kvserver: introduce RangeAppliedState.RaftAppliedIndexTerm

The same field is also introduced in ReplicaState, since ReplicaState
is used in internal data-structures and when sending a state machine
snapshot.

The migration code uses a special unused term value in a
ReplicatedEvalResult to signal to the state machine application
machinery to start populating the term field.

Fixes #75671

Release note: None

docs/generated/settings/settings-for-tenants.txt

@@ -178,4 +178,4 @@ trace.debug.enable	boolean	false	if set, traces for recent requests can be seen
 trace.jaeger.agent	string		the address of a Jaeger agent to receive traces using the Jaeger UDP Thrift protocol, as <host>:<port>. If no port is specified, 6381 will be used.
 trace.opentelemetry.collector	string		address of an OpenTelemetry trace collector to receive traces using the otel gRPC protocol, as <host>:<port>. If no port is specified, 4317 will be used.
 trace.zipkin.collector	string		the address of a Zipkin instance to receive traces, as <host>:<port>. If no port is specified, 9411 will be used.
-version	version	21.2-50	set the active cluster version in the format '<major>.<minor>'
+version	version	21.2-54	set the active cluster version in the format '<major>.<minor>'

docs/generated/settings/settings.html

@@ -186,6 +186,6 @@
 <tr><td><code>trace.jaeger.agent</code></td><td>string</td><td><code></code></td><td>the address of a Jaeger agent to receive traces using the Jaeger UDP Thrift protocol, as <host>:<port>. If no port is specified, 6381 will be used.</td></tr>
 <tr><td><code>trace.opentelemetry.collector</code></td><td>string</td><td><code></code></td><td>address of an OpenTelemetry trace collector to receive traces using the otel gRPC protocol, as <host>:<port>. If no port is specified, 4317 will be used.</td></tr>
 <tr><td><code>trace.zipkin.collector</code></td><td>string</td><td><code></code></td><td>the address of a Zipkin instance to receive traces, as <host>:<port>. If no port is specified, 9411 will be used.</td></tr>
-<tr><td><code>version</code></td><td>version</td><td><code>21.2-50</code></td><td>set the active cluster version in the format '<major>.<minor>'</td></tr>
+<tr><td><code>version</code></td><td>version</td><td><code>21.2-54</code></td><td>set the active cluster version in the format '<major>.<minor>'</td></tr>
 </tbody>
 </table>

pkg/clusterversion/cockroach_versions.go

@@ -251,6 +251,12 @@ const (
 	// EnableProtectedTimestampsForTenant enables the use of protected timestamps
 	// in secondary tenants.
 	EnableProtectedTimestampsForTenant
+	// AddRaftAppliedIndexTermMigration is a migration that causes each range
+	// replica to start populating RangeAppliedState.RaftAppliedIndexTerm field.
+	AddRaftAppliedIndexTermMigration
+	// PostAddRaftAppliedIndexTermMigration is used for asserting that
+	// RaftAppliedIndexTerm is populated.
+	PostAddRaftAppliedIndexTermMigration
 
 	// *************************************************
 	// Step (1): Add new versions here.
@@ -387,6 +393,14 @@ var versionsSingleton = keyedVersions{
 		Key:     EnableProtectedTimestampsForTenant,
 		Version: roachpb.Version{Major: 21, Minor: 2, Internal: 50},
 	},
+	{
+		Key:     AddRaftAppliedIndexTermMigration,
+		Version: roachpb.Version{Major: 21, Minor: 2, Internal: 52},
+	},
+	{
+		Key:     PostAddRaftAppliedIndexTermMigration,
+		Version: roachpb.Version{Major: 21, Minor: 2, Internal: 54},
+	},
 
 	// *************************************************
 	// Step (2): Add new versions here.

