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message delete: Select Message FOR UPDATE when archiving.

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Further commits will start locking the message rows while
adding related fields like reactions or submessages,
to handle races caused by deleting the message itself at the
same time.

The message locking implemented then will create a possibility
of deadlocks, where the related field transaction holds a lock
on the message row, and the message-delete transaction holds a
lock on the database row of the related field (which will also
need to be deleted when the message is deleted), and both
transactions wait for each other.

To prevent such a deadlock, we lock the message itself while
it is being deleted, so that the message-delete transaction
will have to wait till the other transaction (which is about
to delete the related field, and also holds a lock on the
message row) commits.

https://chat.zulip.org/#narrow/near/1185943 has more details.
This commit is contained in:
Abhijeet Prasad Bodas
2021-06-03 19:16:13 +05:30
committed by Tim Abbott
parent 1a9f385e17
commit 5f4113cf60
5 changed files with 31 additions and 5 deletions
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2
zerver/lib/actions.py
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@@ -6156,7 +6156,7 @@ def do_delete_messages(realm: Realm, messages: Iterable[Message]) -> None:
move_messages_to_archive(message_ids, realm=realm, chunk_size=archiving_chunk_size)
event["message_type"] = message_type
send_event(realm, event, users_to_notify)
transaction.on_commit(lambda: send_event(realm, event, users_to_notify))
def do_delete_messages_by_sender(user: UserProfile) -> None: