log: fix data race on log.Output

There was unprotected access to Logger.flag in log.Output which
could lead to data race in cases when log.SetFlags called simultaneously.
For example, "hot" switching on/off debug-mode for Logger by log.SetFlags
while application still writing logs.

Fixes #21935

Change-Id: I36be25f23cad44cde62ed1af28a30d276400e1b8
Reviewed-on: https://go-review.googlesource.com/64710Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/log/log.go

@@ -147,11 +147,7 @@ func (l *Logger) formatHeader(buf *[]byte, t time.Time, file string, line int) {
 // provided for generality, although at the moment on all pre-defined
 // paths it will be 2.
 func (l *Logger) Output(calldepth int, s string) error {
-	// Get time early if we need it.
-	var now time.Time
-	if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
-		now = time.Now()
-	}
+	now := time.Now() // get this early.
 	var file string
 	var line int
 	l.mu.Lock()

src/log/log_test.go

@@ -88,6 +88,17 @@ func TestOutput(t *testing.T) {
 	}
 }
 
+func TestOutputRace(t *testing.T) {
+	var b bytes.Buffer
+	l := New(&b, "", 0)
+	for i := 0; i < 100; i++ {
+		go func() {
+			l.SetFlags(0)
+		}()
+		l.Output(0, "")
+	}
+}
+
 func TestFlagAndPrefixSetting(t *testing.T) {
 	var b bytes.Buffer
 	l := New(&b, "Test:", LstdFlags)