internal/testenv: use cmd.Environ in CleanCmdEnv

In CleanCmdEnv, use cmd.Environ instead of os.Environ, so it
sets the PWD environment variable if cmd.Dir is set. This ensures
the child process sees a canonical path for its working directory.

Change-Id: Ia769552a488dc909eaf6bb7d21937adba06d1072
Reviewed-on: https://go-review.googlesource.com/c/go/+/538215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/internal/testenv/exec.go

@@ -100,11 +100,14 @@ func MustHaveExecPath(t testing.TB, path string) {
 // CleanCmdEnv will fill cmd.Env with the environment, excluding certain
 // variables that could modify the behavior of the Go tools such as
 // GODEBUG and GOTRACEBACK.
+//
+// If the caller wants to set cmd.Dir, set it before calling this function,
+// so PWD will be set correctly in the environment.
 func CleanCmdEnv(cmd *exec.Cmd) *exec.Cmd {
 	if cmd.Env != nil {
 		panic("environment already set")
 	}
-	for _, env := range os.Environ() {
+	for _, env := range cmd.Environ() {
 		// Exclude GODEBUG from the environment to prevent its output
 		// from breaking tests that are trying to parse other command output.
 		if strings.HasPrefix(env, "GODEBUG=") {

src/internal/testenv/testenv_test.go

@@ -163,3 +163,26 @@ func TestMustHaveExec(t *testing.T) {
 		}
 	}
 }
+
+func TestCleanCmdEnvPWD(t *testing.T) {
+	// Test that CleanCmdEnv sets PWD if cmd.Dir is set.
+	switch runtime.GOOS {
+	case "plan9", "windows":
+		t.Skipf("PWD is not used on %s", runtime.GOOS)
+	}
+	dir := t.TempDir()
+	cmd := testenv.Command(t, testenv.GoToolPath(t), "help")
+	cmd.Dir = dir
+	cmd = testenv.CleanCmdEnv(cmd)
+
+	for _, env := range cmd.Env {
+		if strings.HasPrefix(env, "PWD=") {
+			pwd := strings.TrimPrefix(env, "PWD=")
+			if pwd != dir {
+				t.Errorf("unexpected PWD: want %s, got %s", dir, pwd)
+			}
+			return
+		}
+	}
+	t.Error("PWD not set in cmd.Env")
+}