cmd/go: fix data race on cgoLibGccFile

Fixes #4426.

R=dvyukov
CC=golang-dev
https://golang.org/cl/6851099

diff --git a/src/cmd/go/build.go b/src/cmd/go/build.go
index 5c1c783ab483dc1ab6ffbd670fe60512e2853bb8..7e3d2f496ded920275a99056da8e824eb4cc0fbb 100644 (file)
--- a/src/cmd/go/build.go
+++ b/src/cmd/go/build.go
@@ -1478,7 +1478,7 @@ func gccgoCleanPkgpath(p *Package) string {
 func (b *builder) libgcc(p *Package) (string, error) {
        f, err := b.runOut(p.Dir, p.ImportPath, b.gccCmd(p.Dir), "-print-libgcc-file-name")
        if err != nil {
-               return "", nil
+               return "", fmt.Errorf("gcc -print-libgcc-file-name: %v (%s)", err, f)
        }
        return strings.Trim(string(f), "\r\n"), nil
 }
@@ -1542,7 +1542,10 @@ func envList(key string) []string {
 
 var cgoRe = regexp.MustCompile(`[/\\:]`)
 
-var cgoLibGccFile string
+var (
+       cgoLibGccFile     string
+       cgoLibGccFileOnce sync.Once
+)
 
 func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo, outObj []string, err error) {
        if goos != toolGOOS {
@@ -1633,13 +1636,17 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
                        bareLDFLAGS = append(bareLDFLAGS, f)
                }
        }
-       if cgoLibGccFile == "" {
-               var err error
+
+       cgoLibGccFileOnce.Do(func() {
                cgoLibGccFile, err = b.libgcc(p)
-               if err != nil {
-                       return nil, nil, err
+       })
+       if cgoLibGccFile == "" {
+               if err == nil {
+                       err = errors.New("failed to get libgcc filename")
                }
+               return nil, nil, err
        }
+
        var staticLibs []string
        if goos == "windows" {
                // libmingw32 and libmingwex might also use libgcc, so libgcc must come last