don't write the tombstone key. The
				// tombstone key is not expected to be present when normal replica data
				// is present and applySnapshot would delete the key in most cases. If
				// Raft has decided the snapshot shouldn't be applied we would be
				// writing the tombstone key incorrectly.
				r.mu.Lock()
				if r.mu.state.Desc.NextReplicaID > r.mu.minReplicaID {
					r.mu.minReplicaID = r.mu.state.Desc.NextReplicaID
				}
				r.mu.Unlock()
			}

			// Apply the snapshot, as Raft told us to. Preemptive snapshots never
			// subsume replicas (this is guaranteed by Store.canApplySnapshot), so
			// we can simply pass nil for the subsumedRepls parameter.
			if err := r.applySnapshot(
				ctx, inSnap, ready.Snapshot, ready.HardState, nil, /* subsumedRepls */
			); err != nil {
				return roachpb.NewError(err)
			}

			// At this point, the Replica has data but no ReplicaID. We hope
			// that it turns into a "real" Replica by means of receiving Raft
			// messages addressed to it with a ReplicaID, but if that doesn't
			// happen, at some point the Replica GC queue will have to grab it.
			//
			// NB: See the defer at the start of this block for the removal of the
			// placeholder and processing of the range descriptor update.
			return nil
		}

		if err := r.stepRaftGroup(&snapHeader.RaftMessageRequest); err != nil {
			return roachpb.NewError(err)
		}

		if _, expl, err := r.handleRaftReadyRaftMuLocked(inSnap); err != nil {
			fatalOnRaftReadyErr(ctx, expl, err)
		}
		removePlaceholder = false
		return nil
	})
}

// HandleRaftResponse implements the RaftMessageHandler interface. Per the
// interface specification, an error is returned if and only if the underlying
// Raft connection should be closed.
// It requires that s.mu is not held.
func (s *Store) HandleRaftResponse(ctx context.Context, resp *RaftMessageResponse) error {
	ctx = s.AnnotateCtx(ctx)
	repl, replErr := s.GetReplica(resp.RangeID)
	if replErr == nil {
		// Best-effort context annotation of replica.
		ctx = repl.AnnotateCtx(ctx)
	}
	switch val := resp.Union.GetValue().(type) {
	case *roachpb.Error:
		switch tErr := val.GetDetail().(type) {
		case *roachpb.ReplicaTooOldError:
			if replErr != nil {
				// RangeNotFoundErrors are expected here; nothing else is.
				if _, ok := replErr.(*roachpb.RangeNotFoundError); !ok {
					log.Error(ctx, replErr)
				}
				return nil
			}

			repl.mu.Lock()
			// If the replica ID in the error does not match then we know
			// that the replica has been removed and re-added quickly. In
			// that case, we don't want to add it to the replicaGCQueue.
			if tErr.ReplicaID != repl.mu.replicaID {
				repl.mu.Unlock()
				log.Infof(ctx, "replica too old response with old replica ID: %s", tErr.ReplicaID)
				return nil
			}
			// If the replica ID in the error does match, we know the replica
			// will be removed and we can cancel any pending commands. This is
			// sometimes necessary to unblock PushTxn operations that are
			// necessary for the replica GC to succeed.
			repl.cancelPendingCommandsLocked()
			// The replica will be garbage collected soon (we are sure
			// since our replicaID is definitely too old), but in the meantime we
			// already want to bounce all traffic from it. Note that the replica
			// could be re-added with a higher replicaID, in which this error is
			// cleared in setReplicaIDRaftMuLockedMuLocked.
			if repl.mu.destroyStatus.IsAlive() {
				repl.mu.destroyStatus.Set(roachpb.NewRangeNotFoundError(repl.RangeID), destroyReasonRemovalPending)
			}
			repl.mu.Unlock()

			if _, err := s.replicaGCQueue.Add(repl, replicaGCPriorityRemoved); err != nil {
				log.Errorf(ctx, "unable to add to replica GC queue: %s", err)
			} else {
				log.Infof(ctx, "added to replica GC queue (peer suggestion)")
			}
		case *roachpb.RaftGroupDeletedError:
			if replErr != nil {
				// RangeNotFoundErrors are expected here; nothing else is.
				if _, ok := replErr.(*roachpb.RangeNotFoundError); !ok {
					log.Error(ctx, replErr)
				}
				return nil
			}

