Conditionally allocate WaitGroup memory (#2901)

x/merkledb/view.go

@@ -316,7 +316,7 @@ func (v *view) hashChangedNode(n *node) ids.ID {
 		lastKeyByte byte
 
 		// We use [wg] to wait until all descendants of [n] have been updated.
-		wg sync.WaitGroup
+		wg waitGroup
 	)
 
 	if bytesForKey > 0 {
@@ -360,11 +360,16 @@ func (v *view) hashChangedNode(n *node) ids.ID {
 		// Try updating the child and its descendants in a goroutine.
 		if ok := v.db.hashNodesSema.TryAcquire(1); ok {
 			wg.Add(1)
-			go func(childEntry *child) {
+
+			// Passing variables explicitly through the function call rather
+			// than implicitly passing them through the scope of the function
+			// definition allows the passed variables to be allocated on the
+			// stack.
+			go func(wg *sync.WaitGroup, childEntry *child) {
 				childEntry.id = v.hashChangedNode(childNodeChange.after)
 				v.db.hashNodesSema.Release(1)
 				wg.Done()
-			}(childEntry)
+			}(wg.wg, childEntry)
 		} else {
 			// We're at the goroutine limit; do the work in this goroutine.
 			childEntry.id = v.hashChangedNode(childNodeChange.after)

x/merkledb/wait_group.go

@@ -0,0 +1,25 @@
+// Copyright (C) 2019-2024, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package merkledb
+
+import "sync"
+
+// waitGroup is a small wrapper of a sync.WaitGroup that avoids performing a
+// memory allocation when Add is never called.
+type waitGroup struct {
+	wg *sync.WaitGroup
+}
+
+func (wg *waitGroup) Add(delta int) {
+	if wg.wg == nil {
+		wg.wg = new(sync.WaitGroup)
+	}
+	wg.wg.Add(delta)
+}
+
+func (wg *waitGroup) Wait() {
+	if wg.wg != nil {
+		wg.wg.Wait()
+	}
+}

x/merkledb/wait_group_test.go

@@ -0,0 +1,29 @@
+// Copyright (C) 2019-2024, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package merkledb
+
+import "testing"
+
+func Benchmark_WaitGroup_Wait(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var wg waitGroup
+		wg.Wait()
+	}
+}
+
+func Benchmark_WaitGroup_Add(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var wg waitGroup
+		wg.Add(1)
+	}
+}
+
+func Benchmark_WaitGroup_AddDoneWait(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var wg waitGroup
+		wg.Add(1)
+		wg.wg.Done()
+		wg.Wait()
+	}
+}