Add an admin API to check if a username is available

changelog.d/10578.feature

@@ -0,0 +1 @@
+Add an admin API (`GET /_synapse/admin/username_available`) to check if a username is available (regardless of registration settings).

docs/admin_api/user_admin_api.md

@@ -1013,3 +1013,16 @@ The following parameters should be set in the URL:
 - `user_id` - The fully qualified MXID: for example, `@user:server.com`. The user must
   be local.
 
+### Check username availability
+
+The API is:
+
+```
+POST /_synapse/admin/v1/username_availabile?username=$localpart
+```
+
+To use it, you will need to authenticate by providing an `access_token` for a
+server admin: [Admin API](../usage/administration/admin_api)
+
+
+The request and response format is the same as the [/_matrix/client/r0/register/available](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-register-available) API

synapse/rest/admin/__init__.py

@@ -62,6 +62,7 @@
     UserAdminServlet,
     UserMediaRestServlet,
     UserMembershipRestServlet,
+    UsernameAvailableRestServlet,
     UserRegisterServlet,
     UserRestServletV2,
     UsersRestServletV2,
@@ -241,6 +242,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ForwardExtremitiesRestServlet(hs).register(http_server)
     RoomEventContextServlet(hs).register(http_server)
     RateLimitRestServlet(hs).register(http_server)
+    UsernameAvailableRestServlet(hs).register(http_server)
 
 
 def register_servlets_for_client_rest_resource(

synapse/rest/admin/users.py

@@ -1097,3 +1097,28 @@ async def on_DELETE(
         await self.store.delete_ratelimit_for_user(user_id)
 
         return 200, {}
+
+
+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 200, {"available": True}

tests/rest/admin/test_user.py

@@ -22,7 +22,12 @@
 
 import synapse.rest.admin
 from synapse.api.constants import UserTypes
-from synapse.api.errors import Codes, HttpResponseException, ResourceLimitError
+from synapse.api.errors import (
+    Codes,
+    HttpResponseException,
+    ResourceLimitError,
+    SynapseError,
+)
 from synapse.api.room_versions import RoomVersions
 from synapse.rest.client.v1 import login, logout, profile, room
 from synapse.rest.client.v2_alpha import devices, sync
@@ -3406,3 +3411,54 @@ def test_success(self):
         self.assertEqual(200, int(channel.result["code"]), msg=channel.result["body"])
         self.assertNotIn("messages_per_second", channel.json_body)
         self.assertNotIn("burst_count", channel.json_body)
+
+
+class UsernameAvailableTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, hs):
+        self.register_user("admin", "pass", admin=True)
+        self.admin_user_tok = self.login("admin", "pass")
+        self.url = "/_synapse/admin/v1/username_available"
+
+        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 make_homeserver(self, reactor, clock):
+        self.hs = self.setup_test_homeserver()
+        return self.hs
+
+    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
+        """
+
+        async def check_username(username):
+            print("fibble sibble")
+            raise SynapseError(400, "User ID already taken.", errcode=Codes.USER_IN_USE)
+
+        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.")