Reuse existing pagination code for context API

synapse/storage/stream.py

@@ -42,7 +42,7 @@ from synapse.util.caches.descriptors import cached
 from synapse.types import RoomStreamToken
 from synapse.util.caches.stream_change_cache import StreamChangeCache
 from synapse.util.logcontext import make_deferred_yieldable, run_in_background
-from synapse.storage.engines import PostgresEngine, Sqlite3Engine
+from synapse.storage.engines import PostgresEngine
 
 import abc
 import logging
@@ -595,88 +595,28 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             retcols=["stream_ordering", "topological_ordering"],
         )
 
-        token = RoomStreamToken(
+        # Paginating backwards includes the event at the token, but paginating
+        # forward doesn't.
+        before_token = RoomStreamToken(
+            results["topological_ordering"] - 1,
+            results["stream_ordering"],
+        )
+
+        after_token = RoomStreamToken(
             results["topological_ordering"],
             results["stream_ordering"],
         )
 
-        if isinstance(self.database_engine, Sqlite3Engine):
+        rows, start_token = self.paginate_room_events_txn(
-            # SQLite3 doesn't optimise ``(x < a) OR (x = a AND y < b)``
+            txn, room_id, before_token, direction='b', limit=before_limit,
-            # So we give pass it to SQLite3 as the UNION ALL of the two queries.
+        )
-
-            query_before = (
-                "SELECT topological_ordering, stream_ordering, event_id FROM events"
-                " WHERE room_id = ? AND topological_ordering < ?"
-                " UNION ALL"
-                " SELECT topological_ordering, stream_ordering, event_id FROM events"
-                " WHERE room_id = ? AND topological_ordering = ? AND stream_ordering < ?"
-                " ORDER BY topological_ordering DESC, stream_ordering DESC LIMIT ?"
-            )
-            before_args = (
-                room_id, token.topological,
-                room_id, token.topological, token.stream,
-                before_limit,
-            )
-
-            query_after = (
-                "SELECT topological_ordering, stream_ordering, event_id FROM events"
-                " WHERE room_id = ? AND topological_ordering > ?"
-                " UNION ALL"
-                " SELECT topological_ordering, stream_ordering, event_id FROM events"
-                " WHERE room_id = ? AND topological_ordering = ? AND stream_ordering > ?"
-                " ORDER BY topological_ordering ASC, stream_ordering ASC LIMIT ?"
-            )
-            after_args = (
-                room_id, token.topological,
-                room_id, token.topological, token.stream,
-                after_limit,
-            )
-        else:
-            query_before = (
-                "SELECT topological_ordering, stream_ordering, event_id FROM events"
-                " WHERE room_id = ? AND %s"
-                " ORDER BY topological_ordering DESC, stream_ordering DESC LIMIT ?"
-            ) % (upper_bound(token, self.database_engine, inclusive=False),)
-
-            before_args = (room_id, before_limit)
-
-            query_after = (
-                "SELECT topological_ordering, stream_ordering, event_id FROM events"
-                " WHERE room_id = ? AND %s"
-                " ORDER BY topological_ordering ASC, stream_ordering ASC LIMIT ?"
-            ) % (lower_bound(token, self.database_engine, inclusive=False),)
-
-            after_args = (room_id, after_limit)
-
-        txn.execute(query_before, before_args)
-
-        rows = self.cursor_to_dict(txn)
         events_before = [r["event_id"] for r in rows]
 
-        if rows:
+        rows, end_token = self.paginate_room_events_txn(
-            start_token = str(RoomStreamToken(
+            txn, room_id, after_token, direction='f', limit=after_limit,
-                rows[0]["topological_ordering"],
+        )
-                rows[0]["stream_ordering"] - 1,
-            ))
-        else:
-            start_token = str(RoomStreamToken(
-                token.topological,
-                token.stream - 1,
-            ))
-
-        txn.execute(query_after, after_args)
-
-        rows = self.cursor_to_dict(txn)
         events_after = [r["event_id"] for r in rows]
 
-        if rows:
-            end_token = str(RoomStreamToken(
-                rows[-1]["topological_ordering"],
-                rows[-1]["stream_ordering"],
-            ))
-        else:
-            end_token = str(token)
-
         return {
             "before": {
                 "event_ids": events_before,