Re: group locking: incomplete patch, just for discussion

On Thu, Nov 20, 2014 at 12:19 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> Except that it opens us up for all kinds of concurrency bugs. I'm pretty
> strictly set against granting any self exclusive locks en-masse. If we
> do this by default for all granted locks when starting a worker backend
> it'll get *so* much harder to reason about correctness. Suddenly locks
> don't guarantee what they used to anymore. We'll e.g. not be able to
> rely that a CheckTableNotInUse() + AEL makes it safe to
> drop/rewrite/whatever a relation - even if that happens in the main
> backend.

Haven't I responded to this a few times already?

I see no way, even theoretically, that it can be sane for
CheckTableNotInUse() to succeed in a parallel context. Ever. If the
deadlock detector would kill the processes anyway, it doesn't matter,
because CheckTableNotInUse() should do it first, so that we get a
better error and without waiting for deadlock_timeout. So any
scenario that's predicated on the assumption that CheckTableNotInUse()
will succeed in a parallel context is 100% unconvincing to me as an
argument for anything.

--
Robert Haas
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