			// If the replica is talking to a replica that's been deleted, it must be
			// out of date. While this may just mean it's slightly behind, it can
			// also mean that it is so far behind it no longer knows where any of the
			// other replicas are (#23994). Add it to the replica GC queue to do a
			// proper check.
			if _, err := s.replicaGCQueue.Add(repl, replicaGCPriorityDefault); err != nil {
				log.Errorf(ctx, "unable to add to replica GC queue: %s", err)
			} else {
				log.Infof(ctx, "added to replica GC queue (contacted deleted peer)")
			}
		case *roachpb.StoreNotFoundError:
			log.Warningf(ctx, "raft error: node %d claims to not contain store %d for replica %s: %s",
				resp.FromReplica.NodeID, resp.FromReplica.StoreID, resp.FromReplica, val)
			return val.GetDetail() // close Raft connection
		default:
			log.Warningf(ctx, "got error from r%d, replica %s: %s",
				resp.RangeID, resp.FromReplica, val)
		}
	default:
		log.Warningf(ctx, "got unknown raft response type %T from replica %s: %s", val, resp.FromReplica, val)
	}
	return nil
}

// enqueueRaftUpdateCheck asynchronously registers the given range ID to be
// checked for raft updates when the processRaft goroutine is idle.
func (s *Store) enqueueRaftUpdateCheck(rangeID roachpb.RangeID) {
	s.scheduler.EnqueueRaftReady(rangeID)
}

func (s *Store) processRequestQueue(ctx context.Context, rangeID roachpb.RangeID) {
	value, ok := s.replicaQueues.Load(int64(rangeID))
	if !ok {
		return
	}
	q := (*raftRequestQueue)(value)
	q.Lock()
	infos := q.infos
	q.infos = nil
	q.Unlock()

	var lastRepl *Replica
	for i, info := range infos {
		last := i == len(infos)-1
		pErr := s.withReplicaForRequest(info.respStream.Context(), info.req,
			func(ctx context.Context, r *Replica) *roachpb.Error {
				// Save the last Replica we see, since we don't know in advance which
				// requests will fail during Replica retrieval. We want this later
				// so we can handle the Raft Ready state all at once.
				lastRepl = r
				pErr := s.processRaftRequestWithReplica(ctx, r, info.req)
				if last {
					// If this is the last request, we can handle raft.Ready without
					// giving up the lock. Set lastRepl to nil, so we don't handle it
					// down below as well.
					lastRepl = nil
					if _, expl, err := r.handleRaftReadyRaftMuLocked(noSnap); err != nil {
						fatalOnRaftReadyErr(ctx, expl, err)
					}
				}
				return pErr
			})
		if pErr != nil {
			// If we're unable to process the request, clear the request queue. This
			// only happens if we couldn't create the replica because the request was
			// targeted to a removed range. This is also racy and could cause us to
			// drop messages to the deleted range occasionally (#18355), but raft
			// will just retry.
			q.Lock()
			if len(q.infos) == 0 {
				s.replicaQueues.Delete(int64(rangeID))
			}
			q.Unlock()
			if err := info.respStream.Send(newRaftMessageResponse(info.req, pErr)); err != nil {
				// Seems excessive to log this on every occurrence as the other side
				// might have closed.
				log.VEventf(ctx, 1, "error sending error: %s", err)
			}
		}
	}

	// If lastRepl is not nil, that means that some of the requests succeeded during
	// Replica retrieval (withReplicaForRequest) but that the last request did not,
	// otherwise we would have handled this above and set lastRepl to nil.
	if lastRepl != nil {
		// lastRepl will be unlocked when we exit withReplicaForRequest above.
		// It's fine to relock it here (by calling handleRaftReady instead of
		// handleRaftReadyRaftMuLocked) since racing to handle Raft Ready won't
		// have any undesirable results.
		if _, expl, err := lastRepl.handleRaftReady(noSnap); err != nil {
			fatalOnRaftReadyErr(lastRepl.AnnotateCtx(ctx), expl, err)
		}
	}
}

func (s *Store) processReady(ctx context.Context, rangeID roachpb.RangeID) {
	value, ok := s.mu.replicas.Load(int64(rangeID))
	if !ok {
		return
	}

	start := timeutil.Now()
	r := (*Replica)(value)
	stats, expl, err := r.handleRaftReady(noSnap)
	if err != nil {
		log.Fatalf(ctx, "%s: %s", log.Safe(expl), err) // TODO(bdarnell)
	}
	elapsed := timeutil.Since(start)
	s.metrics.RaftWorkingDurationNanos.Inc(elapsed.Nanoseconds())
	// Warn if Raft processing took too long. We use the same duration as we
	// use for warning about excessive raft mutex lock hold times. Long
	// processing time means we'll have starved local replicas of ticks and
	// remote replicas will likely start campaigning.
	if elapsed >= defaultReplicaRaftMuWarnThreshold {
		log.Warningf(ctx, "handle raft ready: %.1fs [processed=%d]",
			elapsed.Seconds(), stats.processed)
	}
	if !r.IsInitialized() {
		// Only an uninitialized replica can have a placeholder since, by
		// definition, an initialized replica will be present in the
		// replicasByKey map. While the replica will usually consume the
		// placeholder itself, that isn't guaranteed and so this invocation
		// here is crucial (i.e. don't remove it).
		//
		// We need to hold raftMu here to prevent removing a placeholder that is
		// actively being used by Store.processRaftRequest.
		r.raftMu.Lock()
		if s.removePlaceholder(ctx, r.RangeID) {
			atomic.AddInt32(&s.counts.droppedPlaceholders, 1)
		}
		r.raftMu.Unlock()
	}
}

