When server leaves room check for stale device lists. (#6801)

When a server leaves a room it may stop sharing a room with remote
users, and thus not get any updates to their device lists. So we need to
check for this case and delete those device lists from the cache.

We don't need to do this if we stop sharing a room because the remote
user leaves the room, because we track that case via looking at
membership changes.

changelog.d/6801.bugfix

@@ -0,0 +1 @@
+Fix bug where Synapse didn't invalidate cache of remote users' devices when Synapse left a room.

synapse/storage/data_stores/main/roommember.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 
 import logging
-from typing import Iterable, List
+from typing import Iterable, List, Set
 
 from six import iteritems, itervalues
 
@@ -40,7 +40,7 @@
     ProfileInfo,
     RoomsForUser,
 )
-from synapse.types import get_domain_from_id
+from synapse.types import Collection, get_domain_from_id
 from synapse.util.async_helpers import Linearizer
 from synapse.util.caches import intern_string
 from synapse.util.caches.descriptors import cached, cachedInlineCallbacks, cachedList
@@ -439,6 +439,39 @@ def _get_rooms_for_user_with_stream_ordering_txn(self, txn, user_id):
 
         return results
 
+    async def get_users_server_still_shares_room_with(
+        self, user_ids: Collection[str]
+    ) -> Set[str]:
+        """Given a list of users return the set that the server still share a
+        room with.
+        """
+
+        if not user_ids:
+            return set()
+
+        def _get_users_server_still_shares_room_with_txn(txn):
+            sql = """
+                SELECT state_key FROM current_state_events
+                WHERE
+                    type = 'm.room.member'
+                    AND membership = 'join'
+                    AND %s
+                GROUP BY state_key
+            """
+
+            clause, args = make_in_list_sql_clause(
+                self.database_engine, "state_key", user_ids
+            )
+
+            txn.execute(sql % (clause,), args)
+
+            return set(row[0] for row in txn)
+
+        return await self.db.runInteraction(
+            "get_users_server_still_shares_room_with",
+            _get_users_server_still_shares_room_with_txn,
+        )
+
     @defer.inlineCallbacks
     def get_rooms_for_user(self, user_id, on_invalidate=None):
         """Returns a set of room_ids the user is currently joined to.

synapse/storage/persist_events.py

@@ -18,7 +18,7 @@
 import itertools
 import logging
 from collections import deque, namedtuple
-from typing import Iterable, List, Optional, Tuple
+from typing import Iterable, List, Optional, Set, Tuple
 
 from six import iteritems
 from six.moves import range
@@ -318,6 +318,11 @@ async def _persist_events(
             # room
             state_delta_for_room = {}
 
+            # Set of remote users which were in rooms the server has left. We
+            # should check if we still share any rooms and if not we mark their
+            # device lists as stale.
+            potentially_left_users = set()  # type: Set[str]
+
             if not backfilled:
                 with Measure(self._clock, "_calculate_state_and_extrem"):
                     # Work out the new "current state" for each room.
@@ -421,7 +426,11 @@ async def _persist_events(
                             # the room then we delete the current state and
                             # extremities.
                             is_still_joined = await self._is_server_still_joined(
-                                room_id, ev_ctx_rm, delta, current_state
+                                room_id,
+                                ev_ctx_rm,
+                                delta,
+                                current_state,
+                                potentially_left_users,
                             )
                             if not is_still_joined:
                                 logger.info("Server no longer in room %s", room_id)
@@ -444,6 +453,8 @@ async def _persist_events(
                 backfilled=backfilled,
             )
 
+            await self._handle_potentially_left_users(potentially_left_users)
+
     async def _calculate_new_extremities(
         self,
         room_id: str,
@@ -688,6 +699,7 @@ async def _is_server_still_joined(
         ev_ctx_rm: List[Tuple[FrozenEvent, EventContext]],
         delta: DeltaState,
         current_state: Optional[StateMap[str]],
+        potentially_left_users: Set[str],
     ) -> bool:
         """Check if the server will still be joined after the given events have
         been persised.
@@ -699,6 +711,9 @@ async def _is_server_still_joined(
                 and what the new current state will be.
             current_state: The new current state if it already been calculated,
                 otherwise None.
+            potentially_left_users: If the server has left the room, then joined
+                remote users will be added to this set to indicate that the
+                server may no longer be sharing a room with them.
         """
 
         if not any(
@@ -741,5 +756,33 @@ async def _is_server_still_joined(
         is_still_joined = any(row["membership"] == Membership.JOIN for row in rows)
         if is_still_joined:
             return True
-        else:
-            return False
+
+        # The server will leave the room, so we go and find out which remote
+        # users will still be joined when we leave.
+        remote_event_ids = [
+            event_id
+            for (typ, state_key,), event_id in current_state.items()
+            if typ == EventTypes.Member and not self.is_mine_id(state_key)
+        ]
+        rows = await self.main_store.get_membership_from_event_ids(remote_event_ids)
+        potentially_left_users.update(
+            row["user_id"] for row in rows if row["membership"] == Membership.JOIN
+        )
+
+        return False
+
+    async def _handle_potentially_left_users(self, user_ids: Set[str]):
+        """Given a set of remote users check if the server still shares a room with
+        them. If not then mark those users' device cache as stale.
+        """
+
+        if not user_ids:
+            return
+
+        joined_users = await self.main_store.get_users_server_still_shares_room_with(
+            user_ids
+        )
+        left_users = user_ids - joined_users
+
+        for user_id in left_users:
+            await self.main_store.mark_remote_user_device_list_as_unsubscribed(user_id)