automatically set default displayname on register

to avoid leaking ugly MXIDs and cluttering up the timeline with
displayname changes as well as membership joins for autojoin rooms
(e.g. the status autojoin rooms), automatically set the displayname
to match the localpart of the mxid upon registration.

synapse/rest/client/v2_alpha/register.py

@@ -20,7 +20,7 @@ import synapse
 import synapse.types
 from synapse.api.auth import get_access_token_from_request, has_access_token
 from synapse.api.constants import LoginType
-from synapse.types import RoomID, RoomAlias
+from synapse.types import RoomID, RoomAlias, get_localpart_from_id
 from synapse.api.errors import SynapseError, Codes, UnrecognizedRequestError
 from synapse.http.servlet import (
     RestServlet, parse_json_object_from_request, assert_params_in_request, parse_string
@@ -343,6 +343,13 @@ class RegisterRestServlet(RestServlet):
                 generate_token=False,
             )
 
+            # before we auto-join, set a default displayname to avoid ugly race
+            # between the client joining rooms and trying to set a displayname
+            localpart = get_localpart_from_id(registered_user_id)
+            yield self.store.set_profile_displayname(
+                localpart, localpart
+            )
+
             # auto-join the user to any rooms we're supposed to dump them into
             fake_requester = synapse.types.create_requester(registered_user_id)
             for r in self.hs.config.auto_join_rooms: