Faster joins: don't stall when a user joins during a fast join (#14606)

Fixes #12801.
Complement tests are at
https://github.com/matrix-org/complement/pull/567.

Avoid blocking on full state when handling a subsequent join into a
partial state room.

Also always perform a remote join into partial state rooms, since we do
not know whether the joining user has been banned and want to avoid
leaking history to banned users.

Signed-off-by: Mathieu Velten <mathieuv@matrix.org>
Co-authored-by: Sean Quah <seanq@matrix.org>
Co-authored-by: David Robertson <davidr@element.io>

1 files changed, 8 insertions, 8 deletions

diff --git a/synapse/handlers/event_auth.py b/synapse/handlers/event_auth.py
index a23a8ce2a1..46dd63c3f0 100644
--- a/synapse/handlers/event_auth.py
+++ b/synapse/handlers/event_auth.py
@@ -202,7 +202,7 @@ class EventAuthHandler:
         state_ids: StateMap[str],
         room_version: RoomVersion,
         user_id: str,
-        prev_member_event: Optional[EventBase],
+        prev_membership: Optional[str],
     ) -> None:
         """
         Check whether a user can join a room without an invite due to restricted join rules.
@@ -214,15 +214,14 @@ class EventAuthHandler:
             state_ids: The state of the room as it currently is.
             room_version: The room version of the room being joined.
             user_id: The user joining the room.
-            prev_member_event: The current membership event for this user.
+            prev_membership: The current membership state for this user. `None` if the
+                user has never joined the room (equivalent to "leave").
 
         Raises:
             AuthError if the user cannot join the room.
         """
         # If the member is invited or currently joined, then nothing to do.
-        if prev_member_event and (
-            prev_member_event.membership in (Membership.JOIN, Membership.INVITE)
-        ):
+        if prev_membership in (Membership.JOIN, Membership.INVITE):
             return
 
         # This is not a room with a restricted join rule, so we don't need to do the
@@ -255,13 +254,14 @@ class EventAuthHandler:
             )
 
     async def has_restricted_join_rules(
-        self, state_ids: StateMap[str], room_version: RoomVersion
+        self, partial_state_ids: StateMap[str], room_version: RoomVersion
     ) -> bool:
         """
         Return if the room has the proper join rules set for access via rooms.
 
         Args:
-            state_ids: The state of the room as it currently is.
+            state_ids: The state of the room as it currently is. May be full or partial
+                state.
             room_version: The room version of the room to query.
 
         Returns:
@@ -272,7 +272,7 @@ class EventAuthHandler:
             return False
 
         # If there's no join rule, then it defaults to invite (so this doesn't apply).
-        join_rules_event_id = state_ids.get((EventTypes.JoinRules, ""), None)
+        join_rules_event_id = partial_state_ids.get((EventTypes.JoinRules, ""), None)
         if not join_rules_event_id:
             return False