pkg/kv/kvserver/batcheval/cmd_end_transaction.go

@@ -1011,9 +1011,23 @@ func splitTriggerHelper(
 		if err != nil {
 			return enginepb.MVCCStats{}, result.Result{}, errors.Wrap(err, "unable to load replica version")
 		}
+		// The RHS should populate RaftAppliedIndexTerm if the LHS is doing so.
+		// Alternatively, we could be more aggressive and also look at the cluster
+		// version, but this is simpler -- if the keyspace occupied by the
+		// original unsplit range has not been migrated yet, by an ongoing
+		// migration, both LHS and RHS will be migrated later.
+		rangeAppliedState, err := sl.LoadRangeAppliedState(ctx, batch)
+		if err != nil {
+			return enginepb.MVCCStats{}, result.Result{},
+				errors.Wrap(err, "unable to load range applied state")
+		}
+		writeRaftAppliedIndexTerm := false
+		if rangeAppliedState.RaftAppliedIndexTerm > 0 {
+			writeRaftAppliedIndexTerm = true
+		}
 		*h.AbsPostSplitRight(), err = stateloader.WriteInitialReplicaState(
 			ctx, batch, *h.AbsPostSplitRight(), split.RightDesc, rightLease,
-			*gcThreshold, replicaVersion,
+			*gcThreshold, replicaVersion, writeRaftAppliedIndexTerm,
 		)
 		if err != nil {
 			return enginepb.MVCCStats{}, result.Result{}, errors.Wrap(err, "unable to write initial Replica state")

pkg/kv/kvserver/batcheval/cmd_migrate.go

@@ -18,6 +18,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/batcheval/result"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvserverpb"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/spanset"
+	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/stateloader"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/storage"
 	"github.com/cockroachdb/errors"
@@ -49,6 +50,8 @@ type migration func(context.Context, storage.ReadWriter, CommandArgs) (result.Re
 
 func init() {
 	_ = registerMigration // prevent unused warning.
+	registerMigration(
+		clusterversion.AddRaftAppliedIndexTermMigration, addRaftAppliedIndexTermMigration)
 }
 
 func registerMigration(key clusterversion.Key, migration migration) {
@@ -89,6 +92,25 @@ func Migrate(
 	return pd, nil
 }
 
+// addRaftAppliedIndexTermMigration migrates the system to start populating
+// the RangeAppliedState.RaftAppliedIndexTerm field.
+func addRaftAppliedIndexTermMigration(
+	ctx context.Context, readWriter storage.ReadWriter, cArgs CommandArgs,
+) (result.Result, error) {
+	return result.Result{
+		Replicated: kvserverpb.ReplicatedEvalResult{
+			State: &kvserverpb.ReplicaState{
+				// Signal the migration by sending a term on the new field that we
+				// want to migrate into. This term is chosen as one that would never
+				// be used in practice (since raftInitialLogTerm is 10), so we can
+				// special-case it below raft and start writing the (real) term to the
+				// AppliedState.
+				RaftAppliedIndexTerm: stateloader.RaftLogTermSignalForAddRaftAppliedIndexTermMigration,
+			},
+		},
+	}, nil
+}
+
 // TestingRegisterMigrationInterceptor is used in tests to register an
 // interceptor for a below-raft migration.
 //

pkg/kv/kvserver/batcheval/result/result.go

@@ -193,10 +193,13 @@ func coalesceBool(lhs *bool, rhs *bool) {
 func (p *Result) MergeAndDestroy(q Result) error {
 	if q.Replicated.State != nil {
 		if q.Replicated.State.RaftAppliedIndex != 0 {
-			return errors.AssertionFailedf("must not specify RaftApplyIndex")
+			return errors.AssertionFailedf("must not specify RaftAppliedIndex")
+		}
+		if q.Replicated.State.RaftAppliedIndexTerm != 0 {
+			return errors.AssertionFailedf("must not specify RaftAppliedIndexTerm")
 		}
 		if q.Replicated.State.LeaseAppliedIndex != 0 {
-			return errors.AssertionFailedf("must not specify RaftApplyIndex")
+			return errors.AssertionFailedf("must not specify LeaseAppliedIndex")
 		}
 		if p.Replicated.State == nil {
 			p.Replicated.State = &kvserverpb.ReplicaState{}