func (s *Store) processTick(ctx context.Context, rangeID roachpb.RangeID) bool {
	value, ok := s.mu.replicas.Load(int64(rangeID))
	if !ok {
		return false
	}
	livenessMap, _ := s.livenessMap.Load().(map[roachpb.NodeID]bool)

	start := timeutil.Now()
	r := (*Replica)(value)
	exists, err := r.tick(livenessMap)
	if err != nil {
		log.Error(ctx, err)
	}
	s.metrics.RaftTickingDurationNanos.Inc(timeutil.Since(start).Nanoseconds())
	return exists // ready
}

// nodeIsLiveCallback is invoked when a node transitions from non-live
// to live. Iterate through all replicas and find any which belong to
// ranges containing the implicated node. Unquiesce if currently
// quiesced. Note that this mechanism can race with concurrent
// invocations of processTick, which may have a copy of the previous
// livenessMap where the now-live node is down. Those instances should
// be rare, however, and we expect the newly live node to eventually
// unquiesce the range.
func (s *Store) nodeIsLiveCallback(nodeID roachpb.NodeID) {
	// Update the liveness map.
	s.livenessMap.Store(s.cfg.NodeLiveness.GetIsLiveMap())

	s.mu.replicas.Range(func(k int64, v unsafe.Pointer) bool {
		r := (*Replica)(v)
		for _, rep := range r.Desc().Replicas {
			if rep.NodeID == nodeID {
				r.unquiesce()
			}
		}
		return true
	})
}

func (s *Store) processRaft(ctx context.Context) {
	if s.cfg.TestingKnobs.DisableProcessRaft {
		return
	}

	s.scheduler.Start(ctx, s.stopper)
	// Wait for the scheduler worker goroutines to finish.
	s.stopper.RunWorker(ctx, s.scheduler.Wait)

	s.stopper.RunWorker(ctx, s.raftTickLoop)
	s.stopper.RunWorker(ctx, s.coalescedHeartbeatsLoop)
	s.stopper.AddCloser(stop.CloserFn(func() {
		s.cfg.Transport.Stop(s.StoreID())
	}))
}

func (s *Store) raftTickLoop(ctx context.Context) {
	ticker := time.NewTicker(s.cfg.RaftTickInterval)
	defer ticker.Stop()

	var rangeIDs []roachpb.RangeID

	for {
		select {
		case <-ticker.C:
			rangeIDs = rangeIDs[:0]
			// Update the liveness map.
			if s.cfg.NodeLiveness != nil {
				nextMap := s.cfg.NodeLiveness.GetIsLiveMap()
				for nodeID, isLive := range nextMap {
					if isLive {
						// Make sure we ask all live nodes for closed timestamp updates.
						s.cfg.ClosedTimestamp.Clients.EnsureClient(nodeID)
						continue
					}
					// Liveness claims that this node is down, but ConnHealth gets the last say
					// because we'd rather quiesce a range too little than one too often.
					//
					// NB: This has false negatives. If a node doesn't have a conn open to it
					// when ConnHealth is called, then ConnHealth will return
					// rpc.ErrNotHeartbeated regardless of whether the node is up or not. That
					// said, for the nodes that matter, we're likely talking to them via the
					// Raft transport, so ConnHealth should usually indicate a real problem if
					// it gives us an error back. The check can also have false positives if the
					// node goes down after populating the map, but that matters even less.
					nextMap[nodeID] = (s.cfg.NodeDialer.ConnHealth(nodeID) == nil)
				}
				s.livenessMap.Store(nextMap)
			}

			s.unquiescedReplicas.Lock()
			// Why do we bother to ever queue a Replica on the Raft scheduler for
			// tick processing? Couldn't we just call Replica.tick() here? Yes, but
			// then a single bad/slow Replica can disrupt tick processing for every
			// Replica on the store which cascades into Raft elections and more
			// disruption.
			for rangeID := range s.unquiescedReplicas.m {
				rangeIDs = append(rangeIDs, rangeID)
			}
			s.unquiescedReplicas.Unlock()

			s.scheduler.EnqueueRaftTick(rangeIDs...)
			s.metrics.RaftTicks.Inc(1)

		case <-s.stopper.ShouldStop():
			return
		}
	}
}

// Since coalesced heartbeats adds latency to heartbeat messages, it is
// beneficial to have it run on a faster cycle than once per tick, so that
// the delay does not impact latency-sensitive features such as quiescence.
func (s *Store) coalescedHeartbeatsLoop(ctx context.Context) {
	ticker := time.NewTicker(s.cfg.CoalescedHeartbeatsInterval)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			s.sendQueuedHeartbeats(ctx)
		case <-s.stopper.ShouldStop():
			return
		}
	}
}

