Can Django's select_for_update deadlock when used on the same record twice?

Question

I have some code that would be a lot simpler with nested calls to select_for_update(), but I'm afraid I'm setting myself up for a deadlock.

Example:

with transaction.atomic():
    uu1 = UniUser.objects.select_for_update().get(pk=uupk)
    with transaction.atomic():
        uu2 = UniUser.objects.select_for_update().get(pk=uupk)  # why no deadlock here??
        uu2.bio = 'i like pizza'
        uu2.save()
    uu1.bio = 'i like pie'
    uu1.save()

I would have expected the second select_for_update() to deadlock since it's trying to lock an already locked record, but this code runs just fine and we end up with 'i like pie' in the bio. Ditto if I remove the inner transaction and just lock the same record twice in the same transaction.

Why is there no deadlock here? Is this safe code?

In case the answer is database specific, I'm using Postgres.

score 3 · Accepted Answer · answered Jul 16 '20 at 20:28

3

The lock is acquired and held by your transaction.

When you attempt to grab it again, nothing happens as your transaction already hods a lock on that row.

answered Jul 16 '20 at 20:28

Mike Organek

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Well that's handy! – mgalgs Jul 16 '20 at 20:35
2

This works because the lock acquired by `SELECT FOR UPDATE` is [reentrant](https://stackoverflow.com/a/1312282/2263517). – knaperek Sep 13 '21 at 11:15

Can Django's select_for_update deadlock when used on the same record twice?

1 Answers1