Re: POC: Cleaning up orphaned files using undo logs

On Wed, Aug 21, 2019 at 6:38 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > FWIW, although I also thought of doing what you are describing here, I
> > think Andres's proposal is probably preferable, because it's simpler.
> > There's not really any reason why we can't take the buffer locks from
> > within the critical section, and that way callers don't have to deal
> > with the extra step.
>
> IIRC, the reason this was done before starting critical section was
> because of coding convention mentioned in src/access/transam/README
> (Section: Write-Ahead Log Coding). It says first pin and exclusive
> lock the shared buffers and then start critical section. It might be
> that we can bypass that convention here, but I guess it is mainly to
> avoid any error in the critical section. I have checked the
> LWLockAcquire path and there doesn't seem to be any reason that it
> will throw error except when the caller has acquired many locks at the
> same time which is not the case here.

Yeah, I think it's fine to deviate from that convention in this
respect. We treat LWLockAcquire() as a no-fail operation in many
places; in my opinion, that elog(ERROR) that we have for too many
LWLocks should be changed to elog(PANIC) precisely because we do treat
LWLockAcquire() as no-fail in lots of places in the code, but I think
I suggested that once and got slapped down, and I haven't had the
energy to fight about it.

--
Robert Haas
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