Add an admin API to check if a username is available (#10578)

This adds a new API GET /_synapse/admin/v1/username_available?username=foo to check if a username is available. It is the counterpart to https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available, except that it works even if registration is disabled.

synapse/rest/admin/username_available.py

@@ -0,0 +1,51 @@
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# 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.
+import logging
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.http.servlet import RestServlet, parse_string
+from synapse.http.site import SynapseRequest
+from synapse.rest.admin._base import admin_patterns, assert_requester_is_admin
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class UsernameAvailableRestServlet(RestServlet):
+    """An admin API to check if a given username is available, regardless of whether registration is enabled.
+
+    Example:
+        GET /_synapse/admin/v1/username_available?username=foo
+        200 OK
+        {
+            "available": true
+        }
+    """
+
+    PATTERNS = admin_patterns("/username_available")
+
+    def __init__(self, hs: "HomeServer"):
+        self.auth = hs.get_auth()
+        self.registration_handler = hs.get_registration_handler()
+
+    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self.auth, request)
+
+        username = parse_string(request, "username", required=True)
+        await self.registration_handler.check_username(username)
+        return HTTPStatus.OK, {"available": True}