test: make a race detector test robust to timing variations

The `external_cgo_thread` test in `runtime/race/output_test.go` was
producing intermittent failures.  The test was performing a sleep,
which may not be enough depending on how long it takes to setup the
callBack goroutine.

Added a synchronization to make sure callBack finishes before main ends.

Whether the increment to racy++ happens first in the callBack
or in main doesn't matter: the race detector should flag the race
regardless.  The output check was changed so that the test passes
regardless of which increment occurs first.

Fixes #43008

Change-Id: I325ec3dea52b3725e739fbf2bd7ae92875d2de10
Reviewed-on: https://go-review.googlesource.com/c/go/+/276752
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>

1 files changed, 10 insertions, 11 deletions

diff --git a/src/runtime/race/output_test.go b/src/runtime/race/output_test.go
index 986667332f..69496874c6 100644
--- a/src/runtime/race/output_test.go
+++ b/src/runtime/race/output_test.go
@@ -284,32 +284,31 @@ static inline void startThread(cb* c) {
 */
 import "C"
 
-import "time"
-
+var done chan bool
 var racy int
 
 //export goCallback
 func goCallback() {
 	racy++
+	done <- true
 }
 
 func main() {
+	done = make(chan bool)
 	var c C.cb
 	C.startThread(&c)
-	time.Sleep(time.Second)
 	racy++
+	<- done
 }
 `, `==================
 WARNING: DATA RACE
-Read at 0x[0-9,a-f]+ by main goroutine:
-  main\.main\(\)
-      .*/main\.go:34 \+0x[0-9,a-f]+
+Read at 0x[0-9,a-f]+ by .*:
+  main\..*
+      .*/main\.go:[0-9]+ \+0x[0-9,a-f]+(?s).*
 
-Previous write at 0x[0-9,a-f]+ by goroutine [0-9]:
-  main\.goCallback\(\)
-      .*/main\.go:27 \+0x[0-9,a-f]+
-  _cgoexp_[0-9a-z]+_goCallback\(\)
-      .*_cgo_gotypes\.go:[0-9]+ \+0x[0-9,a-f]+
+Previous write at 0x[0-9,a-f]+ by .*:
+  main\..*
+      .*/main\.go:[0-9]+ \+0x[0-9,a-f]+(?s).*
 
 Goroutine [0-9] \(running\) created at:
   runtime\.newextram\(\)