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Fix race when persisting an event and deleting a room (#12594)

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This works by taking a row level lock on the `rooms` table at the start of both transactions, ensuring that they don't run at the same time. In the event persistence transaction we also check that there is an entry still in the `rooms` table.

I can't figure out how to do this in SQLite. I was just going to lock the table, but it seems that we don't support that in SQLite either, so I'm *really* confused as to how we maintain integrity in SQLite when using `lock_table`....
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Erik Johnston 2022-05-03 11:47:21 +01:00 • committed by GitHub
parent 01dcf7532d
commit ae7858f184
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3 changed files with 22 additions and 2 deletions
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15
synapse/storage/databases/main/events.py
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@ -47,6 +47,7 @@ from synapse.storage.database import (
)
from synapse.storage.databases.main.events_worker import EventCacheEntry
from synapse.storage.databases.main.search import SearchEntry
from synapse.storage.engines.postgres import PostgresEngine
from synapse.storage.util.id_generators import AbstractStreamIdGenerator
from synapse.storage.util.sequence import SequenceGenerator
from synapse.types import StateMap, get_domain_from_id
@ -364,6 +365,20 @@ class PersistEventsStore:
min_stream_order = events_and_contexts[0][0].internal_metadata.stream_ordering
max_stream_order = events_and_contexts[-1][0].internal_metadata.stream_ordering
# We check that the room still exists for events we're trying to
# persist. This is to protect against races with deleting a room.
#
# Annoyingly SQLite doesn't support row level locking.
if isinstance(self.database_engine, PostgresEngine):
for room_id in {e.room_id for e, _ in events_and_contexts}:
txn.execute(
"SELECT room_version FROM rooms WHERE room_id = ? FOR SHARE",
(room_id,),
)
row = txn.fetchone()
if row is None:
raise Exception(f"Room does not exist {room_id}")
# stream orderings should have been assigned by now
assert min_stream_order
assert max_stream_order