Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Fri, Feb 14, 2020 at 9:13 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Wed, Feb 12, 2020 at 11:53 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> > That's an interesting idea, but it doesn't make the lock acquisition
> > itself interruptible, which seems pretty important to me in this case.
>
> Good point: if you think the contained operation might run too long to
> suit you, then you don't want other backends to be stuck behind it for
> the same amount of time.
>

It is not clear to me why we should add that as a requirement for this
patch when other places like WALWriteLock, etc. have similar coding
patterns and we haven't heard a ton of complaints about making it
interruptable or if there are then I am not aware.

> > I wonder if we could have an LWLockAcquireInterruptibly() or some such
> > that allows the lock acquisition itself to be interruptible. I think
> > that would require some rejiggering but it might be doable.
>
> Yeah, I had the impression from a brief look at LWLockAcquire that
> it was itself depending on not throwing errors partway through.
> But with careful and perhaps-a-shade-slower coding, we could probably
> make a version that didn't require that.
>

If this becomes a requirement to move this patch, then surely we can
do that. BTW, what exactly we need to ensure for it? Is it something
on the lines of ensuring that in error path the state of the lock is
cleared? Are we worried that interrupt handler might do something
which will change the state of lock we are acquiring?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com