diff --git a/vendor/go.uber.org/zap/zapcore/write_syncer.go b/vendor/go.uber.org/zap/zapcore/write_syncer.go
new file mode 100644
index 0000000..209e25f
--- /dev/null
+++ b/vendor/go.uber.org/zap/zapcore/write_syncer.go
@@ -0,0 +1,123 @@
+// Copyright (c) 2016 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package zapcore
+
+import (
+	"io"
+	"sync"
+
+	"go.uber.org/multierr"
+)
+
+// A WriteSyncer is an io.Writer that can also flush any buffered data. Note
+// that *os.File (and thus, os.Stderr and os.Stdout) implement WriteSyncer.
+type WriteSyncer interface {
+	io.Writer
+	Sync() error
+}
+
+// AddSync converts an io.Writer to a WriteSyncer. It attempts to be
+// intelligent: if the concrete type of the io.Writer implements WriteSyncer,
+// we'll use the existing Sync method. If it doesn't, we'll add a no-op Sync.
+func AddSync(w io.Writer) WriteSyncer {
+	switch w := w.(type) {
+	case WriteSyncer:
+		return w
+	default:
+		return writerWrapper{w}
+	}
+}
+
+type lockedWriteSyncer struct {
+	sync.Mutex
+	ws WriteSyncer
+}
+
+// Lock wraps a WriteSyncer in a mutex to make it safe for concurrent use. In
+// particular, *os.Files must be locked before use.
+func Lock(ws WriteSyncer) WriteSyncer {
+	if _, ok := ws.(*lockedWriteSyncer); ok {
+		// no need to layer on another lock
+		return ws
+	}
+	return &lockedWriteSyncer{ws: ws}
+}
+
+func (s *lockedWriteSyncer) Write(bs []byte) (int, error) {
+	s.Lock()
+	n, err := s.ws.Write(bs)
+	s.Unlock()
+	return n, err
+}
+
+func (s *lockedWriteSyncer) Sync() error {
+	s.Lock()
+	err := s.ws.Sync()
+	s.Unlock()
+	return err
+}
+
+type writerWrapper struct {
+	io.Writer
+}
+
+func (w writerWrapper) Sync() error {
+	return nil
+}
+
+type multiWriteSyncer []WriteSyncer
+
+// NewMultiWriteSyncer creates a WriteSyncer that duplicates its writes
+// and sync calls, much like io.MultiWriter.
+func NewMultiWriteSyncer(ws ...WriteSyncer) WriteSyncer {
+	if len(ws) == 1 {
+		return ws[0]
+	}
+	// Copy to protect against https://github.com/golang/go/issues/7809
+	return multiWriteSyncer(append([]WriteSyncer(nil), ws...))
+}
+
+// See https://golang.org/src/io/multi.go
+// When not all underlying syncers write the same number of bytes,
+// the smallest number is returned even though Write() is called on
+// all of them.
+func (ws multiWriteSyncer) Write(p []byte) (int, error) {
+	var writeErr error
+	nWritten := 0
+	for _, w := range ws {
+		n, err := w.Write(p)
+		writeErr = multierr.Append(writeErr, err)
+		if nWritten == 0 && n != 0 {
+			nWritten = n
+		} else if n < nWritten {
+			nWritten = n
+		}
+	}
+	return nWritten, writeErr
+}
+
+func (ws multiWriteSyncer) Sync() error {
+	var err error
+	for _, w := range ws {
+		err = multierr.Append(err, w.Sync())
+	}
+	return err
+}