sync: deflake TestWaitGroupMisuse3

Previous flakes:
https://build.golang.org/log/223365dedb6b6aa0cfdf5afd0a50fd433a16bade
https://build.golang.org/log/edbea4cd3f24e707ef2ae8378559bb0fcc453c22

Dmitry says in email about this:

> The stack trace points to it pretty clearly. Done can indeed unblock
> Wait first and then panic. I guess we need to recover after first
> Done as well.

And it looks like TestWaitGroupMisuse2 was already hardened against
this.  Do the same in TestWaitGroupMisuse3.

Change-Id: I317800c7e46f13c97873f0873c759a489dd5f47d
Reviewed-on: https://go-review.googlesource.com/19183
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/sync/waitgroup_test.go b/src/sync/waitgroup_test.go
index a5816609408d9b3a2aa654a9a356844c8ec31a6e..8ec34fd343bee1332934b676c9b33a598b9c6afa 100644 (file)
--- a/src/sync/waitgroup_test.go
+++ b/src/sync/waitgroup_test.go
@@ -128,13 +128,16 @@ func TestWaitGroupMisuse3(t *testing.T) {
                }
        }()
        defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
-       done := make(chan interface{}, 1)
+       done := make(chan interface{}, 2)
        // The detection is opportunistically, so we want it to panic
        // at least in one run out of a million.
        for i := 0; i < 1e6; i++ {
                var wg WaitGroup
                wg.Add(1)
                go func() {
+                       defer func() {
+                               done <- recover()
+                       }()
                        wg.Done()
                }()
                go func() {
@@ -150,8 +153,10 @@ func TestWaitGroupMisuse3(t *testing.T) {
                        wg.Wait()
                }()
                wg.Wait()
-               if err := <-done; err != nil {
-                       panic(err)
+               for j := 0; j < 2; j++ {
+                       if err := <-done; err != nil {
+                               panic(err)
+                       }
                }
        }
        t.Fatal("Should panic")