More efficiently handle no-op POSITION (#16640)

We may receive `POSITION` commands where we already know that worker has
advanced past that position, so there is no point in handling it.

changelog.d/16640.misc

@@ -0,0 +1 @@
+More efficiently handle no-op `POSITION` over replication.

synapse/replication/tcp/handler.py

@@ -588,6 +588,21 @@ class ReplicationCommandHandler:
 
         logger.debug("Handling '%s %s'", cmd.NAME, cmd.to_line())
 
+        # Check if we can early discard this position. We can only do so for
+        # connected streams.
+        stream = self._streams[cmd.stream_name]
+        if stream.can_discard_position(
+            cmd.instance_name, cmd.prev_token, cmd.new_token
+        ) and self.is_stream_connected(conn, cmd.stream_name):
+            logger.debug(
+                "Discarding redundant POSITION %s/%s %s %s",
+                cmd.instance_name,
+                cmd.stream_name,
+                cmd.prev_token,
+                cmd.new_token,
+            )
+            return
+
         self._add_command_to_stream_queue(conn, cmd)
 
     async def _process_position(
@@ -599,6 +614,18 @@ class ReplicationCommandHandler:
         """
         stream = self._streams[stream_name]
 
+        if stream.can_discard_position(
+            cmd.instance_name, cmd.prev_token, cmd.new_token
+        ) and self.is_stream_connected(conn, cmd.stream_name):
+            logger.debug(
+                "Discarding redundant POSITION %s/%s %s %s",
+                cmd.instance_name,
+                cmd.stream_name,
+                cmd.prev_token,
+                cmd.new_token,
+            )
+            return
+
         # We're about to go and catch up with the stream, so remove from set
         # of connected streams.
         for streams in self._streams_by_connection.values():
@@ -657,6 +684,13 @@ class ReplicationCommandHandler:
 
         self._streams_by_connection.setdefault(conn, set()).add(stream_name)
 
+    def is_stream_connected(
+        self, conn: IReplicationConnection, stream_name: str
+    ) -> bool:
+        """Return if stream has been successfully connected and is ready to
+        receive updates"""
+        return stream_name in self._streams_by_connection.get(conn, ())
+
     def on_REMOTE_SERVER_UP(
         self, conn: IReplicationConnection, cmd: RemoteServerUpCommand
     ) -> None:

synapse/replication/tcp/streams/_base.py

@@ -144,6 +144,16 @@ class Stream:
         """
         raise NotImplementedError()
 
+    def can_discard_position(
+        self, instance_name: str, prev_token: int, new_token: int
+    ) -> bool:
+        """Whether or not a position command for this stream can be discarded.
+
+        Useful for streams that can never go backwards and where we already know
+        the stream ID for the instance has advanced.
+        """
+        return False
+
     def discard_updates_and_advance(self) -> None:
         """Called when the stream should advance but the updates would be discarded,
         e.g. when there are no currently connected workers.
@@ -221,6 +231,14 @@ class _StreamFromIdGen(Stream):
     def minimal_local_current_token(self) -> Token:
         return self._stream_id_gen.get_minimal_local_current_token()
 
+    def can_discard_position(
+        self, instance_name: str, prev_token: int, new_token: int
+    ) -> bool:
+        # These streams can't go backwards, so we know we can ignore any
+        # positions where the tokens are from before the current token.
+
+        return new_token <= self.current_token(instance_name)
+
 
 def current_token_without_instance(
     current_token: Callable[[], int]