Add endpoint to get an event at a given timestamp - MSC3030

synapse/rest/client/v1/room.py

@@ -1389,6 +1389,34 @@ def register_txn_path(servlet, regex_string, http_server, with_get=False):
         )
 
 
+class TimestampLookupRestServlet(RestServlet):
+    PATTERNS = client_patterns("/rooms/(?P<room_id>[^/]*)/timestamp_to_event$")
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastore()
+
+    async def on_GET(
+        self, request: SynapseRequest, room_id: str
+    ) -> Tuple[int, JsonDict]:
+        requester = await self._auth.get_user_by_req(request)
+        await self._auth.check_joined_room(room_id, requester.to_string())
+
+        timestamp = parse_integer(request, "ts")
+        thread_id = parse_integer(request, "thread_id", 0)
+
+        event_id = await self._store.get_event_for_timestamp(
+            room_id,
+            thread_id,
+            timestamp,
+        )
+
+        return 200, {
+            "event_id": event_id,
+        }
+
+
 class RoomSpaceSummaryRestServlet(RestServlet):
     PATTERNS = (
         re.compile(
@@ -1463,6 +1491,7 @@ def register_servlets(hs: "HomeServer", http_server, is_worker=False):
     JoinedRoomsRestServlet(hs).register(http_server)
     RoomAliasListServlet(hs).register(http_server)
     SearchRestServlet(hs).register(http_server)
+    TimestampLookupRestServlet(hs).register(http_server)
 
     # Some servlets only get registered for the main process.
     if not is_worker:

synapse/storage/databases/main/events_worker.py

@@ -1511,3 +1511,43 @@ def _cleanup_old_transaction_ids_txn(txn):
             "_cleanup_old_transaction_ids",
             _cleanup_old_transaction_ids_txn,
         )
+
+    def get_event_for_timestamp(self, room_id, thread_id, timestamp):
+        sql_template = """
+            SELECT event_id, origin_server_ts FROM 
+            WHERE
+                origin_server_ts %s ?
+                AND room_id = ?
+                AND thread_id = ?
+            ORDER BY origin_server_ts
+            LIMIT 1;
+        """
+
+        def f(txn):
+            txn.execute(sql_template % ("<=",), (timestamp, room_id, thread_id))
+            row = txn.fetchone()
+            if row:
+                event_id_before, ts_before = row
+            else:
+                event_id_before, ts_before = None, None
+
+            txn.execute(sql_template % (">=",), (timestamp, room_id, thread_id))
+            row = txn.fetchone()
+            if row:
+                event_id_after, ts_after = row
+            else:
+                event_id_after, ts_after = None, None
+
+            if event_id_before and event_id_after:
+                # Return the closest
+                if (timestamp - ts_before) < (ts_after - timestamp):
+                    return event_id_before
+                else:
+                    return event_id_after
+
+            if event_id_before:
+                return event_id_before
+
+            return event_id_after
+
+        return self.runInteraction("get_event_for_timestamp", f)