Re: Adding REPACK [concurrently]

Hello!

On Thu, Apr 9, 2026 at 4:20 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> But with my proposal to properly teach the deadlock detector about
assuming
> there's a wait edge for the eventual lock upgrade by S1, the first example
> would still work, because the lock upgrade would not be considered a hard
> cycle, and the second example would have S2 error out.

Attached patch contains some (maybe naive) POC for similar approach.
It adds a 'deadlock_protected' flag, which changes how the deadlock
detector cancels backends.

Instead of cancelling the backend entered the deadlock detector - it
cancel some another (nearest hard edge) until it is possible to get the
lock (either by
reordering or directly).

Also, I added test cases with the scenarios you mentioned into the repack
spec - to ensure repack is still working while other backend are cancelled.

> > Anti-wraparound (failsafe) VACUUM is a bit different case [1] (i.e. it
should
> > possibly have higher priority than REPACK), but I think this
prioritization
> > should be implemented in other way than just letting it get in the way
of
> > REPACK (at the time REPACK is nearly finished).
>
> Yea, it makes no sense to interrupt the long running repack, given that
the
> new relation will have much less stuff for vacuum to do.

For now I decided to keep anti-wraparound unaffected because the
feature is may be used not only for repack but also for other potential
scenarios.
But yes, maybe it's worth canceling antiwraparound in the repack case.
Also, checking proc flags to detect antuwraparound requires
LWLockConditionalAcquire(ProcArrayLock) - something I don't like in
deadlock detector.

Best regards,
Mikhail.