// sendQueuedHeartbeatsToNode requires that the s.coalescedMu lock is held. It
// returns the number of heartbeats that were sent.
func (s *Store) sendQueuedHeartbeatsToNode(
	ctx context.Context, beats, resps []RaftHeartbeat, to roachpb.StoreIdent,
) int {
	var msgType raftpb.MessageType

	if len(beats) == 0 && len(resps) == 0 {
		return 0
	} else if len(resps) == 0 {
		msgType = raftpb.MsgHeartbeat
	} else if len(beats) == 0 {
		msgType = raftpb.MsgHeartbeatResp
	} else {
		log.Fatal(ctx, "cannot coalesce both heartbeats and responses")
	}

	chReq := &RaftMessageRequest{
		RangeID: 0,
		ToReplica: roachpb.ReplicaDescriptor{
			NodeID:    to.NodeID,
			StoreID:   to.StoreID,
			ReplicaID: 0,
		},
		FromReplica: roachpb.ReplicaDescriptor{
			NodeID:  s.Ident.NodeID,
			StoreID: s.Ident.StoreID,
		},
		Message: raftpb.Message{
			Type: msgType,
		},
		Heartbeats:     beats,
		HeartbeatResps: resps,
	}

	if log.V(4) {
		log.Infof(ctx, "sending raft request (coalesced) %+v", chReq)
	}

	if !s.cfg.Transport.SendAsync(chReq) {
		for _, beat := range beats {
			if value, ok := s.mu.replicas.Load(int64(beat.RangeID)); ok {
				(*Replica)(value).addUnreachableRemoteReplica(beat.ToReplicaID)
			}
		}
		for _, resp := range resps {
			if value, ok := s.mu.replicas.Load(int64(resp.RangeID)); ok {
				(*Replica)(value).addUnreachableRemoteReplica(resp.ToReplicaID)
			}
		}
		return 0
	}
	return len(beats) + len(resps)
}

func (s *Store) sendQueuedHeartbeats(ctx context.Context) {
	s.coalescedMu.Lock()
	heartbeats := s.coalescedMu.heartbeats
	heartbeatResponses := s.coalescedMu.heartbeatResponses
	s.coalescedMu.heartbeats = map[roachpb.StoreIdent][]RaftHeartbeat{}
	s.coalescedMu.heartbeatResponses = map[roachpb.StoreIdent][]RaftHeartbeat{}
	s.coalescedMu.Unlock()

	var beatsSent int

	for to, beats := range heartbeats {
		beatsSent += s.sendQueuedHeartbeatsToNode(ctx, beats, nil, to)
	}
	for to, resps := range heartbeatResponses {
		beatsSent += s.sendQueuedHeartbeatsToNode(ctx, nil, resps, to)
	}
	s.metrics.RaftCoalescedHeartbeatsPending.Update(int64(beatsSent))
}

var errRetry = errors.New("retry: orphaned replica")

// getOrCreateReplica returns a replica for the given RangeID, creating an
// uninitialized replica if necessary. The caller must not hold the store's
// lock. The returned replica has Replica.raftMu locked and it is the caller's
// responsibility to unlock it.
func (s *Store) getOrCreateReplica(
	ctx context.Context,
	rangeID roachpb.RangeID,
	replicaID roachpb.ReplicaID,
	creatingReplica *roachpb.ReplicaDescriptor,
) (_ *Replica, created bool, _ error) {
	for {
		r, created, err := s.tryGetOrCreateReplica(
			ctx,
			rangeID,
			replicaID,
			creatingReplica,
		)
		if err == errRetry {
			continue
		}
		if err != nil {
			return nil, false, err
		}
		return r, created, err
	}
}

