Add LogToStderr, a programatic way to log exclusively to stderr or not

klog.go

@@ -803,6 +803,14 @@ func SetOutputBySeverity(name string, w io.Writer) {
 	logging.file[sev] = rb
 }
 
+// LogToStderr sets whether to log exclusively to stderr, bypassing outputs
+func LogToStderr(stderr bool) {
+	logging.mu.Lock()
+	defer logging.mu.Unlock()
+
+	logging.toStderr = stderr
+}
+
 // output writes the data to the log files and releases the buffer.
 func (l *loggingT) output(s severity, log logr.InfoLogger, buf *buffer, file string, line int, alsoToStderr bool) {
 	l.mu.Lock()

klog_test.go

@@ -351,6 +351,21 @@ func TestSetOutputDataRace(t *testing.T) {
 	}
 }
 
+func TestLogToOutput(t *testing.T) {
+	logging.toStderr = true
+	defer logging.swap(logging.newBuffers())
+	buf := new(bytes.Buffer)
+	SetOutput(buf)
+	LogToStderr(false)
+
+	Info("Does logging to an output work?")
+
+	str := buf.String()
+	if !strings.Contains(str, "Does logging to an output work?") {
+		t.Fatalf("Expected %q to contain \"Does logging to an output work?\"", str)
+	}
+}
+
 // vGlobs are patterns that match/don't match this file at V=2.
 var vGlobs = map[string]bool{
 	// Easy to test the numeric match here.