io/ioutil: fix data race on rand

Fixes #4212.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6641050

src/pkg/io/ioutil/tempfile.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"sync"
 	"time"
 )
 
@@ -16,18 +17,21 @@ import (
 // chance the file doesn't exist yet - keeps the number of tries in
 // TempFile to a minimum.
 var rand uint32
+var randmu sync.Mutex
 
 func reseed() uint32 {
 	return uint32(time.Now().UnixNano() + int64(os.Getpid()))
 }
 
 func nextSuffix() string {
+	randmu.Lock()
 	r := rand
 	if r == 0 {
 		r = reseed()
 	}
 	r = r*1664525 + 1013904223 // constants from Numerical Recipes
 	rand = r
+	randmu.Unlock()
 	return strconv.Itoa(int(1e9 + r%1e9))[1:]
 }