Make get_current_token accept a direction parameter, which tells whether the source whether we want a token for going 'forwards' or 'backwards'

synapse/handlers/message.py

@@ -89,7 +89,9 @@ class MessageHandler(BaseHandler):
 
         if not pagin_config.from_token:
             pagin_config.from_token = (
-                yield self.hs.get_event_sources().get_current_token()
+                yield self.hs.get_event_sources().get_current_token(
+                    direction='b'
+                )
             )
 
         room_token = RoomStreamToken.parse(pagin_config.from_token.room_key)

synapse/handlers/room.py

@@ -577,8 +577,8 @@ class RoomEventSource(object):
 
         defer.returnValue((events, end_key))
 
-    def get_current_key(self):
-        return self.store.get_room_events_max_id()
+    def get_current_key(self, direction='f'):
+        return self.store.get_room_events_max_id(direction)
 
     @defer.inlineCallbacks
     def get_pagination_rows(self, user, config, key):

synapse/storage/stream.py

@@ -364,9 +364,25 @@ class StreamStore(SQLBaseStore):
         )
 
     @defer.inlineCallbacks
-    def get_room_events_max_id(self):
+    def get_room_events_max_id(self, direction='f'):
         token = yield self._stream_id_gen.get_max_token(self)
-        defer.returnValue("s%d" % (token,))
+        if direction != 'b':
+            defer.returnValue("s%d" % (token,))
+        else:
+            topo = yield self.runInteraction(
+                "_get_max_topological_txn", self._get_max_topological_txn
+            )
+            defer.returnValue("t%d-%d" % (topo, token))
+
+    def _get_max_topological_txn(self, txn):
+        txn.execute(
+            "SELECT MAX(topological_ordering) FROM events"
+            " WHERE outlier = ?",
+            (False,)
+        )
+
+        rows = txn.fetchall()
+        return rows[0][0] if rows else 0
 
     @defer.inlineCallbacks
     def _get_min_token(self):

synapse/streams/events.py

@@ -31,7 +31,7 @@ class NullSource(object):
     def get_new_events_for_user(self, user, from_key, limit):
         return defer.succeed(([], from_key))
 
-    def get_current_key(self):
+    def get_current_key(self, direction='f'):
         return defer.succeed(0)
 
     def get_pagination_rows(self, user, pagination_config, key):
@@ -52,10 +52,10 @@ class EventSources(object):
         }
 
     @defer.inlineCallbacks
-    def get_current_token(self):
+    def get_current_token(self, direction='f'):
         token = StreamToken(
             room_key=(
-                yield self.sources["room"].get_current_key()
+                yield self.sources["room"].get_current_key(direction)
             ),
             presence_key=(
                 yield self.sources["presence"].get_current_key()