Fix incredubly slow back pagination query

If a client didn't specify a from token when paginating backwards
synapse would attempt to query the (global) maximum topological token.
This a) doesn't make much sense since they're room specific and b) there
are no indices that lets postgres do this efficiently.

synapse/handlers/message.py

@@ -82,8 +82,8 @@ class MessageHandler(BaseHandler):
             room_token = pagin_config.from_token.room_key
         else:
             pagin_config.from_token = (
-                yield self.hs.get_event_sources().get_current_token(
-                    direction='b'
+                yield self.hs.get_event_sources().get_current_token_for_room(
+                    room_id=room_id
                 )
             )
             room_token = pagin_config.from_token.room_key

synapse/handlers/room.py

@@ -475,8 +475,11 @@ class RoomEventSource(object):
 
         defer.returnValue((events, end_key))
 
-    def get_current_key(self, direction='f'):
-        return self.store.get_room_events_max_id(direction)
+    def get_current_key(self):
+        return self.store.get_room_events_max_id()
+
+    def get_current_key_for_room(self, room_id):
+        return self.store.get_room_events_max_id(room_id)
 
     @defer.inlineCallbacks
     def get_pagination_rows(self, user, config, key):

synapse/storage/stream.py

@@ -521,13 +521,20 @@ class StreamStore(SQLBaseStore):
         )
 
     @defer.inlineCallbacks
-    def get_room_events_max_id(self, direction='f'):
+    def get_room_events_max_id(self, room_id=None):
+        """Returns the current token for rooms stream.
+
+        By default, it returns the current global stream token. Specifying a
+        `room_id` causes it to return the current room specific topological
+        token.
+        """
         token = yield self._stream_id_gen.get_current_token()
-        if direction != 'b':
+        if room_id is None:
             defer.returnValue("s%d" % (token,))
         else:
             topo = yield self.runInteraction(
-                "_get_max_topological_txn", self._get_max_topological_txn
+                "_get_max_topological_txn", self._get_max_topological_txn,
+                room_id,
             )
             defer.returnValue("t%d-%d" % (topo, token))
 
@@ -579,11 +586,11 @@ class StreamStore(SQLBaseStore):
             lambda r: r[0][0] if r else 0
         )
 
-    def _get_max_topological_txn(self, txn):
+    def _get_max_topological_txn(self, txn, room_id):
         txn.execute(
             "SELECT MAX(topological_ordering) FROM events"
-            " WHERE outlier = ?",
-            (False,)
+            " WHERE room_id = ?",
+            (room_id,)
         )
 
         rows = txn.fetchall()

synapse/streams/events.py

@@ -41,13 +41,39 @@ class EventSources(object):
         self.store = hs.get_datastore()
 
     @defer.inlineCallbacks
-    def get_current_token(self, direction='f'):
+    def get_current_token(self):
         push_rules_key, _ = self.store.get_push_rules_stream_token()
         to_device_key = self.store.get_to_device_stream_token()
 
         token = StreamToken(
             room_key=(
-                yield self.sources["room"].get_current_key(direction)
+                yield self.sources["room"].get_current_key()
+            ),
+            presence_key=(
+                yield self.sources["presence"].get_current_key()
+            ),
+            typing_key=(
+                yield self.sources["typing"].get_current_key()
+            ),
+            receipt_key=(
+                yield self.sources["receipt"].get_current_key()
+            ),
+            account_data_key=(
+                yield self.sources["account_data"].get_current_key()
+            ),
+            push_rules_key=push_rules_key,
+            to_device_key=to_device_key,
+        )
+        defer.returnValue(token)
+
+    @defer.inlineCallbacks
+    def get_current_token_for_room(self, room_id):
+        push_rules_key, _ = self.store.get_push_rules_stream_token()
+        to_device_key = self.store.get_to_device_stream_token()
+
+        token = StreamToken(
+            room_key=(
+                yield self.sources["room"].get_current_key()
             ),
             presence_key=(
                 yield self.sources["presence"].get_current_key()