Faster joins: Fix spurious errors on incremental sync (#15232)

When pushing events in partial state rooms down incremental /sync, we
try to find the `m.room.member` state event for their senders by digging
through their auth events, so that we can present the membership to the
client. Events usually have a membership event in their auth events,
with the exception of the `m.room.create` event and a user's first join
into the room.

When implementing #13477, we took the case of a user's first join into
account, but forgot to handle the `m.room.create` case. This change
fixes that.

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/15232.bugfix

@@ -0,0 +1 @@
+Faster joins: Fix a bug introduced in Synapse 1.66 where spurious "Failed to find memberships ..." errors would be logged.

synapse/handlers/sync.py

@@ -1226,6 +1226,10 @@ class SyncHandler:
                 continue
 
             event_with_membership_auth = events_with_membership_auth[member]
+            is_create = (
+                event_with_membership_auth.is_state()
+                and event_with_membership_auth.type == EventTypes.Create
+            )
             is_join = (
                 event_with_membership_auth.is_state()
                 and event_with_membership_auth.type == EventTypes.Member
@@ -1233,9 +1237,10 @@ class SyncHandler:
                 and event_with_membership_auth.content.get("membership")
                 == Membership.JOIN
             )
-            if not is_join:
+            if not is_create and not is_join:
                 # The event must include the desired membership as an auth event, unless
-                # it's the first join event for a given user.
+                # it's the `m.room.create` event for a room or the first join event for
+                # a given user.
                 missing_members.add(member)
             auth_event_ids.update(event_with_membership_auth.auth_event_ids())