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.

tests/rest/admin/test_username_available.py

@@ -0,0 +1,62 @@
+# Copyright 2021 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 synapse.rest.admin
+from synapse.api.errors import Codes, SynapseError
+from synapse.rest.client.v1 import login
+
+from tests import unittest
+
+
+class UsernameAvailableTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+    ]
+    url = "/_synapse/admin/v1/username_available"
+
+    def prepare(self, reactor, clock, hs):
+        self.register_user("admin", "pass", admin=True)
+        self.admin_user_tok = self.login("admin", "pass")
+
+        async def check_username(username):
+            if username == "allowed":
+                return True
+            raise SynapseError(400, "User ID already taken.", errcode=Codes.USER_IN_USE)
+
+        handler = self.hs.get_registration_handler()
+        handler.check_username = check_username
+
+    def test_username_available(self):
+        """
+        The endpoint should return a 200 response if the username does not exist
+        """
+
+        url = "%s?username=%s" % (self.url, "allowed")
+        channel = self.make_request("GET", url, None, self.admin_user_tok)
+
+        self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
+        self.assertTrue(channel.json_body["available"])
+
+    def test_username_unavailable(self):
+        """
+        The endpoint should return a 200 response if the username does not exist
+        """
+
+        url = "%s?username=%s" % (self.url, "disallowed")
+        channel = self.make_request("GET", url, None, self.admin_user_tok)
+
+        self.assertEqual(400, int(channel.result["code"]), msg=channel.result["body"])
+        self.assertEqual(channel.json_body["errcode"], "M_USER_IN_USE")
+        self.assertEqual(channel.json_body["error"], "User ID already taken.")