textlogger: write unstructured klog output directly

Textlogger can (but doesn't have to) be used to write klog's unstructured
output from calls like Infof directly to the output stream. This has two
advantages:
- traditional output remains exactly the same as before, which is particularly
  important for multi-line messages because those get quoted otherwise
- performance is better because re-encoding the output string is avoided

test/output.go

@@ -39,6 +39,7 @@ import (
 	"github.com/go-logr/logr"
 
 	"k8s.io/klog/v2"
+	"k8s.io/klog/v2/textlogger"
 )
 
 // InitKlog must be called in a test to configure klog for testing.
@@ -577,7 +578,7 @@ func Output(t *testing.T, config OutputConfig) {
 
 			if config.AsBackend {
 				testOutput(t, printWithKlogLine-1, func(buffer *bytes.Buffer) {
-					klog.SetLogger(config.NewLogger(buffer, 10, test.vmodule))
+					setLogger(config.NewLogger(buffer, 10, test.vmodule))
 					printWithKlog(test)
 				})
 				return
@@ -765,10 +766,11 @@ func Output(t *testing.T, config OutputConfig) {
 		for i, test := range tests {
 			t.Run(test.name, func(t *testing.T) {
 				var buffer bytes.Buffer
+				haveWriteKlogBuffer := false
 				if config.NewLogger == nil {
 					klog.SetOutput(&buffer)
 				} else {
-					klog.SetLogger(config.NewLogger(&buffer, 10, ""))
+					haveWriteKlogBuffer = setLogger(config.NewLogger(&buffer, 10, ""))
 					defer klog.ClearLogger()
 				}
 				test.logFunc()
@@ -789,6 +791,7 @@ func Output(t *testing.T, config OutputConfig) {
 				// result, including a trailing newline, to
 				// Logger.Info.
 				if config.NewLogger != nil &&
+					!haveWriteKlogBuffer &&
 					!strings.HasSuffix(test.name, "S") &&
 					!strings.HasSuffix(test.name, "SDepth") {
 					// klog: I output.go:<LINE>] hello world
@@ -851,7 +854,7 @@ func Benchmark(b *testing.B, config OutputConfig) {
 			}
 
 			if config.AsBackend {
-				klog.SetLogger(config.NewLogger(io.Discard, 10, ""))
+				setLogger(config.NewLogger(io.Discard, 10, ""))
 				for i := 0; i < b.N; i++ {
 					printWithKlog(test)
 				}
@@ -871,6 +874,17 @@ func Benchmark(b *testing.B, config OutputConfig) {
 	}
 }
 
+func setLogger(logger logr.Logger) bool {
+	haveWriteKlogBuffer := false
+	var opts []klog.LoggerOption
+	if writer, ok := logger.GetSink().(textlogger.KlogBufferWriter); ok {
+		opts = append(opts, klog.WriteKlogBuffer(writer.WriteKlogBuffer))
+		haveWriteKlogBuffer = true
+	}
+	klog.SetLoggerWithOptions(logger, opts...)
+	return haveWriteKlogBuffer
+}
+
 func copySlice(in []interface{}) []interface{} {
 	return append([]interface{}{}, in...)
 }

textlogger/textlogger.go

@@ -134,6 +134,10 @@ func (l *tlogger) print(err error, s severity.Severity, msg string, kvList []int
 	l.config.co.output.Write(b.Bytes())
 }
 
+func (l *tlogger) WriteKlogBuffer(data []byte) {
+	l.config.co.output.Write(data)
+}
+
 // WithName returns a new logr.Logger with the specified name appended.  klogr
 // uses '/' characters to separate name elements.  Callers should not pass '/'
 // in the provided name string, but this library does not actually enforce that.
@@ -152,5 +156,15 @@ func (l *tlogger) WithValues(kvList ...interface{}) logr.LogSink {
 	return &new
 }
 
+// KlogBufferWriter is implemented by the textlogger LogSink.
+type KlogBufferWriter interface {
+	// WriteKlogBuffer takes a pre-formatted buffer prepared by klog and
+	// writes it unchanged to the output stream. Can be used with
+	// klog.WriteKlogBuffer when setting a logger through
+	// klog.SetLoggerWithOptions.
+	WriteKlogBuffer([]byte)
+}
+
 var _ logr.LogSink = &tlogger{}
 var _ logr.CallDepthLogSink = &tlogger{}
+var _ KlogBufferWriter = &tlogger{}