Add SyncWriter utility type for non-thread-safe writers

log.go

@@ -139,7 +139,13 @@ type Logger struct {
 	counter *uint32
 }
 
-// New creates a root logger with given output writer.
+// New creates a root logger with given output writer. If the output writer implements
+// the LevelWriter interface, the WriteLevel method will be called instead of the Write
+// one.
+//
+// Each logging operation makes a single call to the Writer's Write method. There is no
+// guaranty on access serialization to the Writer. If your Writer is not thread safe,
+// you may consider using sync wrapper.
 func New(w io.Writer) Logger {
 	if w == nil {
 		panic("w is nil")

writer.go

@@ -1,6 +1,9 @@
 package zerolog
 
-import "io"
+import (
+	"io"
+	"sync"
+)
 
 // LevelWriter defines as interface a writer may implement in order
 // to receive level information with payload.
@@ -13,10 +16,44 @@ type levelWriterAdapter struct {
 	io.Writer
 }
 
-func (lw levelWriterAdapter) WriteLevel(level Level, p []byte) (n int, err error) {
+func (lw levelWriterAdapter) WriteLevel(l Level, p []byte) (n int, err error) {
 	return lw.Write(p)
 }
 
+type syncWriter struct {
+	mu sync.Mutex
+	lw LevelWriter
+}
+
+// SyncWriter wraps w so that each call to Write is synchronized with a mutex.
+// This syncer can be the call to writer's Write method is not thread safe.
+// Note that os.File Write operation is using write() syscall which is supposed
+// to be thread-safe on POSIX systems. So there is no need to use this with
+// os.File on such systems as zerolog guaranties to issue a single Write call
+// per log event.
+func SyncWriter(w io.Writer) io.Writer {
+	if lw, ok := w.(LevelWriter); ok {
+		return &syncWriter{lw: lw}
+	}
+	return &syncWriter{lw: levelWriterAdapter{w}}
+}
+
+// Write implements the io.Writer interface.
+func (s *syncWriter) Write(p []byte) (n int, err error) {
+	s.mu.Lock()
+	n, err = s.lw.Write(p)
+	s.mu.Unlock()
+	return
+}
+
+// WriteLevel implements the LevelWriter interface.
+func (s *syncWriter) WriteLevel(l Level, p []byte) (n int, err error) {
+	s.mu.Lock()
+	n, err = s.lw.WriteLevel(l, p)
+	s.mu.Unlock()
+	return
+}
+
 type multiLevelWriter struct {
 	writers []LevelWriter
 }