sync: deflake TestWaitGroupMisuse2

Also runs 100X faster on average, because it takes so many
fewer attempts to trigger the failure.

Fixes #11443.

Change-Id: I8c39ee48bb3ff6c36fa63083e04076771b65a80d
Reviewed-on: https://go-review.googlesource.com/36841
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

diff --git a/src/sync/waitgroup_test.go b/src/sync/waitgroup_test.go
index 8ec34fd343bee1332934b676c9b33a598b9c6afa..c7c391ba230f8884fa2ee104933d3e40daff9883 100644 (file)
--- a/src/sync/waitgroup_test.go
+++ b/src/sync/waitgroup_test.go
@@ -70,11 +70,8 @@ func TestWaitGroupMisuse(t *testing.T) {
 
 func TestWaitGroupMisuse2(t *testing.T) {
        knownRacy(t)
-       if testing.Short() {
-               t.Skip("skipping flaky test in short mode; see issue 11443")
-       }
-       if runtime.NumCPU() <= 2 {
-               t.Skip("NumCPU<=2, skipping: this test requires parallelism")
+       if runtime.NumCPU() <= 4 {
+               t.Skip("NumCPU<=4, skipping: this test requires parallelism")
        }
        defer func() {
                err := recover()
@@ -86,24 +83,37 @@ func TestWaitGroupMisuse2(t *testing.T) {
        }()
        defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
        done := make(chan interface{}, 2)
-       // The detection is opportunistically, so we want it to panic
+       // The detection is opportunistic, so we want it to panic
        // at least in one run out of a million.
        for i := 0; i < 1e6; i++ {
                var wg WaitGroup
+               var here uint32
                wg.Add(1)
                go func() {
                        defer func() {
                                done <- recover()
                        }()
+                       atomic.AddUint32(&here, 1)
+                       for atomic.LoadUint32(&here) != 3 {
+                               // spin
+                       }
                        wg.Wait()
                }()
                go func() {
                        defer func() {
                                done <- recover()
                        }()
+                       atomic.AddUint32(&here, 1)
+                       for atomic.LoadUint32(&here) != 3 {
+                               // spin
+                       }
                        wg.Add(1) // This is the bad guy.
                        wg.Done()
                }()
+               atomic.AddUint32(&here, 1)
+               for atomic.LoadUint32(&here) != 3 {
+                       // spin
+               }
                wg.Done()
                for j := 0; j < 2; j++ {
                        if err := <-done; err != nil {