Alex Vandiver 7720d362ba messages: Only lock UserMessage rows, not other joined tables. ...

By default, `SELECT FOR UPDATE` will also lock any rows which are
`JOIN`ed into the selected rows; in the case of UserMessage rows, this
can mean arbitrary Message rows.

Since the messages themselves are not being changed, it is not
necessary to lock them -- and doing so may lead to deadlocks, in the
case that the UserMessage row is locked for update before the Message,
and some other request has already taken a read lock on the Message
and is blocked on the UserMessage write lock.

Change `select_for_update_query` to explicitly only lock UserMessage.

2024-01-09 13:13:59 -08:00

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