runtime: establish happens-before between goroutine and bubble exit

synctest.Run waits for all bubbled goroutines to exit before returning.
Establish a happens-before relationship between the bubbled goroutines
exiting and Run returning.

For #67434

Change-Id: Ibda7ec2075ae50838c0851e60dc5b3c6f3ca70fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/647755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/synctest/synctest_test.go

@@ -433,6 +433,32 @@ func TestWaitGroup(t *testing.T) {
 	})
 }
 
+func TestHappensBefore(t *testing.T) {
+	var v int
+	synctest.Run(func() {
+		go func() {
+			v++ // 1
+		}()
+		// Wait creates a happens-before relationship on the above goroutine exiting.
+		synctest.Wait()
+		go func() {
+			v++ // 2
+		}()
+	})
+	// Run exiting creates a happens-before relationship on goroutines started in the bubble.
+	synctest.Run(func() {
+		v++ // 3
+		// There is a happens-before relationship between the time.AfterFunc call,
+		// and the func running.
+		time.AfterFunc(0, func() {
+			v++ // 4
+		})
+	})
+	if got, want := v, 4; got != want {
+		t.Errorf("v = %v, want %v", got, want)
+	}
+}
+
 func wantPanic(t *testing.T, want string) {
 	if e := recover(); e != nil {
 		if got := fmt.Sprint(e); got != want {

src/runtime/synctest.go

@@ -182,6 +182,8 @@ func synctestRun(f func()) {
 	sg.active++
 	for {
 		if raceenabled {
+			// Establish a happens-before relationship between a timer being created,
+			// and the timer running.
 			raceacquireg(gp, gp.syncGroup.raceaddr())
 		}
 		unlock(&sg.mu)
@@ -205,6 +207,11 @@ func synctestRun(f func()) {
 
 	total := sg.total
 	unlock(&sg.mu)
+	if raceenabled {
+		// Establish a happens-before relationship between bubbled goroutines exiting
+		// and Run returning.
+		raceacquireg(gp, gp.syncGroup.raceaddr())
+	}
 	if total != 1 {
 		panic("deadlock: all goroutines in bubble are blocked")
 	}