// tryGetOrCreateReplica performs a single attempt at trying to lookup or
// create a replica. It will fail with errRetry if it finds a Replica that has
// been destroyed (and is no longer in Store.mu.replicas) or if during creation
// another goroutine gets there first. In either case, a subsequent call to
// tryGetOrCreateReplica will likely succeed, hence the loop in
// getOrCreateReplica.
func (s *Store) tryGetOrCreateReplica(
	ctx context.Context,
	rangeID roachpb.RangeID,
	replicaID roachpb.ReplicaID,
	creatingReplica *roachpb.ReplicaDescriptor,
) (_ *Replica, created bool, _ error) {
	// The common case: look up an existing (initialized) replica.
	if value, ok := s.mu.replicas.Load(int64(rangeID)); ok {
		repl := (*Replica)(value)
		repl.raftMu.Lock() // not unlocked
		repl.mu.Lock()
		defer repl.mu.Unlock()

		var replTooOldErr error
		if creatingReplica != nil {
			// Drop messages that come from a node that we believe was once a member of
			// the group but has been removed.
			desc := repl.mu.state.Desc
			_, found := desc.GetReplicaDescriptorByID(creatingReplica.ReplicaID)
			// It's not a current member of the group. Is it from the past?
			if !found && creatingReplica.ReplicaID < desc.NextReplicaID {
				replTooOldErr = roachpb.NewReplicaTooOldError(creatingReplica.ReplicaID)
			}
		}

		var err error
		if replTooOldErr != nil {
			err = replTooOldErr
		} else if ds := repl.mu.destroyStatus; ds.reason == destroyReasonRemoved {
			err = errRetry
		} else if ds.reason == destroyReasonCorrupted {
			err = ds.err
		} else {
			err = repl.setReplicaIDRaftMuLockedMuLocked(replicaID)
		}
		if err != nil {
			repl.raftMu.Unlock()
			return nil, false, err
		}
		return repl, false, nil
	}

	// No replica currently exists, so we'll try to create one. Before creating
	// the replica, see if there is a tombstone which would indicate that this is
	// a stale message.
	tombstoneKey := keys.RaftTombstoneKey(rangeID)
	var tombstone roachpb.RaftTombstone
	if ok, err := engine.MVCCGetProto(
		ctx, s.Engine(), tombstoneKey, hlc.Timestamp{}, true, nil, &tombstone,
	); err != nil {
		return nil, false, err
	} else if ok {
		if replicaID != 0 && replicaID < tombstone.NextReplicaID {
			return nil, false, &roachpb.RaftGroupDeletedError{}
		}
	}

	// Create a new replica and lock it for raft processing.
	repl := newReplica(rangeID, s)
	repl.creatingReplica = creatingReplica
	repl.raftMu.Lock() // not unlocked

	// Install the replica in the store's replica map. The replica is in an
	// inconsistent state, but nobody will be accessing it while we hold its
	// locks.
	s.mu.Lock()
	// Grab the internal Replica state lock to ensure nobody mucks with our
	// replica even outside of raft processing. Have to do this after grabbing
	// Store.mu to maintain lock ordering invariant.
	repl.mu.Lock()
	repl.mu.minReplicaID = tombstone.NextReplicaID
	// Add the range to range map, but not replicasByKey since the range's start
	// key is unknown. The range will be added to replicasByKey later when a
	// snapshot is applied. After unlocking Store.mu above, another goroutine
	// might have snuck in and created the replica, so we retry on error.
	if err := s.addReplicaToRangeMapLocked(repl); err != nil {
		repl.mu.Unlock()
		s.mu.Unlock()
		repl.raftMu.Unlock()
		return nil, false, errRetry
	}
	s.mu.uninitReplicas[repl.RangeID] = repl
	s.mu.Unlock()

	desc := &roachpb.RangeDescriptor{
		RangeID: rangeID,
		// TODO(bdarnell): other fields are unknown; need to populate them from
		// snapshot.
	}
	if err := repl.initRaftMuLockedReplicaMuLocked(desc, s.Clock(), replicaID); err != nil {
		// Mark the replica as destroyed and remove it from the replicas maps to
		// ensure nobody tries to use it
		repl.mu.destroyStatus.Set(errors.Wrapf(err, "%s: failed to initialize", repl), destroyReasonRemoved)
		repl.mu.Unlock()
		s.mu.Lock()
		s.unlinkReplicaByRangeIDLocked(rangeID)
		s.mu.Unlock()
		repl.raftMu.Unlock()
		return nil, false, err
	}
	repl.mu.Unlock()
	return repl, true, nil
}

func (s *Store) updateCapacityGauges() error {
	desc, err := s.Descriptor(false /* useCached */)
	if err != nil {
		return err
	}
	s.metrics.Capacity.Update(desc.Capacity.Capacity)
	s.metrics.Available.Update(desc.Capacity.Available)
	s.metrics.Used.Update(desc.Capacity.Used)

	return nil
}

// updateReplicationGauges counts a number of simple replication statistics for
// the ranges in this store.
// TODO(bram): #4564 It may be appropriate to compute these statistics while
// scanning ranges. An ideal solution would be to create incremental events
// whenever availability changes.
func (s *Store) updateReplicationGauges(ctx context.Context) error {
	// Load the system config.
	cfg, ok := s.Gossip().GetSystemConfig()
	if !ok {
		return errors.Errorf("%s: system config not yet available", s)
	}

