Strip "join_authorised_via_users_server" from join events which do not need it. (#10933)

This fixes a "Event not signed by authorising server" error when
transition room member from join -> join, e.g. when updating a
display name or avatar URL for restricted rooms.

synapse/handlers/federation.py

@@ -27,7 +27,12 @@ from unpaddedbase64 import decode_base64
 from twisted.internet import defer
 
 from synapse import event_auth
-from synapse.api.constants import EventTypes, Membership, RejectedReason
+from synapse.api.constants import (
+    EventContentFields,
+    EventTypes,
+    Membership,
+    RejectedReason,
+)
 from synapse.api.errors import (
     AuthError,
     CodeMessageException,
@@ -716,7 +721,7 @@ class FederationHandler(BaseHandler):
 
                 if include_auth_user_id:
                     event_content[
-                        "join_authorised_via_users_server"
+                        EventContentFields.AUTHORISING_USER
                     ] = await self._event_auth_handler.get_user_which_could_invite(
                         room_id,
                         state_ids,

synapse/handlers/room_member.py

@@ -573,6 +573,14 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                 errcode=Codes.BAD_JSON,
             )
 
+        # The event content should *not* include the authorising user as
+        # it won't be properly signed. Strip it out since it might come
+        # back from a client updating a display name / avatar.
+        #
+        # This only applies to restricted rooms, but there should be no reason
+        # for a client to include it. Unconditionally remove it.
+        content.pop(EventContentFields.AUTHORISING_USER, None)
+
         effective_membership_state = action
         if action in ["kick", "unban"]:
             effective_membership_state = "leave"
@@ -939,7 +947,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         # be included in the event content in order to efficiently validate
         # the event.
         content[
-            "join_authorised_via_users_server"
+            EventContentFields.AUTHORISING_USER
         ] = await self.event_auth_handler.get_user_which_could_invite(
             room_id,
             current_state_ids,