remove WriteSyncer's automatic lock

options.go

@@ -59,11 +59,13 @@ func Fields(fs ...zapcore.Field) Option {
 }
 
 // ErrorOutput sets the destination for errors generated by the logger. The
-// supplied WriteSyncer is automatically wrapped with a mutex, so it need not be
-// safe for concurrent use.
+// supplied WriteSyncer must be safe for concurrent use.
+//
+// The Open and zapcore.Lock functions are the simplest ways to make files safe
+// for concurrent use.
 func ErrorOutput(w zapcore.WriteSyncer) Option {
 	return optionFunc(func(log *Logger) {
-		log.errorOutput = zapcore.Lock(w)
+		log.errorOutput = w
 	})
 }
 

writer.go

@@ -75,8 +75,8 @@ func open(paths []string) ([]zapcore.WriteSyncer, func(), error) {
 	return writers, close, errs.AsError()
 }
 
-// CombineWriteSyncers combines the passed set of WriteSyncer objects into a
-// locked WriteSyncer.
+// CombineWriteSyncers combines multiple WriteSyncers into a single, locked
+// WriteSyncer.
 func CombineWriteSyncers(writers ...zapcore.WriteSyncer) zapcore.WriteSyncer {
 	if len(writers) == 0 {
 		return zapcore.AddSync(ioutil.Discard)

zapcore/core.go

@@ -61,7 +61,7 @@ func NewCore(enc Encoder, ws WriteSyncer, enab LevelEnabler) Core {
 	return &ioCore{
 		LevelEnabler: enab,
 		enc:          enc,
-		out:          Lock(ws),
+		out:          ws,
 	}
 }