From c3319445aa0d891921f0236ff5bd7a8d879be4f8 Mon Sep 17 00:00:00 2001
From: Stephen Drake <stephen@xenolith.net>
Date: Mon, 12 Sep 2016 19:24:07 +0200
Subject: [PATCH] Prevent stdout / stderr race condition in limitedBuffer.

Signed-off-by: Stephen Drake <stephen@xenolith.net>
---
 daemon/health.go | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/daemon/health.go b/daemon/health.go
index 9f7c76ee14..87d6a6a4e3 100644
--- a/daemon/health.go
+++ b/daemon/health.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"runtime"
 	"strings"
+	"sync"
 	"time"
 
 	"golang.org/x/net/context"
@@ -271,11 +272,15 @@ func (d *Daemon) stopHealthchecks(c *container.Container) {
 // Buffer up to maxOutputLen bytes. Further data is discarded.
 type limitedBuffer struct {
 	buf       bytes.Buffer
+	mu        sync.Mutex
 	truncated bool // indicates that data has been lost
 }
 
 // Append to limitedBuffer while there is room.
 func (b *limitedBuffer) Write(data []byte) (int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
 	bufLen := b.buf.Len()
 	dataLen := len(data)
 	keep := min(maxOutputLen-bufLen, dataLen)
@@ -290,6 +295,9 @@ func (b *limitedBuffer) Write(data []byte) (int, error) {
 
 // The contents of the buffer, with "..." appended if it overflowed.
 func (b *limitedBuffer) String() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
 	out := b.buf.String()
 	if b.truncated {
 		out = out + "..."