pkg/kv/kvserver/below_raft_protos_test.go

@@ -78,7 +78,7 @@ var belowRaftGoldenProtos = map[reflect.Type]fixture{
 			return enginepb.NewPopulatedRangeAppliedState(r, false)
 		},
 		emptySum:     615555020845646359,
-		populatedSum: 17354515720541950025,
+		populatedSum: 12125419916111069931,
 	},
 	reflect.TypeOf(&raftpb.HardState{}): {
 		populatedConstructor: func(r *rand.Rand) protoutil.Message {

pkg/kv/kvserver/kvserverpb/state.proto

@@ -134,6 +134,10 @@ message ReplicaState {
   // "follower reads" at or below this timestamp.
   util.hlc.Timestamp raft_closed_timestamp = 13 [(gogoproto.nullable) = false];
 
+  // The term corresponding to RaftAppliedIndex. This is derived from
+  // RangeAppliedState.RaftAppliedIndexTerm.
+  uint64 raft_applied_index_term = 14;
+
   reserved 8, 9, 10;
 }
 

pkg/kv/kvserver/replica_application_state_machine.go

@@ -19,6 +19,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/closedts/ctpb"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvserverbase"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvserverpb"
+	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/stateloader"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/storage"
 	"github.com/cockroachdb/cockroach/pkg/storage/enginepb"
@@ -849,9 +850,23 @@ func (b *replicaAppBatch) runPreApplyTriggersAfterStagingWriteBatch(
 func (b *replicaAppBatch) stageTrivialReplicatedEvalResult(
 	ctx context.Context, cmd *replicatedCmd,
 ) {
-	if raftAppliedIndex := cmd.ent.Index; raftAppliedIndex != 0 {
-		b.state.RaftAppliedIndex = raftAppliedIndex
+	raftAppliedIndex := cmd.ent.Index
+	if raftAppliedIndex == 0 {
+		log.Fatalf(ctx, "raft entry with index 0")
+	}
+	b.state.RaftAppliedIndex = raftAppliedIndex
+	rs := cmd.decodedRaftEntry.replicatedResult().State
+	// We are post migration or this replicatedCmd is doing the migration.
+	if b.state.RaftAppliedIndexTerm > 0 || (rs != nil &&
+		rs.RaftAppliedIndexTerm == stateloader.RaftLogTermSignalForAddRaftAppliedIndexTermMigration) {
+		// Once we populate b.state.RaftAppliedIndexTerm it will flow into the
+		// persisted RangeAppliedState and into the in-memory representation in
+		// Replica.mu.state. The latter is used to initialize b.state, so future
+		// calls to this method will see that the migration has already happened
+		// and will continue to populate the term.
+		b.state.RaftAppliedIndexTerm = cmd.ent.Term
 	}
+
 	if leaseAppliedIndex := cmd.leaseIndex; leaseAppliedIndex != 0 {
 		b.state.LeaseAppliedIndex = leaseAppliedIndex
 	}
@@ -915,6 +930,9 @@ func (b *replicaAppBatch) ApplyToStateMachine(ctx context.Context) error {
 	// Update the replica's applied indexes, mvcc stats and closed timestamp.
 	r.mu.Lock()
 	r.mu.state.RaftAppliedIndex = b.state.RaftAppliedIndex
+	// RaftAppliedIndexTerm will be non-zero only when the
+	// AddRaftAppliedIndexTermMigration has happened.
+	r.mu.state.RaftAppliedIndexTerm = b.state.RaftAppliedIndexTerm
 	r.mu.state.LeaseAppliedIndex = b.state.LeaseAppliedIndex
 
