From fb71758054e7cd2eace15c75474540e93277d3db Mon Sep 17 00:00:00 2001
From: Koichi Shiraishi <zchee.io@gmail.com>
Date: Fri, 25 Jun 2021 03:53:20 +0900
Subject: [PATCH] zapcore: add stopped for Stop method called exactly once
 (#966)

---
 zapcore/buffered_write_syncer.go      | 20 ++++++++++++++++----
 zapcore/buffered_write_syncer_test.go |  8 ++++++++
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/zapcore/buffered_write_syncer.go b/zapcore/buffered_write_syncer.go
index f08ca7e81..0c1436f76 100644
--- a/zapcore/buffered_write_syncer.go
+++ b/zapcore/buffered_write_syncer.go
@@ -74,6 +74,7 @@ type BufferedWriteSyncer struct {
 	writer      *bufio.Writer
 	ticker      *time.Ticker
 	stop        chan struct{} // closed when flushLoop should stop
+	stopped     bool          // whether Stop() has run
 	done        chan struct{} // closed when flushLoop has stopped
 }
 
@@ -155,9 +156,9 @@ func (s *BufferedWriteSyncer) flushLoop() {
 
 // Stop closes the buffer, cleans up background goroutines, and flushes
 // remaining unwritten data.
-//
-// This must be called exactly once.
-func (s *BufferedWriteSyncer) Stop() error {
+func (s *BufferedWriteSyncer) Stop() (err error) {
+	var stopped bool
+
 	// Critical section.
 	func() {
 		s.mu.Lock()
@@ -167,10 +168,21 @@ func (s *BufferedWriteSyncer) Stop() error {
 			return
 		}
 
+		stopped = s.stopped
+		if stopped {
+			return
+		}
+		s.stopped = true
+
 		s.ticker.Stop()
 		close(s.stop) // tell flushLoop to stop
 		<-s.done      // and wait until it has
 	}()
 
-	return s.Sync()
+	// Don't call Sync on consecutive Stops.
+	if !stopped {
+		err = s.Sync()
+	}
+
+	return err
 }
diff --git a/zapcore/buffered_write_syncer_test.go b/zapcore/buffered_write_syncer_test.go
index 9c28076bb..72d4d6f88 100644
--- a/zapcore/buffered_write_syncer_test.go
+++ b/zapcore/buffered_write_syncer_test.go
@@ -53,6 +53,14 @@ func TestBufferWriter(t *testing.T) {
 		assert.Equal(t, "foo", buf.String(), "Unexpected log string")
 	})
 
+	t.Run("stop twice", func(t *testing.T) {
+		ws := &BufferedWriteSyncer{WS: &ztest.FailWriter{}}
+		_, err := ws.Write([]byte("foo"))
+		require.NoError(t, err, "Unexpected error writing to WriteSyncer.")
+		assert.Error(t, ws.Stop(), "Expected stop to fail.")
+		assert.NoError(t, ws.Stop(), "Expected stop to not fail.")
+	})
+
 	t.Run("wrap twice", func(t *testing.T) {
 		buf := &bytes.Buffer{}
 		bufsync := &BufferedWriteSyncer{WS: AddSync(buf)}