This change fixes a rare bug where initial /syncs would fail with a
`KeyError` under the following circumstances:
1. A user fast joins a remote room.
2. The user is kicked from the room before the room's full state has
been synced.
3. A second local user fast joins the room.
4. Events are backfilled into the room, with a negative stream
ordering, but higher topological ordering than the original user's
leave. It's not clear how backfill happened here, since it is
expected to be equivalent to syncing the full state.
5. The second local user leaves the room before the room's full state
has been synced. The homeserver does not complete the sync.
6. The original user performs an initial /sync with lazy_load_members
enabled.
* Because they were kicked from the room, the room is included in
the /sync response even though the include_leave option is not
specified.
* To populate the room's timeline, `_load_filtered_recents` /
`get_recent_events_for_room` fetches events with a lower stream
ordering than the leave event and picks the ones with the highest
topological orderings (which are most recent). This captures the
backfilled events after the leave, since they have a negative
stream ordering. These events are filtered out of the timeline,
since the user was not in the room at the time and cannot view
them. The sync code ends up with an empty timeline for the room
that notably does not include the user's leave event.
This seems buggy, but at least we don't disclose events the user
isn't allowed to see.
* Normally, `compute_state_delta` would fetch the state at the
start and end of the room's timeline to generate the sync
response. Since the timeline is empty, it fetches the state at
`min(now, last event in the room)`, which corresponds with the
second user's leave. The state during the entirety of the second
user's membership does not include the membership for the first
user because of partial state.
This part is also questionable, since we are fetching state from
outside the bounds of the user's membership.
* `compute_state_delta` then tries and fails to find the user's
membership in the auth events of timeline events. Because there
is no timeline event whose auth events are expected to contain
the user's membership, a `KeyError` is raised.
Signed-off-by: Sean Quah <seanq@matrix.org>