Refactor get_user_ids_changed to pull less from DB

When a client asks for users whose devices have changed since a token we
used to pull *all* users from the database since the token, which could
easily be thousands of rows for old tokens.

This PR changes this to only check for changes for users the client is
actually interested in.

Fixes #5553

synapse/handlers/device.py

@@ -101,9 +101,13 @@ class DeviceWorkerHandler(BaseHandler):
 
         room_ids = yield self.store.get_rooms_for_user(user_id)
 
-        # First we check if any devices have changed
+        # First we check if any devices have changed for users that we share
+        # rooms with.
+        users_who_share_room = yield self.store.get_users_who_share_room_with_user(
+            user_id
+        )
         changed = yield self.store.get_user_whose_devices_changed(
-            from_token.device_list_key
+            from_token.device_list_key, users_who_share_room
         )
 
         # Then work out if any users have since joined
@@ -188,10 +192,6 @@ class DeviceWorkerHandler(BaseHandler):
                         break
 
         if possibly_changed or possibly_left:
-            users_who_share_room = yield self.store.get_users_who_share_room_with_user(
-                user_id
-            )
-
             # Take the intersection of the users whose devices may have changed
             # and those that actually still share a room with the user
             possibly_joined = possibly_changed & users_who_share_room

synapse/handlers/sync.py

@@ -1062,10 +1062,6 @@ class SyncHandler(object):
         since_token = sync_result_builder.since_token
 
         if since_token and since_token.device_list_key:
-            changed = yield self.store.get_user_whose_devices_changed(
-                since_token.device_list_key
-            )
-
             # TODO: Be more clever than this, i.e. remove users who we already
             # share a room with?
             for room_id in newly_joined_rooms:
@@ -1076,21 +1072,23 @@ class SyncHandler(object):
                 left_users = yield self.state.get_current_users_in_room(room_id)
                 newly_left_users.update(left_users)
 
-            # TODO: Check that these users are actually new, i.e. either they
-            # weren't in the previous sync *or* they left and rejoined.
-            changed.update(newly_joined_or_invited_users)
-
-            if not changed and not newly_left_users:
-                defer.returnValue(DeviceLists(changed=[], left=newly_left_users))
-
             users_who_share_room = yield self.store.get_users_who_share_room_with_user(
                 user_id
             )
 
+            # TODO: Check that these users are actually new, i.e. either they
+            # weren't in the previous sync *or* they left and rejoined.
+            changed = users_who_share_room & set(newly_joined_or_invited_users)
+
+            changed_users = yield self.store.get_user_whose_devices_changed(
+                since_token.device_list_key, users_who_share_room
+            )
+
+            changed.update(changed_users)
+
             defer.returnValue(
                 DeviceLists(
-                    changed=users_who_share_room & changed,
-                    left=set(newly_left_users) - users_who_share_room,
+                    changed=changed, left=set(newly_left_users) - users_who_share_room
                 )
             )
         else:

synapse/storage/devices.py

@@ -391,22 +391,53 @@ class DeviceWorkerStore(SQLBaseStore):
 
         return now_stream_id, []
 
-    @defer.inlineCallbacks
-    def get_user_whose_devices_changed(self, from_key):
-        """Get set of users whose devices have changed since `from_key`.
+    def get_user_whose_devices_changed(self, from_key, user_ids):
+        """Get set of users whose devices have changed since `from_key` that
+        are in the given list of user_ids.
+
+        Args:
+            user_ids (Iterable[str])
+            from_key: The device lists stream token
+
+        Returns:
+            Deferred[set[str]]: The set of user_ids whose devices have changed
+            since `from_key`
         """
         from_key = int(from_key)
-        changed = self._device_list_stream_cache.get_all_entities_changed(from_key)
-        if changed is not None:
-            defer.returnValue(set(changed))
+
+        # Get set of users who *may* have changed. Users not in the returned
+        # list have definitely not changed.
+        to_check = list(
+            self._device_list_stream_cache.get_entities_changed(user_ids, from_key)
+        )
+
+        if not to_check:
+            return defer.succeed(set())
+
+        # We now check the database for all users in `to_check`, in batches.
+        batch_size = 100
+        chunks = [
+            to_check[i : i + batch_size] for i in range(0, len(to_check), batch_size)
+        ]
 
         sql = """
-            SELECT DISTINCT user_id FROM device_lists_stream WHERE stream_id > ?
+            SELECT DISTINCT user_id FROM device_lists_stream
+            WHERE stream_id > ?
+            AND user_id IN (%s)
         """
-        rows = yield self._execute(
-            "get_user_whose_devices_changed", None, sql, from_key
+
+        def _get_user_whose_devices_changed_txn(txn):
+            changes = set()
+
+            for chunk in chunks:
+                txn.execute(sql % (",".join("?" for _ in chunk),), [from_key] + chunk)
+                changes.update(user_id for user_id, in txn)
+
+            return changes
+
+        return self.runInteraction(
+            "get_user_whose_devices_changed", _get_user_whose_devices_changed_txn
         )
-        defer.returnValue(set(row[0] for row in rows))
 
     def get_all_device_list_changes_for_remotes(self, from_key, to_key):
         """Return a list of `(stream_id, user_id, destination)` which is the