	var (
		raftLeaderCount               int64
		leaseHolderCount              int64
		leaseExpirationCount          int64
		leaseEpochCount               int64
		raftLeaderNotLeaseHolderCount int64
		quiescentCount                int64
		averageQueriesPerSecond       float64
		averageWritesPerSecond        float64

		rangeCount                int64
		unavailableRangeCount     int64
		underreplicatedRangeCount int64
		behindCount               int64
	)

	timestamp := s.cfg.Clock.Now()
	var livenessMap map[roachpb.NodeID]bool
	if s.cfg.NodeLiveness != nil {
		livenessMap = s.cfg.NodeLiveness.GetIsLiveMap()
	}

	newStoreReplicaVisitor(s).Visit(func(rep *Replica) bool {
		metrics := rep.Metrics(ctx, timestamp, cfg, livenessMap)
		if metrics.Leader {
			raftLeaderCount++
			if metrics.LeaseValid && !metrics.Leaseholder {
				raftLeaderNotLeaseHolderCount++
			}
		}
		if metrics.Leaseholder {
			leaseHolderCount++
			switch metrics.LeaseType {
			case roachpb.LeaseNone:
			case roachpb.LeaseExpiration:
				leaseExpirationCount++
			case roachpb.LeaseEpoch:
				leaseEpochCount++
			}
		}
		if metrics.Quiescent {
			quiescentCount++
		}
		if metrics.RangeCounter {
			rangeCount++
			if metrics.Unavailable {
				unavailableRangeCount++
			}
			if metrics.Underreplicated {
				underreplicatedRangeCount++
			}
		}
		behindCount += metrics.BehindCount
		if qps, dur := rep.leaseholderStats.avgQPS(); dur >= MinStatsDuration {
			averageQueriesPerSecond += qps
		}
		if wps, dur := rep.writeStats.avgQPS(); dur >= MinStatsDuration {
			averageWritesPerSecond += wps
		}
		return true // more
	})

	s.metrics.RaftLeaderCount.Update(raftLeaderCount)
	s.metrics.RaftLeaderNotLeaseHolderCount.Update(raftLeaderNotLeaseHolderCount)
	s.metrics.LeaseHolderCount.Update(leaseHolderCount)
	s.metrics.LeaseExpirationCount.Update(leaseExpirationCount)
	s.metrics.LeaseEpochCount.Update(leaseEpochCount)
	s.metrics.QuiescentCount.Update(quiescentCount)
	s.metrics.AverageQueriesPerSecond.Update(averageWritesPerSecond)
	s.metrics.AverageWritesPerSecond.Update(averageWritesPerSecond)
	s.recordNewPerSecondStats(averageQueriesPerSecond, averageWritesPerSecond)

	s.metrics.RangeCount.Update(rangeCount)
	s.metrics.UnavailableRangeCount.Update(unavailableRangeCount)
	s.metrics.UnderReplicatedRangeCount.Update(underreplicatedRangeCount)
	s.metrics.RaftLogFollowerBehindCount.Update(behindCount)

	return nil
}

// updateCommandQueueGauges updates a number of simple statistics for
// the CommandQueues of each replica in this store.
func (s *Store) updateCommandQueueGauges() error {
	var (
		maxCommandQueueSize       int64
		maxCommandQueueWriteCount int64
		maxCommandQueueReadCount  int64
		maxCommandQueueTreeSize   int64
		maxCommandQueueOverlaps   int64
		combinedCommandQueueSize  int64
		combinedCommandWriteCount int64
		combinedCommandReadCount  int64
	)
	newStoreReplicaVisitor(s).Visit(func(rep *Replica) bool {
		rep.cmdQMu.Lock()

		writes := rep.cmdQMu.queues[spanset.SpanGlobal].localMetrics.writeCommands
		writes += rep.cmdQMu.queues[spanset.SpanLocal].localMetrics.writeCommands

		reads := rep.cmdQMu.queues[spanset.SpanGlobal].localMetrics.readCommands
		reads += rep.cmdQMu.queues[spanset.SpanLocal].localMetrics.readCommands

		treeSize := int64(rep.cmdQMu.queues[spanset.SpanGlobal].treeSize())
		treeSize += int64(rep.cmdQMu.queues[spanset.SpanLocal].treeSize())

		maxOverlaps := rep.cmdQMu.queues[spanset.SpanGlobal].localMetrics.maxOverlapsSeen
		if locMax := rep.cmdQMu.queues[spanset.SpanLocal].localMetrics.maxOverlapsSeen; locMax > maxOverlaps {
			maxOverlaps = locMax
		}
		rep.cmdQMu.queues[spanset.SpanGlobal].localMetrics.maxOverlapsSeen = 0
		rep.cmdQMu.queues[spanset.SpanLocal].localMetrics.maxOverlapsSeen = 0
		rep.cmdQMu.Unlock()

