os: add check for ERROR_BAD_NETPATH in windows IsNotExist

Otherwise IsNotExist does not account for not existent servers and shares.

Fixes #12374

Change-Id: I37f6850198f91dcb02a4a917b793339d7e30e934
Reviewed-on: https://go-review.googlesource.com/14579Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/error_windows.go

@@ -19,6 +19,8 @@ func isExist(err error) bool {
 		err == syscall.ERROR_FILE_EXISTS || err == ErrExist
 }
 
+const _ERROR_BAD_NETPATH = syscall.Errno(53)
+
 func isNotExist(err error) bool {
 	switch pe := err.(type) {
 	case nil:
@@ -29,6 +31,7 @@ func isNotExist(err error) bool {
 		err = pe.Err
 	}
 	return err == syscall.ERROR_FILE_NOT_FOUND ||
+		err == _ERROR_BAD_NETPATH ||
 		err == syscall.ERROR_PATH_NOT_FOUND || err == ErrNotExist
 }
 

src/os/os_windows_test.go

@@ -126,3 +126,23 @@ func TestStartProcessAttr(t *testing.T) {
 	defer p.Wait()
 	t.Fatalf("StartProcess expected to fail, but succeeded.")
 }
+
+func TestShareNotExistError(t *testing.T) {
+	if testing.Short() {
+		t.Skip("slow test that uses network; skipping")
+	}
+	_, err := os.Stat(`\\no_such_server\no_such_share\no_such_file`)
+	if err == nil {
+		t.Fatal("stat succeeded, but expected to fail")
+	}
+	if !os.IsNotExist(err) {
+		t.Fatalf("os.Stat failed with %q, but os.IsNotExist(err) is false", err)
+	}
+}
+
+func TestBadNetPathError(t *testing.T) {
+	const ERROR_BAD_NETPATH = syscall.Errno(53)
+	if !os.IsNotExist(ERROR_BAD_NETPATH) {
+		t.Fatal("os.IsNotExist(syscall.Errno(53)) is false, but want true")
+	}
+}