os: fix data race in epipecheck()

Fixes #3860.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6443051

src/pkg/os/file_posix.go

@@ -7,6 +7,7 @@
 package os
 
 import (
+	"sync/atomic"
 	"syscall"
 	"time"
 )
@@ -15,12 +16,11 @@ func sigpipe() // implemented in package runtime
 
 func epipecheck(file *File, e error) {
 	if e == syscall.EPIPE {
-		file.nepipe++
-		if file.nepipe >= 10 {
+		if atomic.AddInt32(&file.nepipe, 1) >= 10 {
 			sigpipe()
 		}
 	} else {
-		file.nepipe = 0
+		atomic.StoreInt32(&file.nepipe, 0)
 	}
 }
 

src/pkg/os/file_unix.go

@@ -24,7 +24,7 @@ type file struct {
 	fd      int
 	name    string
 	dirinfo *dirInfo // nil unless directory being read
-	nepipe  int      // number of consecutive EPIPE in Write
+	nepipe  int32    // number of consecutive EPIPE in Write
 }
 
 // Fd returns the integer Unix file descriptor referencing the open file.

src/pkg/os/file_windows.go

@@ -25,7 +25,6 @@ type file struct {
 	fd      syscall.Handle
 	name    string
 	dirinfo *dirInfo   // nil unless directory being read
-	nepipe  int        // number of consecutive EPIPE in Write
 	l       sync.Mutex // used to implement windows pread/pwrite
 }