Alex Vandiver 1184bdc934 push_notifications: Lock message while we mark it pending for push. ...

Deleting a message can race with sending a push notification for it.
b47535d8bb handled the case where the Message row has gone away --
but in such cases, it is also possible for `access_message` to
succeed, but for the save of `user_message.flags` to fail, because the
UserMessage row has been deleted by then.

Take a lock on the Message row over the accesses of, and updates to,
the relevant UserMessage row.  This guarantees that the
message's (non-)existence is consistent across that transaction.

Partial fix for #16502.

2023-05-18 11:53:21 -07:00

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