cmd/go: set $PWD when running commands

This makes os.Getwd inside those commands much faster.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7324055

src/cmd/go/build.go

@@ -1226,7 +1226,7 @@ func (b *builder) runOut(dir string, desc string, cmdargs ...interface{}) ([]byt
 		cmd.Stdout = &buf
 		cmd.Stderr = &buf
 		cmd.Dir = dir
-		// TODO: cmd.Env
+		cmd.Env = envForDir(cmd.Dir)
 		err := cmd.Run()
 
 		// cmd.Run will fail on Unix if some other process has the binary

src/cmd/go/main.go

@@ -379,6 +379,25 @@ func runOut(dir string, cmdargs ...interface{}) []byte {
 	return out
 }
 
+// envForDir returns a copy of the environment
+// suitable for running in the given directory.
+// The environment is the current process's environment
+// but with an updated $PWD, so that an os.Getwd in the
+// child will be faster.
+func envForDir(dir string) []string {
+	env := os.Environ()
+	for i, kv := range env {
+		if strings.HasPrefix(kv, "PWD=") {
+			env[i] = "PWD=" + dir
+			return env
+		}
+	}
+	// Internally we only use rooted paths, so dir is rooted.
+	// Even if dir is not rooted, no harm done.
+	env = append(env, "PWD="+dir)
+	return env
+}
+
 // matchPattern(pattern)(name) reports whether
 // name matches pattern.  Pattern is a limited glob
 // pattern in which '...' means 'any string' and there

src/cmd/go/test.go

@@ -635,6 +635,7 @@ func (b *builder) runTest(a *action) error {
 
 	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Dir = a.p.Dir
+	cmd.Env = envForDir(cmd.Dir)
 	var buf bytes.Buffer
 	if testStreamOutput {
 		cmd.Stdout = os.Stdout
@@ -647,7 +648,7 @@ func (b *builder) runTest(a *action) error {
 	// If there are any local SWIG dependencies, we want to load
 	// the shared library from the build directory.
 	if a.p.usesSwig() {
-		env := os.Environ()
+		env := cmd.Env
 		found := false
 		prefix := "LD_LIBRARY_PATH="
 		for i, v := range env {

src/cmd/go/vcs.go

@@ -190,6 +190,7 @@ func (v *vcsCmd) run1(dir string, cmdline string, keyval []string, verbose bool)
 
 	cmd := exec.Command(v.cmd, args...)
 	cmd.Dir = dir
+	cmd.Env = envForDir(cmd.Dir)
 	if buildX {
 		fmt.Printf("cd %s\n", dir)
 		fmt.Printf("%s %s\n", v.cmd, strings.Join(args, " "))