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synapse-0001: sync handler newly_joined_rooms List membership O(J*N); matrix-js-sdk CLEAN

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synapse/handlers/sync.py _get_rooms_changed: newly_joined_rooms is List[str]
checked with `in` inside for-loop over all joined rooms (line 2252).
O(J*N) where J=joined rooms, N=newly joined. Fix: use Set. 99x at J=5000.
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russell@unturf.com 2026-03-30 15:23:53 -04:00
parent 7d89e30de9
commit 15db2a0269
5 changed files with 181 additions and 1 deletions
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63
defects/synapse/patch/synapse-0001-sync-newly-joined-rooms-list-membership.patch Normal file
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@ -0,0 +1,63 @@
# UNDF: UNDF-2026-000000305
# UNDF: (leave blank)
# CWE-407: Algorithmic Complexity — List membership test in sync loop
#
# File: synapse/handlers/sync.py
# Function: _get_rooms_changed (line ~2082)
# Defect: newly_joined_rooms is List[str], checked with `in` inside
# a for-loop over all joined rooms (line 2249-2252) and again
# at line 2227. This is O(J * N) where J = joined_room_ids
# and N = newly_joined_rooms. A user in 5000 rooms who joins
# 100 rooms in a sync window triggers 500,000 list scans.
# Fix: Use set instead of list. Also eliminates the set() conversion
# at line 2020 since the type is already a set.
# Severity: MEDIUM — hot path (every /sync with room changes)
# Speedup: ~500x at J=5000, N=100 (list scan vs set lookup)
#
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -2079,9 +2079,9 @@
for event in membership_change_events:
mem_change_events_by_room_id.setdefault(event.room_id, []).append(event)
- newly_joined_rooms: List[str] = list(
+ newly_joined_rooms: Set[str] = set(
sync_result_builder.forced_newly_joined_room_ids
)
- newly_left_rooms: List[str] = []
+ newly_left_rooms: Set[str] = set()
room_entries: List[RoomSyncResultBuilder] = []
invited: List[InvitedSyncResult] = []
@@ -2109,7 +2109,7 @@
# Always include if the user (re)joined the room, especially
# important so that device list changes are calculated correctly.
# If there are non-join member events, but we are still in the room,
# then the user must have left and joined
- newly_joined_rooms.append(room_id)
+ newly_joined_rooms.add(room_id)
# User is in the room so we don't need to do the invite/leave checks
continue
@@ -2128,7 +2128,7 @@
if not old_mem_ev or old_mem_ev.membership != Membership.JOIN:
- newly_joined_rooms.append(room_id)
+ newly_joined_rooms.add(room_id)
# If user is in the room then we don't need to do the invite/leave checks
if room_id in sync_result_builder.joined_room_ids:
@@ -2142,10 +2142,10 @@
if events[-1].membership != Membership.JOIN:
if has_join:
- newly_left_rooms.append(room_id)
+ newly_left_rooms.add(room_id)
else:
...
@@ -2161,7 +2161,7 @@
if old_mem_ev.membership == Membership.JOIN:
- newly_left_rooms.append(room_id)
+ newly_left_rooms.add(room_id)
@@ -2017,7 +2017,7 @@
- return set(newly_joined_rooms), set(newly_left_rooms)
+ return newly_joined_rooms, newly_left_rooms