 	// Sanity check that the RaftClosedTimestamp doesn't go backwards.
@@ -978,7 +996,7 @@ func (b *replicaAppBatch) addAppliedStateKeyToBatch(ctx context.Context) error {
 	// lease index along with the mvcc stats, all in one key.
 	loader := &b.r.raftMu.stateLoader
 	return loader.SetRangeAppliedState(
-		ctx, b.batch, b.state.RaftAppliedIndex, b.state.LeaseAppliedIndex,
+		ctx, b.batch, b.state.RaftAppliedIndex, b.state.LeaseAppliedIndex, b.state.RaftAppliedIndexTerm,
 		b.state.Stats, &b.state.RaftClosedTimestamp,
 	)
 }

pkg/kv/kvserver/replica_raftstorage.go

@@ -386,6 +386,15 @@ func (r *Replica) GetLeaseAppliedIndex() uint64 {
 // r.mu is held. Note that the returned snapshot is a placeholder and
 // does not contain any of the replica data. The snapshot is actually generated
 // (and sent) by the Raft snapshot queue.
+//
+// More specifically, this method is called by etcd/raft in
+// (*raftLog).snapshot. Raft expects that it generates the snapshot (by
+// calling Snapshot) and that "sending" the result actually sends the
+// snapshot. In CockroachDB, that message is intercepted (at the sender) and
+// instead we add the replica (the raft leader) to the raft snapshot queue,
+// and when its turn comes we look at the raft state for followers that want a
+// snapshot, and then send one. That actual sending path does not call this
+// Snapshot method.
 func (r *replicaRaftStorage) Snapshot() (raftpb.Snapshot, error) {
 	r.mu.AssertHeld()
 	appliedIndex := r.mu.state.RaftAppliedIndex
@@ -418,11 +427,27 @@ func (r *Replica) GetSnapshot(
 	// the corresponding Raft command not applied yet).
 	r.raftMu.Lock()
 	snap := r.store.engine.NewSnapshot()
-	r.mu.Lock()
-	appliedIndex := r.mu.state.RaftAppliedIndex
-	// Cleared when OutgoingSnapshot closes.
-	r.addSnapshotLogTruncationConstraintLocked(ctx, snapUUID, appliedIndex, recipientStore)
-	r.mu.Unlock()
+	{
+		r.mu.Lock()
+		// We will fetch the applied index later again, from snap. The
+		// appliedIndex fetched here is narrowly used for adding a log truncation
+		// constraint to prevent log entries > appliedIndex from being removed.
+		// Note that the appliedIndex maintained in Replica actually lags the one
+		// in the engine, since replicaAppBatch.ApplyToStateMachine commits the
+		// engine batch and then acquires Replica.mu to update
+		// Replica.mu.state.RaftAppliedIndex. The use of a possibly stale value
+		// here is harmless since using a lower index in this constraint, than the
+		// actual snapshot index, preserves more from a log truncation
+		// perspective.
+		//
+		// TODO(sumeer): despite the above justification, this is unnecessarily
+		// complicated. Consider loading the RaftAppliedIndex from the snap for
+		// this use case.
+		appliedIndex := r.mu.state.RaftAppliedIndex
+		// Cleared when OutgoingSnapshot closes.
+		r.addSnapshotLogTruncationConstraintLocked(ctx, snapUUID, appliedIndex, recipientStore)
+		r.mu.Unlock()
+	}
 	r.raftMu.Unlock()
 
 	release := func() {
@@ -568,6 +593,12 @@ func snapshot(
 	}
 
 	term, err := term(ctx, rsl, snap, rangeID, eCache, state.RaftAppliedIndex)
+	// If we've migrated to populating RaftAppliedIndexTerm, check that the term
+	// from the two sources are equal.
+	if state.RaftAppliedIndexTerm != 0 && term != state.RaftAppliedIndexTerm {
+		return OutgoingSnapshot{},
+			errors.AssertionFailedf("unequal terms %d != %d", term, state.RaftAppliedIndexTerm)
+	}
 	if err != nil {
 		return OutgoingSnapshot{}, errors.Wrapf(err, "failed to fetch term of %d", state.RaftAppliedIndex)
 	}
@@ -920,6 +951,12 @@ func (r *Replica) applySnapshot(
 		log.Fatalf(ctx, "snapshot RaftAppliedIndex %d doesn't match its metadata index %d",
 			state.RaftAppliedIndex, nonemptySnap.Metadata.Index)
 	}
+	// If we've migrated to populating RaftAppliedIndexTerm, check that the term
+	// from the two sources are equal.
+	if state.RaftAppliedIndexTerm != 0 && state.RaftAppliedIndexTerm != nonemptySnap.Metadata.Term {
+		log.Fatalf(ctx, "snapshot RaftAppliedIndexTerm %d doesn't match its metadata term %d",
+			state.RaftAppliedIndexTerm, nonemptySnap.Metadata.Term)
+	}
 
