fusefrontend: handle empty xattrs efficiently

We handle empty files by storing an actual empty file
on disk. Handle xattrs similarily and encrypt the
empty value to the empty value.

internal/fusefrontend/fs.go

@@ -348,7 +348,11 @@ func (fs *FS) StatFs(path string) *fuse.StatfsOut {
 
 // decryptSymlinkTarget: "cData64" is base64-decoded and decrypted
 // like file contents (GCM).
+// The empty string decrypts to the empty string.
 func (fs *FS) decryptSymlinkTarget(cData64 string) (string, error) {
+	if cData64 == "" {
+		return "", nil
+	}
 	cData, err := fs.nameTransform.B64.DecodeString(cData64)
 	if err != nil {
 		return "", err
@@ -409,7 +413,11 @@ func (fs *FS) Unlink(path string, context *fuse.Context) (code fuse.Status) {
 
 // encryptSymlinkTarget: "data" is encrypted like file contents (GCM)
 // and base64-encoded.
+// The empty string encrypts to the empty string.
 func (fs *FS) encryptSymlinkTarget(data string) (cData64 string) {
+	if data == "" {
+		return ""
+	}
 	cData := fs.contentEnc.EncryptBlock([]byte(data), 0, nil)
 	cData64 = fs.nameTransform.B64.EncodeToString(cData)
 	return cData64

tests/xattr/xattr_integration_test.go

@@ -66,6 +66,60 @@ func TestXattrSetGetRm(t *testing.T) {
 	}
 }
 
+func TestXattrSetEmpty(t *testing.T) {
+	attr := "user.foo"
+	fn := test_helpers.DefaultPlainDir + "/TestXattrSetEmpty1"
+	err := ioutil.WriteFile(fn, nil, 0700)
+	if err != nil {
+		t.Fatalf("creating empty file failed: %v", err)
+	}
+	// Make sure it does not exist already
+	_, err = xattr.Get(fn, attr)
+	if err == nil {
+		t.Fatal("we should have got an error here")
+	}
+	// Set empty value
+	err = xattr.Set(fn, attr, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Read back
+	val, err := xattr.Get(fn, attr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(val) != 0 {
+		t.Errorf("wrong length: want=0 have=%d", len(val))
+	}
+	// Overwrite empty value with something
+	val1 := []byte("xyz123")
+	err = xattr.Set(fn, attr, val1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Read back
+	val2, err := xattr.Get(fn, attr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(val1, val2) {
+		t.Fatalf("wrong readback value: %v != %v", val1, val2)
+	}
+	// Overwrite something with empty value
+	err = xattr.Set(fn, attr, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Read back
+	val, err = xattr.Get(fn, attr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(val) != 0 {
+		t.Errorf("wrong length: want=0 have=%d", len(val2))
+	}
+}
+
 func TestXattrList(t *testing.T) {
 	fn := test_helpers.DefaultPlainDir + "/TestXattrList"
 	err := ioutil.WriteFile(fn, nil, 0700)