Add some debug about processing read receipts.

I'm hoping to establish which rooms are having lots of RRs sent for them, and
how old the events are when they are sent.

changelog.d/4798.misc

+Add some debug about processing read receipts.

synapse/storage/receipts.py

@@ -346,15 +346,23 @@ class ReceiptsStore(ReceiptsWorkerStore):
 
     def insert_linearized_receipt_txn(self, txn, room_id, receipt_type,
                                       user_id, event_id, data, stream_id):
+        """Inserts a read-receipt into the database if it's newer than the current RR
+
+        Returns: int|None
+            None if the RR is older than the current RR
+            otherwise, the rx timestamp of the event that the RR corresponds to
+                (or 0 if the event is unknown)
+        """
         res = self._simple_select_one_txn(
             txn,
             table="events",
-            retcols=["topological_ordering", "stream_ordering"],
+            retcols=["stream_ordering", "received_ts"],
             keyvalues={"event_id": event_id},
             allow_none=True
         )
 
         stream_ordering = int(res["stream_ordering"]) if res else None
+        rx_ts = res["received_ts"] if res else 0
 
         # We don't want to clobber receipts for more recent events, so we
         # have to compare orderings of existing receipts
@@ -373,7 +381,7 @@ class ReceiptsStore(ReceiptsWorkerStore):
                         "one for later event %s",
                         event_id, eid,
                     )
-                    return False
+                    return None
 
         txn.call_after(
             self.get_receipts_for_room.invalidate, (room_id, receipt_type)
@@ -429,7 +437,7 @@ class ReceiptsStore(ReceiptsWorkerStore):
                 stream_ordering=stream_ordering,
             )
 
-        return True
+        return rx_ts
 
     @defer.inlineCallbacks
     def insert_receipt(self, room_id, receipt_type, user_id, event_ids, data):
@@ -466,7 +474,7 @@ class ReceiptsStore(ReceiptsWorkerStore):
 
         stream_id_manager = self._receipts_id_gen.get_next()
         with stream_id_manager as stream_id:
-            have_persisted = yield self.runInteraction(
+            event_ts = yield self.runInteraction(
                 "insert_linearized_receipt",
                 self.insert_linearized_receipt_txn,
                 room_id, receipt_type, user_id, linearized_event_id,
@@ -474,8 +482,14 @@ class ReceiptsStore(ReceiptsWorkerStore):
                 stream_id=stream_id,
             )
 
-            if not have_persisted:
-                defer.returnValue(None)
+        if event_ts is None:
+            defer.returnValue(None)
+
+        now = self._clock.time_msec()
+        logger.debug(
+            "RR for event %s in %s (%i ms old)",
+            linearized_event_id, room_id, now - event_ts,
+        )
 
         yield self.insert_graph_receipt(
             room_id, receipt_type, user_id, event_ids, data