feat: add fsx file exist and write file funcs

fsx/fsx.go

@@ -31,3 +31,41 @@ func ReadFile(path string) ([]byte, error) {
 
 	return nil, err
 }
+
+func FileExists(path string) (bool, error) {
+	_, err := os.Stat(path)
+	if err == nil {
+		return true, nil
+	}
+
+	if errors.Is(err, os.ErrNotExist) {
+		return false, nil
+	}
+
+	if errors.Is(err, os.ErrPermission) {
+		if isSnapProcess() {
+			return false, fmt.Errorf("%w: %s", ErrSnapPermission, path)
+		}
+
+		return false, fmt.Errorf("%w: %s", ErrPermission, path)
+	}
+
+	return false, err
+}
+
+func WriteFile(path string, data []byte, perm os.FileMode) error {
+	err := os.WriteFile(path, data, perm)
+	if err == nil {
+		return nil
+	}
+
+	if errors.Is(err, os.ErrPermission) {
+		if isSnapProcess() {
+			return fmt.Errorf("%w: %s", ErrSnapPermission, path)
+		}
+
+		return fmt.Errorf("%w: %s", ErrPermission, path)
+	}
+
+	return err
+}

fsx/fsx_test.go

@@ -82,3 +82,146 @@ func TestReadFile_PermissionDenied_Snap(t *testing.T) {
 		t.Errorf("expected ErrSnapPermission, got: %v", err)
 	}
 }
+
+func TestFileExists_Exists(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.txt")
+	if err := os.WriteFile(path, []byte("hello"), 0600); err != nil {
+		t.Fatalf("failed to create test file: %v", err)
+	}
+
+	exists, err := fsx.FileExists(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !exists {
+		t.Error("expected file to exist")
+	}
+}
+
+func TestFileExists_NotExists(t *testing.T) {
+	exists, err := fsx.FileExists("/nonexistent/path/file.txt")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if exists {
+		t.Error("expected file to not exist")
+	}
+}
+
+func TestFileExists_PermissionDenied_Generic(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("skipping permission test: running as root")
+	}
+
+	t.Setenv("SNAP", "")
+	t.Setenv("SNAP_NAME", "")
+	t.Setenv("SNAP_USER_DATA", "")
+
+	dir := t.TempDir()
+	if err := os.Chmod(dir, 0000); err != nil {
+		t.Fatalf("failed to chmod dir: %v", err)
+	}
+	t.Cleanup(func() { os.Chmod(dir, 0700) })
+
+	path := filepath.Join(dir, "file.txt")
+	_, err := fsx.FileExists(path)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, fsx.ErrPermission) {
+		t.Errorf("expected ErrPermission, got: %v", err)
+	}
+}
+
+func TestFileExists_PermissionDenied_Snap(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("skipping permission test: running as root")
+	}
+
+	t.Setenv("SNAP", "/snap/myapp/current")
+	t.Setenv("SNAP_NAME", "myapp")
+	t.Setenv("SNAP_USER_DATA", "")
+
+	dir := t.TempDir()
+	if err := os.Chmod(dir, 0000); err != nil {
+		t.Fatalf("failed to chmod dir: %v", err)
+	}
+	t.Cleanup(func() { os.Chmod(dir, 0700) })
+
+	path := filepath.Join(dir, "file.txt")
+	_, err := fsx.FileExists(path)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, fsx.ErrSnapPermission) {
+		t.Errorf("expected ErrSnapPermission, got: %v", err)
+	}
+}
+
+func TestWriteFile_Success(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "out.txt")
+
+	if err := fsx.WriteFile(path, []byte("world"), 0600); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("failed to read back file: %v", err)
+	}
+	if string(data) != "world" {
+		t.Errorf("expected %q, got %q", "world", string(data))
+	}
+}
+
+func TestWriteFile_PermissionDenied_Generic(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("skipping permission test: running as root")
+	}
+
+	t.Setenv("SNAP", "")
+	t.Setenv("SNAP_NAME", "")
+	t.Setenv("SNAP_USER_DATA", "")
+
+	dir := t.TempDir()
+	if err := os.Chmod(dir, 0000); err != nil {
+		t.Fatalf("failed to chmod dir: %v", err)
+	}
+	t.Cleanup(func() { os.Chmod(dir, 0700) })
+
+	path := filepath.Join(dir, "out.txt")
+	err := fsx.WriteFile(path, []byte("data"), 0600)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, fsx.ErrPermission) {
+		t.Errorf("expected ErrPermission, got: %v", err)
+	}
+}
+
+func TestWriteFile_PermissionDenied_Snap(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skip("skipping permission test: running as root")
+	}
+
+	t.Setenv("SNAP", "/snap/myapp/current")
+	t.Setenv("SNAP_NAME", "myapp")
+	t.Setenv("SNAP_USER_DATA", "")
+
+	dir := t.TempDir()
+	if err := os.Chmod(dir, 0000); err != nil {
+		t.Fatalf("failed to chmod dir: %v", err)
+	}
+	t.Cleanup(func() { os.Chmod(dir, 0700) })
+
+	path := filepath.Join(dir, "out.txt")
+	err := fsx.WriteFile(path, []byte("data"), 0600)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, fsx.ErrSnapPermission) {
+		t.Errorf("expected ErrSnapPermission, got: %v", err)
+	}
+}