		cqSize := writes + reads
		if cqSize > maxCommandQueueSize {
			maxCommandQueueSize = cqSize
		}
		if writes > maxCommandQueueWriteCount {
			maxCommandQueueWriteCount = writes
		}
		if reads > maxCommandQueueReadCount {
			maxCommandQueueReadCount = reads
		}
		if treeSize > maxCommandQueueTreeSize {
			maxCommandQueueTreeSize = treeSize
		}
		if maxOverlaps > maxCommandQueueOverlaps {
			maxCommandQueueOverlaps = maxOverlaps
		}

		combinedCommandQueueSize += cqSize
		combinedCommandWriteCount += writes
		combinedCommandReadCount += reads
		return true // more
	})

	s.metrics.MaxCommandQueueSize.Update(maxCommandQueueSize)
	s.metrics.MaxCommandQueueWriteCount.Update(maxCommandQueueWriteCount)
	s.metrics.MaxCommandQueueReadCount.Update(maxCommandQueueReadCount)
	s.metrics.MaxCommandQueueTreeSize.Update(maxCommandQueueTreeSize)
	s.metrics.MaxCommandQueueOverlaps.Update(maxCommandQueueOverlaps)
	s.metrics.CombinedCommandQueueSize.Update(combinedCommandQueueSize)
	s.metrics.CombinedCommandWriteCount.Update(combinedCommandWriteCount)
	s.metrics.CombinedCommandReadCount.Update(combinedCommandReadCount)

	return nil
}

// ComputeMetrics immediately computes the current value of store metrics which
// cannot be computed incrementally. This method should be invoked periodically
// by a higher-level system which records store metrics.
func (s *Store) ComputeMetrics(ctx context.Context, tick int) error {
	ctx = s.AnnotateCtx(ctx)
	if err := s.updateCapacityGauges(); err != nil {
		return err
	}
	if err := s.updateReplicationGauges(ctx); err != nil {
		return err
	}
	if err := s.updateCommandQueueGauges(); err != nil {
		return err
	}

	// Get the latest RocksDB stats.
	stats, err := s.engine.GetStats()
	if err != nil {
		return err
	}
	s.metrics.updateRocksDBStats(*stats)

	// If we're using RocksDB, log the sstable overview.
	if rocksdb, ok := s.engine.(*engine.RocksDB); ok {
		sstables := rocksdb.GetSSTables()
		s.metrics.RdbNumSSTables.Update(int64(sstables.Len()))
		readAmp := sstables.ReadAmplification()
		s.metrics.RdbReadAmplification.Update(int64(readAmp))
		// Log this metric infrequently.
		if tick%60 == 0 /* every 10m */ {
			log.Infof(ctx, "sstables (read amplification = %d):\n%s", readAmp, sstables)
			log.Infof(ctx, "%s\nestimated_pending_compaction_bytes: %s",
				rocksdb.GetCompactionStats(), humanizeutil.IBytes(stats.PendingCompactionBytesEstimate))
		}
	}
	return nil
}

// StoreKeySpanStats carries the result of a stats computation over a key range.
type StoreKeySpanStats struct {
	ReplicaCount         int
	MVCC                 enginepb.MVCCStats
	ApproximateDiskBytes uint64
}

// ComputeStatsForKeySpan computes the aggregated MVCCStats for all replicas on
// this store which contain any keys in the supplied range.
func (s *Store) ComputeStatsForKeySpan(startKey, endKey roachpb.RKey) (StoreKeySpanStats, error) {
	var result StoreKeySpanStats

	newStoreReplicaVisitor(s).Visit(func(repl *Replica) bool {
		desc := repl.Desc()
		if bytes.Compare(startKey, desc.EndKey) >= 0 || bytes.Compare(desc.StartKey, endKey) >= 0 {
			return true // continue
		}
		result.MVCC.Add(repl.GetMVCCStats())
		result.ReplicaCount++
		return true
	})

	var err error
	result.ApproximateDiskBytes, err = s.engine.ApproximateDiskBytes(startKey.AsRawKey(), endKey.AsRawKey())
	return result, err
}

// AllocatorDryRun runs the given replica through the allocator without actually
// carrying out any changes, returning all trace messages collected along the way.
// Intended to help power a debug endpoint.
func (s *Store) AllocatorDryRun(
	ctx context.Context, repl *Replica,
) ([]tracing.RecordedSpan, error) {
	sysCfg, ok := s.cfg.Gossip.GetSystemConfig()
	if !ok {
		return nil, errors.New("allocator dry runs require a valid system config")
	}
	ctx, collect, cancel := tracing.ContextWithRecordingSpan(ctx, "allocator dry run")
	defer cancel()
	canTransferLease := func() bool { return true }
	_, err := s.replicateQueue.processOneChange(
		ctx, repl, sysCfg, canTransferLease, true /* dryRun */, false /* disableStatsBasedRebalancing */)
	if err != nil {
		log.Eventf(ctx, "error simulating allocator on replica %s: %s", repl, err)
	}
	return collect(), nil
}

