Wait for a POSITION on the right connection before accepting RDATA

... otherwise we can believe we're up to date when we're not.

synapse/replication/tcp/handler.py

@@ -81,9 +81,6 @@ class ReplicationCommandHandler:
         self._instance_id = hs.get_instance_id()
         self._instance_name = hs.get_instance_name()
 
-        # Set of streams that we've caught up with.
-        self._streams_connected = set()  # type: Set[str]
-
         self._streams = {
             stream.NAME: stream(hs) for stream in STREAMS_MAP.values()
         }  # type: Dict[str, Stream]
@@ -103,6 +100,9 @@ class ReplicationCommandHandler:
         # outgoing replication commands to.)
         self._connections = []  # type: List[AbstractConnection]
 
+        # For each connection, the incoming streams that are coming from that connection
+        self._streams_by_connection = {}  # type: Dict[AbstractConnection, Set[str]]
+
         LaterGauge(
             "synapse_replication_tcp_resource_total_connections",
             "",
@@ -258,12 +258,14 @@ class ReplicationCommandHandler:
         #   2. so we don't race with getting a POSITION command and fetching
         #      missing RDATA.
         with await self._position_linearizer.queue(cmd.stream_name):
-            if stream_name not in self._streams_connected:
-                # If the stream isn't marked as connected then we haven't seen a
-                # `POSITION` command yet, and so we may have missed some rows.
+            # make sure that we've processed a POSITION for this stream *on this
+            # connection*. (A POSITION on another connection is no good, as there
+            # is no guarantee that we have seen all the intermediate updates.)
+            sbc = self._streams_by_connection.get(conn)
+            if not sbc or stream_name not in sbc:
                 # Let's drop the row for now, on the assumption we'll receive a
                 # `POSITION` soon and we'll catch up correctly then.
-                logger.warning(
+                logger.info(
                     "Discarding RDATA for unconnected stream %s -> %s",
                     stream_name,
                     cmd.token,
@@ -298,30 +300,33 @@ class ReplicationCommandHandler:
             # Ignore POSITION that are just our own echoes
             return
 
-        stream = self._streams.get(cmd.stream_name)
+        logger.info("Handling '%s %s'", cmd.NAME, cmd.to_line())
+
+        stream_name = cmd.stream_name
+        stream = self._streams.get(stream_name)
         if not stream:
-            logger.error("Got POSITION for unknown stream: %s", cmd.stream_name)
+            logger.error("Got POSITION for unknown stream: %s", stream_name)
             return
 
         # We protect catching up with a linearizer in case the replication
         # connection reconnects under us.
-        with await self._position_linearizer.queue(cmd.stream_name):
+        with await self._position_linearizer.queue(stream_name):
             # We're about to go and catch up with the stream, so remove from set
             # of connected streams.
-            self._streams_connected.discard(cmd.stream_name)
+            for streams in self._streams_by_connection.values():
+                streams.discard(stream_name)
 
             # We clear the pending batches for the stream as the fetching of the
             # missing updates below will fetch all rows in the batch.
-            self._pending_batches.pop(cmd.stream_name, [])
+            self._pending_batches.pop(stream_name, [])
 
             # Find where we previously streamed up to.
             current_token = self._replication_data_handler.get_streams_to_replicate().get(
-                cmd.stream_name
+                stream_name
             )
             if current_token is None:
                 logger.warning(
-                    "Got POSITION for stream we're not subscribed to: %s",
-                    cmd.stream_name,
+                    "Got POSITION for stream we're not subscribed to: %s", stream_name,
                 )
                 return
 
@@ -330,6 +335,12 @@ class ReplicationCommandHandler:
             # between then and now.
             missing_updates = cmd.token != current_token
             while missing_updates:
+                logger.info(
+                    "Fetching replication rows for '%s' between %i and %i",
+                    stream_name,
+                    current_token,
+                    cmd.token,
+                )
                 (
                     updates,
                     current_token,
@@ -343,13 +354,15 @@ class ReplicationCommandHandler:
 
                 for token, rows in _batch_updates(updates):
                     await self.on_rdata(
-                        cmd.stream_name, token, [stream.parse_row(row) for row in rows],
+                        stream_name, token, [stream.parse_row(row) for row in rows],
                     )
 
-            # We've now caught up to position sent to us, notify handler.
-            await self._replication_data_handler.on_position(cmd.stream_name, cmd.token)
+            logger.info("Caught up with stream '%s' to %i", stream_name, cmd.token)
 
-            self._streams_connected.add(cmd.stream_name)
+            # We've now caught up to position sent to us, notify handler.
+            await self._replication_data_handler.on_position(stream_name, cmd.token)
+
+            self._streams_by_connection.setdefault(conn, set()).add(stream_name)
 
     async def on_REMOTE_SERVER_UP(
         self, conn: AbstractConnection, cmd: RemoteServerUpCommand
@@ -407,6 +420,12 @@ class ReplicationCommandHandler:
     def lost_connection(self, connection: AbstractConnection):
         """Called when a connection is closed/lost.
         """
+        # we no longer need _streams_by_connection for this connection.
+        streams = self._streams_by_connection.pop(connection, None)
+        if streams:
+            logger.info(
+                "Lost replication connection; streams now disconnected: %s", streams
+            )
         try:
             self._connections.remove(connection)
         except ValueError:

synapse/replication/tcp/redis.py

@@ -144,7 +144,7 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
         Args:
             cmd (Command)
         """
-        run_as_background_process("send-cmd", self._send_command, cmd)
+        run_as_background_process("send-cmd", self._async_send_command, cmd)
 
     async def _async_send_command(self, cmd: Command):
         """Encode a replication command and send it over our outbound connection"""