Keep some auto-locking

config.go

@@ -162,8 +162,8 @@ func (cfg Config) Build(opts ...Option) (*Logger, error) {
 	}
 
 	log := New(
-		zapcore.NewCore(enc, zapcore.Lock(sink), cfg.Level),
-		cfg.buildOptions(zapcore.Lock(errSink))...,
+		zapcore.NewCore(enc, sink, cfg.Level),
+		cfg.buildOptions(errSink)...,
 	)
 	if len(opts) > 0 {
 		log = log.WithOptions(opts...)

options.go

@@ -59,8 +59,10 @@ 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 = w

writer.go

@@ -75,11 +75,11 @@ func open(paths []string) ([]zapcore.WriteSyncer, func(), error) {
 	return writers, close, errs.AsError()
 }
 
-// CombineWriteSyncers combines the passed set of WriteSyncer objects into a
+// CombineWriteSyncers combines the passed set of WriteSyncers into a single
 // locked WriteSyncer.
 func CombineWriteSyncers(writers ...zapcore.WriteSyncer) zapcore.WriteSyncer {
 	if len(writers) == 0 {
 		return zapcore.AddSync(ioutil.Discard)
 	}
-	return zapcore.NewMultiWriteSyncer(writers...)
+	return zapcore.Lock(zapcore.NewMultiWriteSyncer(writers...))
 }