[release-branch.go1.12] os: remove unreadable directories in RemoveAll

Updates #30555
Fixes #30579

Change-Id: Ib894b4f3cdba23a18a69c9470cf69ceb83591a4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/165057
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit c7465929)
Reviewed-on: https://go-review.googlesource.com/c/go/+/165058
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/removeall_at.go

@@ -92,7 +92,8 @@ func removeAllFrom(parent *File, path string) error {
 			if IsNotExist(err) {
 				return nil
 			}
-			return err
+			recurseErr = err
+			break
 		}
 
 		names, readErr := file.Readdirnames(request)

src/os/removeall_test.go

@@ -372,3 +372,33 @@ func TestRemoveAllButReadOnly(t *testing.T) {
 		}
 	}
 }
+
+func TestRemoveUnreadableDir(t *testing.T) {
+	switch runtime.GOOS {
+	case "nacl", "js", "windows":
+		t.Skipf("skipping test on %s", runtime.GOOS)
+	}
+
+	if Getuid() == 0 {
+		t.Skip("skipping test when running as root")
+	}
+
+	t.Parallel()
+
+	tempDir, err := ioutil.TempDir("", "TestRemoveAllButReadOnly-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(tempDir)
+
+	target := filepath.Join(tempDir, "d0", "d1", "d2")
+	if err := MkdirAll(target, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := Chmod(target, 0300); err != nil {
+		t.Fatal(err)
+	}
+	if err := RemoveAll(filepath.Join(tempDir, "d0")); err != nil {
+		t.Fatal(err)
+	}
+}