Refactor username parsing function of clientapi:login (#432)

* Refactor username parse function of login

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

* Add tests for userutil

Signed-off-by: Anant Prakash <anantprakashjsr@gmail.com>

src/github.com/matrix-org/dendrite/clientapi/userutil/userutil.go

@@ -0,0 +1,43 @@
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package userutil
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// ParseUsernameParam extracts localpart from usernameParam.
+// usernameParam can either be a user ID or just the localpart/username.
+// If serverName is passed, it is verified against the domain obtained from usernameParam (if present)
+// Returns error in case of invalid usernameParam.
+func ParseUsernameParam(usernameParam string, expectedServerName *gomatrixserverlib.ServerName) (string, error) {
+	localpart := usernameParam
+
+	if strings.HasPrefix(usernameParam, "@") {
+		lp, domain, err := gomatrixserverlib.SplitID('@', usernameParam)
+
+		if err != nil {
+			return "", errors.New("Invalid username")
+		}
+
+		if expectedServerName != nil && domain != *expectedServerName {
+			return "", errors.New("User ID does not belong to this server")
+		}
+
+		localpart = lp
+	}
+	return localpart, nil
+}