sql: refactor ConcurrentWriteBuffer into ConcurrentBufferGuard

This commit refactors the pattern used in ConcurrentWriterBuffer used in
transaction ID cache into a generic helper data structure to allow for
future reuse.

Partially address cockroachdb#74487

Release note: None

pkg/BUILD.bazel

@@ -241,6 +241,7 @@ ALL_TESTS = [
     "//pkg/sql/colflow/colrpc:colrpc_test",
     "//pkg/sql/colflow:colflow_test",
     "//pkg/sql/colmem:colmem_test",
+    "//pkg/sql/contention/contentionutils:contentionutils_test",
     "//pkg/sql/contention/txnidcache:txnidcache_test",
     "//pkg/sql/contention:contention_test",
     "//pkg/sql/covering:covering_test",

pkg/sql/contention/contentionutils/BUILD.bazel

@@ -0,0 +1,21 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "contentionutils",
+    srcs = ["concurrent_buffer_guard.go"],
+    importpath = "github.com/cockroachdb/cockroach/pkg/sql/contention/contentionutils",
+    visibility = ["//visibility:public"],
+    deps = ["//pkg/util/syncutil"],
+)
+
+go_test(
+    name = "contentionutils_test",
+    srcs = ["concurrent_buffer_guard_test.go"],
+    embed = [":contentionutils"],
+    deps = [
+        "//pkg/util/syncutil",
+        "//pkg/util/uuid",
+        "@com_github_stretchr_testify//require",
+        "@org_golang_x_exp//rand",
+    ],
+)

