`get_missing_events` does not respect `min_depth`

[timedoutuk]

Description

The on_get_missing_events handler does not respect the min_depth field of the POST /_matrix/federation/v1/get_missing_events/{roomId} endpoint:

synapse/synapse/federation/transport/server/federation.py

Lines 627 to 650 in 306d8b2

	class FederationGetMissingEventsServlet(BaseFederationServerServlet):
	PATH = "/get_missing_events/(?P<room_id>[^/]*)"
	CATEGORY = "Federation requests"
	async def on_POST(
	self,
	origin: str,
	content: JsonDict,
	query: dict[bytes, list[bytes]],
	room_id: str,
	) -> tuple[int, JsonDict]:
	limit = int(content.get("limit", 10))
	earliest_events = content.get("earliest_events", [])
	latest_events = content.get("latest_events", [])
	result = await self.handler.on_get_missing_events(
	origin,
	room_id=room_id,
	earliest_events=earliest_events,
	latest_events=latest_events,
	limit=limit,
	)
	return 200, result

This means that Synapse will return potentially significantly more events than the sender requested, wasting time and potentially resulting in even more roundtrips.

Steps to reproduce

• call /get_missing_events with a min_depth that is less than some of the events that would be returned
• synapse returns a bunch of events below min_depth

Homeserver

N/A

Synapse Version

N/A

Installation Method

I don't know

Database

N/A

Workers

Multiple workers

Platform

N/A

Configuration

N/A

Relevant log output

2026-05-30T18:28:29.460190Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Attempting to gap fill missing events elapsed=12.959471226s limit=50 via=servername.example iteration=101 discovered=1648 min_depth=68402
2026-05-30T18:28:29.460346Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk:request:fed{dest="servername.example"}: conduwuit_service::federation::execute: Sending request method=POST url=https://matrix.servername.example:8448/_matrix/federation/v1/get_missing_events/!roomid
2026-05-30T18:28:29.534368Z TRACE transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk:request:fed{dest="servername.example"}: conduwuit_service::federation::execute: Received response from https://matrix.servername.example:8448 status=200 OK method=POST request_url=https://matrix.servername.example:8448/_matrix/federation/v1/get_missing_events/!roomid response_url=https://matrix.servername.example:8448/_matrix/federation/v1/get_missing_events/!roomid
2026-05-30T18:28:29.534395Z TRACE transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk:request:fed{dest="servername.example"}: conduwuit_service::federation::execute: Waiting for response body...
2026-05-30T18:28:29.535236Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk:request:fed{dest="servername.example"}: conduwuit_service::federation::execute: Got 200 for POST https://matrix.servername.example:8448/_matrix/federation/v1/get_missing_events/!roomid
2026-05-30T18:28:29.535282Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Got 20 events back from remote elapsed=13.034563404s
2026-05-30T18:28:29.535471Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48622 below min_depth 68402 elapsed=13.034752457s
2026-05-30T18:28:29.535545Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48622 below min_depth 68402 elapsed=13.034826528s
2026-05-30T18:28:29.535608Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48622 below min_depth 68402 elapsed=13.034888886s
2026-05-30T18:28:29.535702Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48622 below min_depth 68402 elapsed=13.034983079s
2026-05-30T18:28:29.535758Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48625 below min_depth 68402 elapsed=13.035038577s
2026-05-30T18:28:29.535846Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48622 below min_depth 68402 elapsed=13.035126887s
2026-05-30T18:28:29.535890Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48622 below min_depth 68402 elapsed=13.035171184s
2026-05-30T18:28:29.535933Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48590 below min_depth 68402 elapsed=13.035214707s
2026-05-30T18:28:29.535987Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48587 below min_depth 68402 elapsed=13.035268002s
2026-05-30T18:28:29.536095Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035376758s
2026-05-30T18:28:29.536152Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035433022s
2026-05-30T18:28:29.536200Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48633 below min_depth 68402 elapsed=13.035481615s
2026-05-30T18:28:29.536280Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035561297s
2026-05-30T18:28:29.536330Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035611791s
2026-05-30T18:28:29.536376Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035657714s
2026-05-30T18:28:29.536430Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035711498s
2026-05-30T18:28:29.536482Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035763367s
2026-05-30T18:28:29.536565Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035846586s
2026-05-30T18:28:29.536623Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035904578s
2026-05-30T18:28:29.536668Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Received PDU with depth 48629 below min_depth 68402 elapsed=13.035948729s
2026-05-30T18:28:29.536675Z TRACE transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}:get_missing_events_bulk: conduwuit_service::rooms::event_handler::fetch_and_handle_outliers: Finished get_missing_events elapsed=13.035956504s
2026-05-30T18:28:29.536682Z DEBUG transaction{id="txnid" origin="servername.example"}:pdu{room_id=!roomid event_id=$eventid}: conduwuit_service::rooms::event_handler::fetch_prev: Fetched 1659 missing events elapsed=13.037008972s

Anything else that would be useful to know?

Synapse does request a min_depth too:

synapse/synapse/federation/transport/client.py

Lines 780 to 792 in 306d8b2

	path = _create_v1_path("/get_missing_events/%s", room_id)
	return await self.client.post_json(
	destination=destination,
	path=path,
	data={
	"limit": int(limit),
	"min_depth": int(min_depth),
	"earliest_events": earliest_events,
	"latest_events": latest_events,
	},
	timeout=timeout,
	)

and looks like it expects that field to be respected:

synapse/synapse/handlers/federation_event.py

Lines 307 to 329 in 306d8b2

	if missing_prevs:
	# We only backfill backwards to the min depth.
	min_depth = await self._store.get_min_depth(pdu.room_id)
	logger.debug("min_depth: %d", min_depth)
	if min_depth is not None and pdu.depth > min_depth:
	# If we're missing stuff, ensure we only fetch stuff one
	# at a time.
	logger.info(
	"Acquiring room lock to fetch %d missing prev_events: %s",
	len(missing_prevs),
	shortstr(missing_prevs),
	)
	async with self._room_pdu_linearizer.queue(pdu.room_id):
	logger.info(
	"Acquired room lock to fetch %d missing prev_events",
	len(missing_prevs),
	)
	try:
	await self._get_missing_events_for_pdu(
	origin, pdu, prevs, min_depth
	)

and min_depth is even mentioned soon after too:

synapse/synapse/handlers/federation_event.py

Lines 344 to 352 in 306d8b2

	if missing_prevs:
	# since this event was pushed to us, it is possible for it to
	# become the only forward-extremity in the room, and we would then
	# trust its state to be the state for the whole room. This is very
	# bad. Further, if the event was pushed to us, there is no excuse
	# for us not to have all the prev_events. (XXX: apart from
	# min_depth?)
	#
	# We therefore reject any such events.