Fix a few race conditions in the state calculation

Be a bit more careful about how we calculate the state to be returned by
/sync. In a few places, it was possible for /sync to return slightly later
state than that represented by the next_batch token and the timeline. In
particular, the following cases were susceptible:

* On a full state sync, for an active room
* During a per-room incremental sync with a timeline gap
* When the user has just joined a room. (Refactor check_joined_room to make it
  less magical)

Also, use store.get_state_for_events() (and thus the existing stategroups) to
calculate the state corresponding to a particular sync position, rather than
state_handler.compute_event_context(), which recalculates from first principles
(and tends to miss some state).

Merged from PR https://github.com/matrix-org/synapse/pull/372

2 files changed, 77 insertions, 60 deletions

diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 8b154fa7e7..6dc9d0fb92 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -254,9 +254,7 @@ class SyncHandler(BaseHandler):
             room_id, sync_config, now_token, since_token=timeline_since_token
         )
 
-        current_state = yield self.state_handler.get_current_state(
-            room_id
-        )
+        current_state = yield self.get_state_at(room_id, now_token)
 
         defer.returnValue(JoinedSyncResult(
             room_id=room_id,
@@ -353,14 +351,12 @@ class SyncHandler(BaseHandler):
             room_id, sync_config, leave_token, since_token=timeline_since_token
         )
 
-        leave_state = yield self.store.get_state_for_events(
-            [leave_event_id], None
-        )
+        leave_state = yield self.store.get_state_for_event(leave_event_id)
 
         defer.returnValue(ArchivedSyncResult(
             room_id=room_id,
             timeline=batch,
-            state=leave_state[leave_event_id],
+            state=leave_state,
             private_user_data=self.private_user_data_for_room(
                 room_id, tags_by_room
             ),
@@ -424,6 +420,9 @@ class SyncHandler(BaseHandler):
         if len(room_events) <= timeline_limit:
             # There is no gap in any of the rooms. Therefore we can just
             # partition the new events by room and return them.
+            logger.debug("Got %i events for incremental sync - not limited",
+                         len(room_events))
+
             invite_events = []
             leave_events = []
             events_by_room_id = {}
@@ -439,9 +438,11 @@ class SyncHandler(BaseHandler):
 
             for room_id in joined_room_ids:
                 recents = events_by_room_id.get(room_id, [])
+                logger.debug("Events for room %s: %r", room_id, recents)
                 state = {
                     (event.type, event.state_key): event
                     for event in recents if event.is_state()}
+                limited = False
 
                 if recents:
                     prev_batch = now_token.copy_and_replace(
@@ -450,9 +451,13 @@ class SyncHandler(BaseHandler):
                 else:
                     prev_batch = now_token
 
-                state, limited = yield self.check_joined_room(
-                    sync_config, room_id, state
-                )
+                just_joined = yield self.check_joined_room(sync_config, state)
+                if just_joined:
+                    logger.debug("User has just joined %s: needs full state",
+                                 room_id)
+                    state = yield self.get_state_at(room_id, now_token)
+                    # the timeline is inherently limited if we've just joined
+                    limited = True
 
                 room_sync = JoinedSyncResult(
                     room_id=room_id,
@@ -467,10 +472,15 @@ class SyncHandler(BaseHandler):
                         room_id, tags_by_room
                     ),
                 )
+                logger.debug("Result for room %s: %r", room_id, room_sync)
+
                 if room_sync:
                     joined.append(room_sync)
 
         else:
+            logger.debug("Got %i events for incremental sync - hit limit",
+                         len(room_events))
+
             invite_events = yield self.store.get_invites_for_user(
                 sync_config.user.to_string()
             )
@@ -563,6 +573,8 @@ class SyncHandler(BaseHandler):
         Returns:
             A Deferred JoinedSyncResult
         """
+        logger.debug("Doing incremental sync for room %s between %s and %s",
+                     room_id, since_token, now_token)
 
         # TODO(mjark): Check for redactions we might have missed.
 
@@ -572,30 +584,26 @@ class SyncHandler(BaseHandler):
 
         logging.debug("Recents %r", batch)
 
-        # TODO(mjark): This seems racy since this isn't being passed a
-        # token to indicate what point in the stream this is
-        current_state = yield self.state_handler.get_current_state(
-            room_id
-        )
+        current_state = yield self.get_state_at(room_id, now_token)
 
-        state_at_previous_sync = yield self.get_state_at_previous_sync(
-            room_id, since_token=since_token
+        state_at_previous_sync = yield self.get_state_at(
+            room_id, stream_position=since_token
         )
 
-        state_events_delta = yield self.compute_state_delta(
+        state = yield self.compute_state_delta(
             since_token=since_token,
             previous_state=state_at_previous_sync,
             current_state=current_state,
         )
 
