sync: deflake TestWaitGroupMisuse2

We need to yield to the runtime every now and again to avoid
deadlock. This doesn't show up on most machines because the test
only runs when you have 5 or more CPUs.

Fixes #20072.

Change-Id: Ibf5ed370e919943395f3418487188df0b2be160b
Reviewed-on: https://go-review.googlesource.com/112978
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/sync/waitgroup_test.go

@@ -68,6 +68,21 @@ func TestWaitGroupMisuse(t *testing.T) {
 	t.Fatal("Should panic")
 }
 
+// pollUntilEqual blocks until v, loaded atomically, is
+// equal to the target.
+func pollUntilEqual(v *uint32, target uint32) {
+	for {
+		for i := 0; i < 1e3; i++ {
+			if atomic.LoadUint32(v) == target {
+				return
+			}
+		}
+		// yield to avoid deadlock with the garbage collector
+		// see issue #20072
+		runtime.Gosched()
+	}
+}
+
 func TestWaitGroupMisuse2(t *testing.T) {
 	knownRacy(t)
 	if runtime.NumCPU() <= 4 {
@@ -94,9 +109,7 @@ func TestWaitGroupMisuse2(t *testing.T) {
 				done <- recover()
 			}()
 			atomic.AddUint32(&here, 1)
-			for atomic.LoadUint32(&here) != 3 {
-				// spin
-			}
+			pollUntilEqual(&here, 3)
 			wg.Wait()
 		}()
 		go func() {
@@ -104,16 +117,12 @@ func TestWaitGroupMisuse2(t *testing.T) {
 				done <- recover()
 			}()
 			atomic.AddUint32(&here, 1)
-			for atomic.LoadUint32(&here) != 3 {
-				// spin
-			}
+			pollUntilEqual(&here, 3)
 			wg.Add(1) // This is the bad guy.
 			wg.Done()
 		}()
 		atomic.AddUint32(&here, 1)
-		for atomic.LoadUint32(&here) != 3 {
-			// spin
-		}
+		pollUntilEqual(&here, 3)
 		wg.Done()
 		for j := 0; j < 2; j++ {
 			if err := <-done; err != nil {