Discard retention policies when retrieving state

changelog.d/6436.bugfix

@@ -0,0 +1 @@
+Fix a bug where a room could become unusable with a low retention policy and a low activity.

synapse/handlers/message.py

@@ -138,7 +138,7 @@ def get_state_events(
                 raise NotFoundError("Can't find event for token %s" % (at_token,))
 
             visible_events = yield filter_events_for_client(
-                self.storage, user_id, last_events
+                self.storage, user_id, last_events, apply_retention_policies=False
             )
 
             event = last_events[0]

synapse/visibility.py

@@ -44,7 +44,12 @@
 
 @defer.inlineCallbacks
 def filter_events_for_client(
-    storage: Storage, user_id, events, is_peeking=False, always_include_ids=frozenset()
+    storage: Storage,
+    user_id,
+    events,
+    is_peeking=False,
+    always_include_ids=frozenset(),
+    apply_retention_policies=True,
 ):
     """
     Check which events a user is allowed to see
@@ -59,6 +64,10 @@ def filter_events_for_client(
             events
         always_include_ids (set(event_id)): set of event ids to specifically
             include (unless sender is ignored)
+        apply_retention_policies (bool): Whether to filter out events that's older than
+            allowed by the room's retention policy. Useful when this function is called
+            to e.g. check whether a user should be allowed to see the state at a given
+            event rather than to know if it should send an event to a user's client(s).
 
     Returns:
         Deferred[list[synapse.events.EventBase]]
@@ -86,13 +95,14 @@ def filter_events_for_client(
 
     erased_senders = yield storage.main.are_users_erased((e.sender for e in events))
 
-    room_ids = set(e.room_id for e in events)
-    retention_policies = {}
+    if apply_retention_policies:
+        room_ids = set(e.room_id for e in events)
+        retention_policies = {}
 
-    for room_id in room_ids:
-        retention_policies[room_id] = yield storage.main.get_retention_policy_for_room(
-            room_id
-        )
+        for room_id in room_ids:
+            retention_policies[
+                room_id
+            ] = yield storage.main.get_retention_policy_for_room(room_id)
 
     def allowed(event):
         """
@@ -113,7 +123,7 @@ def allowed(event):
 
         # Don't try to apply the room's retention policy if the event is a state event, as
         # MSC1763 states that retention is only considered for non-state events.
-        if not event.is_state():
+        if apply_retention_policies and not event.is_state():
             retention_policy = retention_policies[event.room_id]
             max_lifetime = retention_policy.get("max_lifetime")