-        state_events_delta, _ = yield self.check_joined_room(
-            sync_config, room_id, state_events_delta
-        )
+        just_joined = yield self.check_joined_room(sync_config, state)
+        if just_joined:
+            state = yield self.get_state_at(room_id, now_token)
 
         room_sync = JoinedSyncResult(
             room_id=room_id,
             timeline=batch,
-            state=state_events_delta,
+            state=state,
             ephemeral=ephemeral_by_room.get(room_id, []),
             private_user_data=self.private_user_data_for_room(
                 room_id, tags_by_room
@@ -627,16 +635,12 @@ class SyncHandler(BaseHandler):
 
         logging.debug("Recents %r", batch)
 
-        # TODO(mjark): This seems racy since this isn't being passed a
-        # token to indicate what point in the stream this is
-        leave_state = yield self.store.get_state_for_events(
-            [leave_event.event_id], None
+        state_events_at_leave = yield self.store.get_state_for_event(
+            leave_event.event_id
         )
 
-        state_events_at_leave = leave_state[leave_event.event_id]
-
-        state_at_previous_sync = yield self.get_state_at_previous_sync(
-            leave_event.room_id, since_token=since_token
+        state_at_previous_sync = yield self.get_state_at(
+            leave_event.room_id, stream_position=since_token
         )
 
         state_events_delta = yield self.compute_state_delta(
@@ -659,26 +663,36 @@ class SyncHandler(BaseHandler):
         defer.returnValue(room_sync)
 
     @defer.inlineCallbacks
-    def get_state_at_previous_sync(self, room_id, since_token):
-        """ Get the room state at the previous sync the client made.
-        Returns:
-            A Deferred map from ((type, state_key)->Event)
+    def get_state_after_event(self, event):
+        """
+        Get the room state after the given event
+
+        :param synapse.events.EventBase event: event of interest
+        :return: A Deferred map from ((type, state_key)->Event)
+        """
+        state = yield self.store.get_state_for_event(event.event_id)
+        if event.is_state():
+            state = state.copy()
+            state[(event.type, event.state_key)] = event
+        defer.returnValue(state)
+
+    @defer.inlineCallbacks
+    def get_state_at(self, room_id, stream_position):
+        """ Get the room state at a particular stream position
+        :param str room_id: room for which to get state
+        :param StreamToken stream_position: point at which to get state
+        :returns: A Deferred map from ((type, state_key)->Event)
         """
         last_events, token = yield self.store.get_recent_events_for_room(
-            room_id, end_token=since_token.room_key, limit=1,
+            room_id, end_token=stream_position.room_key, limit=1,
         )
 
         if last_events:
-            last_event = last_events[0]
-            last_context = yield self.state_handler.compute_event_context(
-                last_event
-            )
-            if last_event.is_state():
-                state = last_context.current_state.copy()
-                state[(last_event.type, last_event.state_key)] = last_event
-            else:
-                state = last_context.current_state
+            last_event = last_events[-1]
+            state = yield self.get_state_after_event(last_event)
+
         else:
+            # no events in this room - so presumably no state
             state = {}
         defer.returnValue(state)
 
@@ -706,31 +720,20 @@ class SyncHandler(BaseHandler):
                 state_delta[key] = event
         return state_delta
 
-    @defer.inlineCallbacks
-    def check_joined_room(self, sync_config, room_id, state_delta):
+    def check_joined_room(self, sync_config, state_delta):
         """
-        Check if the user has just joined the given room. If so, return the
-        full state for the room, instead of the delta since the last sync.
+        Check if the user has just joined the given room (so should
+        be given the full state)
 
         :param sync_config:
-        :param room_id:
         :param dict[(str,str), synapse.events.FrozenEvent] state_delta: the
            difference in state since the last sync
 
         :returns A deferred Tuple (state_delta, limited)
         """
-        joined = False
-        limited = False
-
         join_event = state_delta.get((
             EventTypes.Member, sync_config.user.to_string()), None)
         if join_event is not None:
             if join_event.content["membership"] == Membership.JOIN:
-                joined = True
-
-        if joined:
-            state_delta = yield self.state_handler.get_current_state(room_id)
-            # the timeline is inherently limited if we've just joined
-            limited = True
-
-        defer.returnValue((state_delta, limited))
+                return True
+        return False
diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index acfb322a53..80e9b63f50 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -237,6 +237,20 @@ class StateStore(SQLBaseStore):
 
         defer.returnValue({event: event_to_state[event] for event in event_ids})
 
+    @defer.inlineCallbacks
+    def get_state_for_event(self, event_id, types=None):
+        """
+        Get the state dict corresponding to a particular event
+
+        :param str event_id: event whose state should be returned
+        :param list[(str, str)]|None types: List of (type, state_key) tuples
+            which are used to filter the state fetched. May be None, which
+            matches any key
+        :return: a deferred dict from (type, state_key) -> state_event
+        """
+        state_map = yield self.get_state_for_events([event_id], types)
+        defer.returnValue(state_map[event_id])
+
     @cached(num_args=2, lru=True, max_entries=10000)
     def _get_state_group_for_event(self, room_id, event_id):
         return self._simple_select_one_onecol(