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![Sean Quah](/assets/img/avatar_default.png)
Whenever we want to persist an event, we first compute an event context, which includes the state at the event and a flag indicating whether the state is partial. After a lot of processing, we finally try to store the event in the database, which can fail for partial state events when the containing room has been un-partial stated in the meantime. We detect the race as a foreign key constraint failure in the data store layer and turn it into a special `PartialStateConflictError` exception, which makes its way up to the method in which we computed the event context. To make things difficult, the exception needs to cross a replication request: `/fed_send_events` for events coming over federation and `/send_event` for events from clients. We transport the `PartialStateConflictError` as a `409 Conflict` over replication and turn `409`s back into `PartialStateConflictError`s on the worker making the request. All client events go through `EventCreationHandler.handle_new_client_event`, which is called in *a lot* of places. Instead of trying to update all the code which creates client events, we turn the `PartialStateConflictError` into a `429 Too Many Requests` in `EventCreationHandler.handle_new_client_event` and hope that clients take it as a hint to retry their request. On the federation event side, there are 7 places which compute event contexts. 4 of them use outlier event contexts: `FederationEventHandler._auth_and_persist_outliers_inner`, `FederationHandler.do_knock`, `FederationHandler.on_invite_request` and `FederationHandler.do_remotely_reject_invite`. These events won't have the partial state flag, so we do not need to do anything for then. The remaining 3 paths which create events are `FederationEventHandler.process_remote_join`, `FederationEventHandler.on_send_membership_event` and `FederationEventHandler._process_received_pdu`. We can't experience the race in `process_remote_join`, unless we're handling an additional join into a partial state room, which currently blocks, so we make no attempt to handle it correctly. `on_send_membership_event` is only called by `FederationServer._on_send_membership_event`, so we catch the `PartialStateConflictError` there and retry just once. `_process_received_pdu` is called by `on_receive_pdu` for incoming events and `_process_pulled_event` for backfill. The latter should never try to persist partial state events, so we ignore it. We catch the `PartialStateConflictError` in `on_receive_pdu` and retry just once. Refering to the graph of code paths in https://github.com/matrix-org/synapse/issues/12988#issuecomment-1156857648 may make the above make more sense. Signed-off-by: Sean Quah <seanq@matrix.org>
159 lines
5.5 KiB
Python
159 lines
5.5 KiB
Python
# Copyright 2018 New Vector Ltd
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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import logging
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from typing import TYPE_CHECKING, List, Tuple
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from twisted.web.server import Request
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from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
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from synapse.events import EventBase, make_event_from_dict
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from synapse.events.snapshot import EventContext
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from synapse.http.server import HttpServer
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from synapse.http.servlet import parse_json_object_from_request
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from synapse.replication.http._base import ReplicationEndpoint
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from synapse.types import JsonDict, Requester, UserID
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from synapse.util.metrics import Measure
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if TYPE_CHECKING:
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from synapse.server import HomeServer
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from synapse.storage.databases.main import DataStore
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logger = logging.getLogger(__name__)
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class ReplicationSendEventRestServlet(ReplicationEndpoint):
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"""Handles events newly created on workers, including persisting and
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notifying.
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The API looks like:
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POST /_synapse/replication/send_event/:event_id/:txn_id
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{
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"event": { .. serialized event .. },
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"room_version": .., // "1", "2", "3", etc: the version of the room
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// containing the event
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"event_format_version": .., // 1,2,3 etc: the event format version
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"internal_metadata": { .. serialized internal_metadata .. },
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"outlier": true|false,
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"rejected_reason": .., // The event.rejected_reason field
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"context": { .. serialized event context .. },
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"requester": { .. serialized requester .. },
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"ratelimit": true,
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"extra_users": [],
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}
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200 OK
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{ "stream_id": 12345, "event_id": "$abcdef..." }
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Responds with a 409 when a `PartialStateConflictError` is raised due to an event
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context that needs to be recomputed due to the un-partial stating of a room.
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The returned event ID may not match the sent event if it was deduplicated.
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"""
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NAME = "send_event"
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PATH_ARGS = ("event_id",)
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def __init__(self, hs: "HomeServer"):
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super().__init__(hs)
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self.event_creation_handler = hs.get_event_creation_handler()
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self.store = hs.get_datastores().main
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self._storage_controllers = hs.get_storage_controllers()
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self.clock = hs.get_clock()
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@staticmethod
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async def _serialize_payload( # type: ignore[override]
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event_id: str,
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store: "DataStore",
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event: EventBase,
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context: EventContext,
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requester: Requester,
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ratelimit: bool,
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extra_users: List[UserID],
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) -> JsonDict:
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"""
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Args:
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event_id
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store
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requester
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event
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context
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ratelimit
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extra_users: Any extra users to notify about event
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"""
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serialized_context = await context.serialize(event, store)
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payload = {
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"event": event.get_pdu_json(),
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"room_version": event.room_version.identifier,
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"event_format_version": event.format_version,
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"internal_metadata": event.internal_metadata.get_dict(),
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"outlier": event.internal_metadata.is_outlier(),
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"rejected_reason": event.rejected_reason,
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"context": serialized_context,
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"requester": requester.serialize(),
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"ratelimit": ratelimit,
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"extra_users": [u.to_string() for u in extra_users],
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}
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return payload
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async def _handle_request( # type: ignore[override]
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self, request: Request, event_id: str
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) -> Tuple[int, JsonDict]:
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with Measure(self.clock, "repl_send_event_parse"):
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content = parse_json_object_from_request(request)
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event_dict = content["event"]
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room_ver = KNOWN_ROOM_VERSIONS[content["room_version"]]
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internal_metadata = content["internal_metadata"]
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rejected_reason = content["rejected_reason"]
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event = make_event_from_dict(
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event_dict, room_ver, internal_metadata, rejected_reason
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)
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event.internal_metadata.outlier = content["outlier"]
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requester = Requester.deserialize(self.store, content["requester"])
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context = EventContext.deserialize(
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self._storage_controllers, content["context"]
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)
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ratelimit = content["ratelimit"]
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extra_users = [UserID.from_string(u) for u in content["extra_users"]]
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logger.info(
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"Got event to send with ID: %s into room: %s", event.event_id, event.room_id
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)
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event = await self.event_creation_handler.persist_and_notify_client_event(
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requester, event, context, ratelimit=ratelimit, extra_users=extra_users
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)
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return (
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200,
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{
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"stream_id": event.internal_metadata.stream_ordering,
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"event_id": event.event_id,
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},
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)
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def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
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ReplicationSendEventRestServlet(hs).register(http_server)
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