[release-branch.go1.15] testing: fix Cleanup race with Logf and Errorf

Updates #40908
Fixes #41034

Change-Id: I25561a3f18e730a50e6fbf85aa7bd85bf1b73b6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/250078
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 00a053bd4b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/250617
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Michał Łowicki <mlowicki@gmail.com>

src/cmd/go/testdata/script/testing_issue40908.txt

@@ -0,0 +1,21 @@
+[short] skip
+[!race] skip
+
+go test -race testrace
+
+-- testrace/race_test.go --
+package testrace
+
+import "testing"
+
+func TestRace(t *testing.T) {
+	helperDone := make(chan struct{})
+	go func() {
+		t.Logf("Something happened before cleanup.")
+		close(helperDone)
+	}()
+
+	t.Cleanup(func() {
+		<-helperDone
+	})
+}

src/testing/testing.go

@@ -851,11 +851,15 @@ func (c *common) Cleanup(f func()) {
 	c.cleanup = func() {
 		if oldCleanup != nil {
 			defer func() {
+				c.mu.Lock()
 				c.cleanupPc = oldCleanupPc
+				c.mu.Unlock()
 				oldCleanup()
 			}()
 		}
+		c.mu.Lock()
 		c.cleanupName = callerName(0)
+		c.mu.Unlock()
 		f()
 	}
 	var pc [maxStackLen]uintptr