// ManuallyEnqueue runs the given replica through the requested queue,
// returning all trace events collected along the way as well as the error
// message returned from the queue's process method, if any.  Intended to help
// power an admin debug endpoint.
func (s *Store) ManuallyEnqueue(
	ctx context.Context, queueName string, repl *Replica, skipShouldQueue bool,
) ([]tracing.RecordedSpan, string, error) {
	var queue queueImpl
	var needsLease bool
	for _, replicaQueue := range s.scanner.queues {
		if strings.ToLower(replicaQueue.Name()) == strings.ToLower(queueName) {
			queue = replicaQueue.(queueImpl)
			needsLease = replicaQueue.NeedsLease()
		}
	}
	if queue == nil {
		return nil, "", errors.Errorf("unknown queue type %q", queueName)
	}

	sysCfg, ok := s.cfg.Gossip.GetSystemConfig()
	if !ok {
		return nil, "", errors.New("cannot run queue without a valid system config; make sure the cluster " +
			"has been initialized and all nodes connected to it")
	}

	// Many queues are only meant to be run on leaseholder replicas, so attempt to
	// take the lease here or bail out early if a different replica has it.
	if needsLease {
		hasLease, pErr := repl.getLeaseForGossip(ctx)
		if pErr != nil {
			return nil, "", pErr.GoError()
		}
		if !hasLease {
			return nil, fmt.Sprintf("replica %v does not have the range lease", repl), nil
		}
	}

	ctx, collect, cancel := tracing.ContextWithRecordingSpan(
		ctx, fmt.Sprintf("manual %s queue run", queueName))
	defer cancel()

	if !skipShouldQueue {
		log.Eventf(ctx, "running %s.shouldQueue", queueName)
		shouldQueue, priority := queue.shouldQueue(ctx, s.cfg.Clock.Now(), repl, sysCfg)
		log.Eventf(ctx, "shouldQueue=%v, priority=%f", shouldQueue, priority)
		if !shouldQueue {
			return collect(), "", nil
		}
	}

	log.Eventf(ctx, "running %s.process", queueName)
	err := queue.process(ctx, repl, sysCfg)
	if err != nil {
		return collect(), err.Error(), nil
	}
	return collect(), "", nil
}

// WriteClusterVersion writes the given cluster version to the store-local cluster version key.
func WriteClusterVersion(
	ctx context.Context, writer engine.ReadWriter, cv cluster.ClusterVersion,
) error {
	return engine.MVCCPutProto(ctx, writer, nil, keys.StoreClusterVersionKey(), hlc.Timestamp{}, nil, &cv)
}

// ReadClusterVersion reads the the cluster version from the store-local version key.
func ReadClusterVersion(ctx context.Context, reader engine.Reader) (cluster.ClusterVersion, error) {
	var cv cluster.ClusterVersion
	_, err := engine.MVCCGetProto(ctx, reader, keys.StoreClusterVersionKey(), hlc.Timestamp{}, true, nil, &cv)
	return cv, err
}

// The methods below can be used to control a store's queues. Stopping a queue
// is only meant to happen in tests.

func (s *Store) setGCQueueActive(active bool) {
	s.gcQueue.SetDisabled(!active)
}
func (s *Store) setMergeQueueActive(active bool) {
	s.mergeQueue.SetDisabled(!active)
}
func (s *Store) setRaftLogQueueActive(active bool) {
	s.raftLogQueue.SetDisabled(!active)
}
func (s *Store) setReplicaGCQueueActive(active bool) {
	s.replicaGCQueue.SetDisabled(!active)
}
func (s *Store) setReplicateQueueActive(active bool) {
	s.replicateQueue.SetDisabled(!active)
}
func (s *Store) setSplitQueueActive(active bool) {
	s.splitQueue.SetDisabled(!active)
}
func (s *Store) setTimeSeriesMaintenanceQueueActive(active bool) {
	s.tsMaintenanceQueue.SetDisabled(!active)
}
func (s *Store) setRaftSnapshotQueueActive(active bool) {
	s.raftSnapshotQueue.SetDisabled(!active)
}
func (s *Store) setConsistencyQueueActive(active bool) {
	s.consistencyQueue.SetDisabled(!active)
}
func (s *Store) setScannerActive(active bool) {
	s.scanner.SetDisabled(!active)
}