pkg/sql/contention/contentionutils/concurrent_buffer_guard.go

@@ -0,0 +1,153 @@
+// Copyright 2022 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package contentionutils
+
+import (
+	"sync"
+	"sync/atomic"
+
+	"github.com/cockroachdb/cockroach/pkg/util/syncutil"
+)
+
+// CapacityLimiter is used to specify the capacity of the buffer. This allows
+// the size of the buffer to change during runtime.
+type CapacityLimiter func() int64
+
+// ConcurrentBufferGuard is a helper data structure that can be used to
+// implement optimized concurrent linear write buffer.
+//
+// Note: this is a rather awkward implementation to work around the fact that
+// Golang doesn't have generic (as of 1.17). Ideally, this would be implemented
+// as a generic data structure as something like:
+//
+// template<typename T>
+// class ConcurrentBuffer<T> {
+//   std::vector<T> buffer;
+//   ...
+// public:
+//   void write(T val);
+//   std::vector<T> read() const;
+// };
+//
+// To work around the lacking of generic, ConcurrentBufferGuard is designed to
+// be embedded into higher-level structs that implements the buffer read/write
+// operations, where the buffer's access is done in the higher-level structs.
+type ConcurrentBufferGuard struct {
+	flushSyncLock syncutil.RWMutex
+	flushDone     sync.Cond
+
+	limiter          CapacityLimiter
+	onBufferFullSync onBufferFullHandler
+
+	// atomicIdx is the index pointing into the fixed-length array within the
+	// msgBlock.This should only be accessed using atomic package.
+	atomicIdx int64
+}
+
+// onBufferFullHandler is called when the buffer is full. ConcurrentBufferGuard
+// will handle the locking process to block all inflight writer requests. This
+// means that onBufferFullHandler can safely assume that it is executed with
+// exclusive access to the guarded buffer. The callback receives an integer
+// index (currentWriterIndex) indicating the index where buffer is filled to.
+type onBufferFullHandler func(currentWriterIndex int64)
+
+// bufferWriteOp is called to perform a synchronized write to the guarded
+// buffer. ConcurrentBufferGuard passes in a writerIdx into the callback.
+// The callback can safely use the writerIdx to write to the guarded buffer
+// without further synchronization.
+type bufferWriteOp func(writerIdx int64)
+
+// NewConcurrentBufferGuard returns a new instance of ConcurrentBufferGuard.
+func NewConcurrentBufferGuard(
+	limiter CapacityLimiter, fullHandler onBufferFullHandler,
+) *ConcurrentBufferGuard {
+	writeBuffer := &ConcurrentBufferGuard{
+		limiter:          limiter,
+		onBufferFullSync: fullHandler,
+	}
+	writeBuffer.flushDone.L = writeBuffer.flushSyncLock.RLocker()
+	return writeBuffer
+}
+
+// AtomicWrite executes the bufferWriterOp atomically, where bufferWriterOp
+// is a write operation into a shared linear buffer.
+//
+// Any write requests initially starts by holding a read lock (flushSyncLock)
+// and then reserves a write-index to the shared buffer (a fixed-length array).
+// If the reserved index is valid, AtomicWrite immediately executes the
+// bufferWriteOp with the reserved index. However, if the reserved index is not
+// valid, (that is, array index out of bound), there are two scenarios:
+// 1. If the reserved index == size of the array, then the caller of AtomicWrite()
+//    method is responsible for executing the onBufferFullHandler() callback. The
+//    caller does so by upgrading the read-lock to a write-lock, therefore
+//    blocks all future writers. After the callback is executed, the write-lock
+//    is then downgraded to a read-lock.
+// 2. If the reserved index > size of the array, then the caller of AtomicWrite()
+//    is blocked until the array is flushed. This is achieved by waiting on the
+//    conditional variable (flushDone) while holding onto the read-lock. After
+//    the flush is completed, the writer is unblocked and allowed to retry.
+func (c *ConcurrentBufferGuard) AtomicWrite(op bufferWriteOp) {
+	size := c.limiter()
+	c.flushSyncLock.RLock()
+	defer c.flushSyncLock.RUnlock()
+	for {
+		reservedIdx := c.reserveMsgBlockIndex()
+		if reservedIdx < size {
+			op(reservedIdx)
+			return
+		} else if reservedIdx == size {
+			c.syncRLocked()
+		} else {
+			c.flushDone.Wait()
+		}
+	}
+}
+
+// ForceSync blocks all inflight and upcoming write operation, to allow
+// the onBufferFullHandler to be executed. This can be used to preemptively
+// flushes the buffer.
+func (c *ConcurrentBufferGuard) ForceSync() {
+	c.flushSyncLock.Lock()
+	c.syncWLocked()
+	c.flushSyncLock.Unlock()
+}
+
+func (c *ConcurrentBufferGuard) syncRLocked() {
+	// We upgrade the read-lock to a write-lock, then when we are done flushing,
+	// the lock is downgraded to a read-lock.
+	c.flushSyncLock.RUnlock()
+	defer c.flushSyncLock.RLock()
+	c.flushSyncLock.Lock()
+	defer c.flushSyncLock.Unlock()
+	c.syncWLocked()
+}
+
+func (c *ConcurrentBufferGuard) syncWLocked() {
+	c.onBufferFullSync(c.currentWriterIndex())
+	c.flushDone.Broadcast()
+	c.rewindBuffer()
+}
+
+func (c *ConcurrentBufferGuard) rewindBuffer() {
+	atomic.StoreInt64(&c.atomicIdx, 0)
+}
+
+func (c *ConcurrentBufferGuard) reserveMsgBlockIndex() int64 {
+	return atomic.AddInt64(&c.atomicIdx, 1) - 1 // since array is 0-indexed.
+}
+
+func (c *ConcurrentBufferGuard) currentWriterIndex() int64 {
+	sizeLimit := c.limiter()
+	if curIdx := atomic.LoadInt64(&c.atomicIdx); curIdx < sizeLimit {
+		return curIdx
+	}
+	return sizeLimit
+}

