log: Update NewSyncWriter to work with term.IsTerminal.

log/sync.go

@@ -1,4 +1,4 @@
-package log
+package log
 
 import (
 	"io"
@@ -36,24 +36,59 @@ func (l *SwapLogger) Swap(logger Logger) {
 	l.logger.Store(loggerStruct{logger})
 }
 
-// SyncWriter synchronizes concurrent writes to an io.Writer.
-type SyncWriter struct {
-	mu sync.Mutex
-	w  io.Writer
+// NewSyncWriter returns a new writer that is safe for concurrent use by
+// multiple goroutines. Writes to the returned writer are passed on to w. If
+// another write is already in progress, the calling goroutine blocks until
+// the writer is available.
+//
+// If w implements the following interface, so does the returned writer.
+//
+//    interface {
+//        Fd() uintptr
+//    }
+func NewSyncWriter(w io.Writer) io.Writer {
+	switch w := w.(type) {
+	case fdWriter:
+		return &fdSyncWriter{fdWriter: w}
+	default:
+		return &syncWriter{Writer: w}
+	}
+}
+
+// syncWriter synchronizes concurrent writes to an io.Writer.
+type syncWriter struct {
+	sync.Mutex
+	io.Writer
+}
+
+// Write writes p to the underlying io.Writer. If another write is already in
+// progress, the calling goroutine blocks until the syncWriter is available.
+func (w *syncWriter) Write(p []byte) (n int, err error) {
+	w.Lock()
+	n, err = w.Writer.Write(p)
+	w.Unlock()
+	return n, err
+}
+
+// fdWriter is an io.Writer that also has an Fd method. The most common
+// example of an fdWriter is an *os.File.
+type fdWriter interface {
+	io.Writer
+	Fd() uintptr
 }
 
-// NewSyncWriter returns a new SyncWriter. The returned writer is safe for
-// concurrent use by multiple goroutines.
-func NewSyncWriter(w io.Writer) *SyncWriter {
-	return &SyncWriter{w: w}
+// fdSyncWriter synchronizes concurrent writes to an fdWriter.
+type fdSyncWriter struct {
+	sync.Mutex
+	fdWriter
 }
 
 // Write writes p to the underlying io.Writer. If another write is already in
-// progress, the calling goroutine blocks until the SyncWriter is available.
-func (w *SyncWriter) Write(p []byte) (n int, err error) {
-	w.mu.Lock()
-	n, err = w.w.Write(p)
-	w.mu.Unlock()
+// progress, the calling goroutine blocks until the fdSyncWriter is available.
+func (w *fdSyncWriter) Write(p []byte) (n int, err error) {
+	w.Lock()
+	n, err = w.fdWriter.Write(p)
+	w.Unlock()
 	return n, err
 }
 

log/sync_test.go

@@ -1,8 +1,9 @@
-package log_test
+package log_test
 
 import (
 	"bytes"
 	"io"
+	"os"
 	"testing"
 
 	"github.com/go-kit/kit/log"
@@ -70,3 +71,13 @@ func TestSyncWriterConcurrency(t *testing.T) {
 	w = log.NewSyncWriter(w)
 	testConcurrency(t, log.NewLogfmtLogger(w), 10000)
 }
+
+func TestSyncWriterFd(t *testing.T) {
+	_, ok := log.NewSyncWriter(os.Stdout).(interface {
+		Fd() uintptr
+	})
+
+	if !ok {
+		t.Error("NewSyncWriter does not pass through Fd method")
+	}
+}