sync: add WaitGroup.Go

Fixes #63796

Change-Id: I2a941275dd64ef858cbf02d31a759fdc5c082ceb
Reviewed-on: https://go-review.googlesource.com/c/go/+/662635
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

api/next/63796.txt

@@ -0,0 +1 @@
+pkg sync, method (*WaitGroup) Go(func()) #63769

doc/next/6-stdlib/99-minor/sync/63769.md

@@ -0,0 +1,2 @@
+[WaitGroup] has added a new method [WaitGroup.Go],
+that makes the common pattern of creating and counting goroutines more convenient.

src/sync/waitgroup.go

@@ -10,12 +10,35 @@ import (
 	"unsafe"
 )
 
-// A WaitGroup waits for a collection of goroutines to finish.
-// The main goroutine calls [WaitGroup.Add] to set the number of
+// A WaitGroup is a counting semaphore typically used to wait
+// for a group of goroutines to finish.
+//
+// The main goroutine calls [WaitGroup.Add] to set (or increase) the number of
 // goroutines to wait for. Then each of the goroutines
 // runs and calls [WaitGroup.Done] when finished. At the same time,
 // [WaitGroup.Wait] can be used to block until all goroutines have finished.
 //
+// This is a typical pattern of WaitGroup usage to
+// synchronize 3 goroutines, each calling the function f:
+//
+//	var wg sync.WaitGroup
+//	for range 3 {
+//	   wg.Add(1)
+//	   go func() {
+//	       defer wg.Done()
+//	       f()
+//	   }()
+//	}
+//	wg.Wait()
+//
+// For convenience, the [WaitGroup.Go] method simplifies this pattern to:
+//
+//	var wg sync.WaitGroup
+//	for range 3 {
+//	   wg.Go(f)
+//	}
+//	wg.Wait()
+//
 // A WaitGroup must not be copied after first use.
 //
 // In the terminology of [the Go memory model], a call to [WaitGroup.Done]
@@ -127,3 +150,23 @@ func (wg *WaitGroup) Wait() {
 		}
 	}
 }
+
+// Go calls f in a new goroutine and adds that task to the WaitGroup.
+// When f returns, the task is removed from the WaitGroup.
+//
+// If the WaitGroup is empty, Go must happen before a [WaitGroup.Wait].
+// Typically, this simply means Go is called to start tasks before Wait is called.
+// If the WaitGroup is not empty, Go may happen at any time.
+// This means a goroutine started by Go may itself call Go.
+// If a WaitGroup is reused to wait for several independent sets of tasks,
+// new Go calls must happen after all previous Wait calls have returned.
+//
+// In the terminology of [the Go memory model](https://go.dev/ref/mem),
+// the return from f "synchronizes before" the return of any Wait call that it unblocks.
+func (wg *WaitGroup) Go(f func()) {
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		f()
+	}()
+}

src/sync/waitgroup_test.go

@@ -98,6 +98,18 @@ func TestWaitGroupAlign(t *testing.T) {
 	x.wg.Wait()
 }
 
+func TestWaitGroupGo(t *testing.T) {
+	wg := &WaitGroup{}
+	var i int
+	wg.Go(func() {
+		i++
+	})
+	wg.Wait()
+	if i != 1 {
+		t.Fatalf("got %d, want 1", i)
+	}
+}
+
 func BenchmarkWaitGroupUncontended(b *testing.B) {
 	type PaddedWaitGroup struct {
 		WaitGroup