Re: Fixing a couple of buglets in how VACUUM sets visibility map bits

On Mon, Jan 9, 2023 at 12:58 PM Peter Geoghegan <pg@bowt.ie> wrote:
> I didn't realize that affected visibilitymap_set() calls could
> generate useless set-VM WAL records until you pointed it out. That's
> far more likely to happen than the race condition that I described --
> it has nothing at all to do with concurrency. That's what clinches it
> for me.

I didn't spend as much time on this as I'd like to so far, but I think
that this concern about visibilitymap_set() actually turns out to not
apply. The visibilitymap_set() call in question is gated by a
"!VM_ALL_FROZEN()", which is enough to avoid the problem with writing
useless VM set records.

That doesn't make me doubt my original concern about races where the
all-frozen bit can be set, without setting the all-visible bit, and
without accounting for the fact that it changed underneath us. That
scenario will have !VM_ALL_FROZEN(), so that won't save us. (And we
won't test VM_ALL_VISIBLE() or PD_ALL_VISIBLE in a way that is
sufficient to realize that all_visible_according_to_vm is stale.
prunestate.all_visible being set doesn't reliably indicate that it's not stale,
but lazy_scan_heap incorrectly believes that it really does work that way.)


--
Peter Geoghegan