os: in Getwd, $PWD override syscall.Getwd

This makes os.Getwd mimic C getwd on OS X,
and possibly other systems. The change on OS X
was a regression from 1.2 to 1.3.

Fixes #8400.

LGTM=bradfitz
R=iant, bradfitz
CC=golang-codereviews
https://golang.org/cl/118970043

src/pkg/os/getwd.go

@@ -23,22 +23,12 @@ var useSyscallwd = func(error) bool { return true }
 // reached via multiple paths (due to symbolic links),
 // Getwd may return any one of them.
 func Getwd() (dir string, err error) {
-	// If the operating system provides a Getwd call, use it.
-	if syscall.ImplementsGetwd {
-		s, e := syscall.Getwd()
-		if useSyscallwd(e) {
-			return s, NewSyscallError("getwd", e)
-		}
-	}
-
-	// Otherwise, we're trying to find our way back to ".".
+	// Clumsy but widespread kludge:
+	// if $PWD is set and matches ".", use it.
 	dot, err := Stat(".")
 	if err != nil {
 		return "", err
 	}
-
-	// Clumsy but widespread kludge:
-	// if $PWD is set and matches ".", use it.
 	dir = Getenv("PWD")
 	if len(dir) > 0 && dir[0] == '/' {
 		d, err := Stat(dir)
@@ -47,6 +37,15 @@ func Getwd() (dir string, err error) {
 		}
 	}
 
+	// If the operating system provides a Getwd call, use it.
+	// Otherwise, we're trying to find our way back to ".".
+	if syscall.ImplementsGetwd {
+		s, e := syscall.Getwd()
+		if useSyscallwd(e) {
+			return s, NewSyscallError("getwd", e)
+		}
+	}
+
 	// Apply same kludge but to cached dir instead of $PWD.
 	getwdCache.Lock()
 	dir = getwdCache.dir

src/pkg/os/os_test.go

@@ -811,8 +811,8 @@ func TestChdirAndGetwd(t *testing.T) {
 		t.Fatalf("Open .: %s", err)
 	}
 	// These are chosen carefully not to be symlinks on a Mac
-	// (unlike, say, /var, /etc, and /tmp).
-	dirs := []string{"/", "/usr/bin"}
+	// (unlike, say, /var, /etc), except /tmp, which we handle below.
+	dirs := []string{"/", "/usr/bin", "/tmp"}
 	// /usr/bin does not usually exist on Plan 9 or Android.
 	switch runtime.GOOS {
 	case "android":
@@ -820,6 +820,7 @@ func TestChdirAndGetwd(t *testing.T) {
 	case "plan9":
 		dirs = []string{"/", "/usr"}
 	}
+	oldwd := Getenv("PWD")
 	for mode := 0; mode < 2; mode++ {
 		for _, d := range dirs {
 			if mode == 0 {
@@ -833,7 +834,11 @@ func TestChdirAndGetwd(t *testing.T) {
 				err = fd1.Chdir()
 				fd1.Close()
 			}
+			if d == "/tmp" {
+				Setenv("PWD", "/tmp")
+			}
 			pwd, err1 := Getwd()
+			Setenv("PWD", oldwd)
 			err2 := fd.Chdir()
 			if err2 != nil {
 				// We changed the current directory and cannot go back.