testing/race: fixing intermittent test failure

Test NoRaceMutexPureHappensBefore in runtime/race/testdata/mutex_test.go
expects the second spawned goroutine to run after the first.  The test
attempts to force this scheduling with a 10 millisecond wait.  Following
a suggestion by Bryan Mills, we force this scheduling using a shared
variable whose access take place within the existing mutex.

Fixes #35745.

Change-Id: Ib23ec51492ecfeed4752e020401dd25755a669ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/291292
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/runtime/race/testdata/mutex_test.go b/src/runtime/race/testdata/mutex_test.go
index cbed2d370c..9dbed9a2c9 100644
--- a/src/runtime/race/testdata/mutex_test.go
+++ b/src/runtime/race/testdata/mutex_test.go
@@ -78,16 +78,23 @@ func TestNoRaceMutexPureHappensBefore(t *testing.T) {
 	var mu sync.Mutex
 	var x int16 = 0
 	_ = x
+	written := false
 	ch := make(chan bool, 2)
 	go func() {
 		x = 1
 		mu.Lock()
+		written = true
 		mu.Unlock()
 		ch <- true
 	}()
 	go func() {
-		<-time.After(1e5)
+		time.Sleep(100 * time.Microsecond)
 		mu.Lock()
+		for !written {
+			mu.Unlock()
+			time.Sleep(100 * time.Microsecond)
+			mu.Lock()
+		}
 		mu.Unlock()
 		x = 1
 		ch <- true