cmd/go: fix data race during build

Fixes #2695.

R=golang-dev, mpimenov, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/5545052

diff --git a/src/cmd/go/build.go b/src/cmd/go/build.go
index 4a046391db818071d3ad6e281e9251ea5513a882..77a64f406e316a10f85e31baa86a176ffbeb6ce2 100644 (file)
--- a/src/cmd/go/build.go
+++ b/src/cmd/go/build.go
@@ -873,6 +873,8 @@ func (b *builder) runOut(dir string, desc string, cmdargs ...interface{}) ([]byt
 
 // mkdir makes the named directory.
 func (b *builder) mkdir(dir string) error {
+       b.exec.Lock()
+       defer b.exec.Unlock()
        // We can be a little aggressive about being
        // sure directories exist.  Skip repeated calls.
        if b.mkdirCache[dir] {