pkg/sql/contention/contentionutils/concurrent_buffer_guard_test.go

@@ -0,0 +1,167 @@
+// Copyright 2022 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package contentionutils
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/util/syncutil"
+	"github.com/cockroachdb/cockroach/pkg/util/uuid"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/exp/rand"
+)
+
+type pair struct {
+	k uuid.UUID
+	v int
+}
+
+// testAsyncBuffer is a simple asynchronous lock-free buffer implemented using
+// ConcurrentBufferGuard. It serves two purposes:
+// 1. provide a simple testing interface to test ConcurrentBufferGuard.
+// 2. provide a simple example on how ConcurrentBufferGuard can be used.
+type testAsyncBuffer struct {
+	guard *ConcurrentBufferGuard
+
+	writerBuffer []pair
+
+	// zeroBuffer is used to quickly reset writerBuffer using Golang's builtin
+	// copy.
+	zeroBuffer []pair
+
+	// validation is an anonymous struct that synchronizes the writes to the
+	// testAsyncBuffer for testing purposes. Alternatively, this can be
+	// implemented using Golang's channel.
+	validation struct {
+		syncutil.RWMutex
+		readMap map[uuid.UUID]int
+	}
+}
+
+// newTestBuffer creates a new testAsyncBuffer. The sizeLimit params specify
+// the size of the writerBuffer before it gets flushed.
+func newTestBuffer(sizeLimit int64) *testAsyncBuffer {
+	t := &testAsyncBuffer{
+		writerBuffer: make([]pair, sizeLimit),
+		zeroBuffer:   make([]pair, sizeLimit),
+	}
+
+	t.validation.readMap = make(map[uuid.UUID]int)
+
+	t.guard = NewConcurrentBufferGuard(
+		func() int64 {
+			return sizeLimit
+		}, /* limiter */
+		func(currentWriterIdx int64) {
+			t.validation.Lock()
+			for idx := int64(0); idx < currentWriterIdx; idx++ {
+				p := t.writerBuffer[idx]
+				t.validation.readMap[p.k] = p.v
+			}
+			t.validation.Unlock()
+
+			// Resets t.writerBuffer.
+			copy(t.writerBuffer, t.zeroBuffer)
+		}, /* onBufferFullSync */
+	)
+
+	return t
+}
+
+func (ta *testAsyncBuffer) write(v pair) {
+	ta.guard.AtomicWrite(func(writerIdx int64) {
+		ta.writerBuffer[writerIdx] = v
+	})
+}
+
+func (ta *testAsyncBuffer) sync() {
+	ta.guard.ForceSync()
+}
+
+func (ta *testAsyncBuffer) assert(t *testing.T, expectedMap map[uuid.UUID]int) {
+	t.Helper()
+
+	ta.validation.RLock()
+	defer ta.validation.RUnlock()
+
+	for k, v := range expectedMap {
+		actual, ok := ta.validation.readMap[k]
+		require.True(t, ok,
+			"expected %s to exist, but it was not found", k.String())
+		require.Equal(t, v, actual, "expected to found pair %s:%d, but "+
+			"found %s:%d", k.String(), v, k.String(), actual)
+	}
+}
+
+func TestConcurrentWriterGuard(t *testing.T) {
+	numOfConcurrentWriters := []int{1, 2, 4, 16, 32}
+	bufferSizeLimit := []int64{1, 2, 5, 10, 20, 48}
+	for _, concurrentWriters := range numOfConcurrentWriters {
+		t.Run(fmt.Sprintf("concurrentWriter=%d", concurrentWriters), func(t *testing.T) {
+			for _, sizeLimit := range bufferSizeLimit {
+				t.Run(fmt.Sprintf("bufferSizeLimit=%d", sizeLimit), func(t *testing.T) {
+					runConcurrentWriterGuard(t, concurrentWriters, sizeLimit)
+				})
+			}
+		})
+	}
+}
+
+func runConcurrentWriterGuard(t *testing.T, concurrentWriters int, sizeLimit int64) {
+	start := make(chan struct{})
+	buf := newTestBuffer(sizeLimit)
+
+	expectedMaps := make(chan map[uuid.UUID]int, concurrentWriters)
+
+	var wg sync.WaitGroup
+
+	for writerCnt := 0; writerCnt < concurrentWriters; writerCnt++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			input, expected := randomGeneratedInput()
+			expectedMaps <- expected
+
+			<-start
+
+			for _, val := range input {
+				buf.write(val)
+			}
+		}()
+	}
+	close(start)
+
+	wg.Wait()
+
+	buf.sync()
+	for writerIdx := 0; writerIdx < concurrentWriters; writerIdx++ {
+		expected := <-expectedMaps
+		buf.assert(t, expected)
+	}
+}
+
+func randomGeneratedInput() (input []pair, expected map[uuid.UUID]int) {
+	const inputSize = 2000
+	input = make([]pair, 0, inputSize)
+	expected = make(map[uuid.UUID]int)
+
+	p := pair{}
+	for i := 0; i < inputSize; i++ {
+		p.k = uuid.FastMakeV4()
+		p.v = rand.Int()
+		input = append(input, p)
+		expected[p.k] = p.v
+	}
+
+	return input, expected
+}

pkg/sql/contention/txnidcache/BUILD.bazel

@@ -15,6 +15,7 @@ go_library(
         "//pkg/roachpb:with-mocks",
         "//pkg/settings",
         "//pkg/settings/cluster",
+        "//pkg/sql/contention/contentionutils",
         "//pkg/util/cache",
         "//pkg/util/encoding",
         "//pkg/util/metric",