 	// The on-disk state is now committed, but the corresponding in-memory state
 	// has not yet been updated. Any errors past this point must therefore be
@@ -975,6 +1012,11 @@ func (r *Replica) applySnapshot(
 	// feelings about this ever change, we can add a LastIndex field to
 	// raftpb.SnapshotMetadata.
 	r.mu.lastIndex = state.RaftAppliedIndex
+
+	// TODO(sumeer): We should be able to set this to
+	// nonemptySnap.Metadata.Term. See
+	// https://reviewable.io/reviews/cockroachdb/cockroach/75675#-MukTjIhAlmWs8rSi-kY:-MukTjIhAlmWs8rSi-kZ:bf8moge
+	// for a discussion regarding this.
 	r.mu.lastTerm = invalidLastTerm
 	r.mu.raftLogSize = 0
 	// Update the store stats for the data in the snapshot.

pkg/kv/kvserver/stateloader/initial.go

@@ -13,6 +13,7 @@ package stateloader
 import (
 	"context"
 
+	"github.com/cockroachdb/cockroach/pkg/clusterversion"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvserverpb"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/storage"
@@ -29,6 +30,15 @@ import (
 const (
 	raftInitialLogIndex = 10
 	raftInitialLogTerm  = 5
+
+	// RaftLogTermSignalForAddRaftAppliedIndexTermMigration is never persisted
+	// in the state machine or in HardState. It is only used in
+	// AddRaftAppliedIndexTermMigration to signal to the below raft code that
+	// the migration should happen when applying the raft log entry that
+	// contains ReplicatedEvalResult.State.RaftAppliedIndexTerm equal to this
+	// value. It is less than raftInitialLogTerm since that ensures it will
+	// never be used under normal operation.
+	RaftLogTermSignalForAddRaftAppliedIndexTermMigration = 3
 )
 
 // WriteInitialReplicaState sets up a new Range, but without writing an
@@ -46,6 +56,7 @@ func WriteInitialReplicaState(
 	lease roachpb.Lease,
 	gcThreshold hlc.Timestamp,
 	replicaVersion roachpb.Version,
+	writeRaftAppliedIndexTerm bool,
 ) (enginepb.MVCCStats, error) {
 	rsl := Make(desc.RangeID)
 	var s kvserverpb.ReplicaState
@@ -54,6 +65,9 @@ func WriteInitialReplicaState(
 		Index: raftInitialLogIndex,
 	}
 	s.RaftAppliedIndex = s.TruncatedState.Index
+	if writeRaftAppliedIndexTerm {
+		s.RaftAppliedIndexTerm = s.TruncatedState.Term
+	}
 	s.Desc = &roachpb.RangeDescriptor{
 		RangeID: desc.RangeID,
 	}
@@ -102,9 +116,12 @@ func WriteInitialRangeState(
 	initialGCThreshold := hlc.Timestamp{}
 	initialMS := enginepb.MVCCStats{}
 
+	writeRaftAppliedIndexTerm :=
+		clusterversion.ClusterVersion{Version: replicaVersion}.IsActiveVersion(
+			clusterversion.ByKey(clusterversion.AddRaftAppliedIndexTermMigration))
 	if _, err := WriteInitialReplicaState(
 		ctx, readWriter, initialMS, desc, initialLease, initialGCThreshold,
-		replicaVersion,
+		replicaVersion, writeRaftAppliedIndexTerm,
 	); err != nil {
 		return err
 	}