Re: post-recovery amcheck expectations

On Tue, Oct 24, 2023 at 07:03:34PM -0700, Peter Geoghegan wrote:
> On Mon, Oct 23, 2023 at 7:28 PM Noah Misch <noah@leadboat.com> wrote:
> > > That makes sense to me. I believe that it's not possible to have a
> > > string of consecutive sibling pages that are all half-dead (regardless
> > > of the BlockNumber order of sibling pages, even). But I'd probably
> > > have written the fix in roughly the same way. Although...maybe you
> > > should try to detect a string of half-dead pages? Hard to say if it's
> > > worth the trouble.
> >
> > I imagined a string of half-dead siblings could arise in structure like this:
> >
> >  *               1
> >  *          /    |    \
> >  *         4 <-> 2 <-> 3
> >
> > With events like this:
> >
> > - DELETE renders blk 4 deletable.
> > - Crash with concurrent VACUUM, leaving 4 half-dead after having visited 1-4.
> > - DELETE renders blk 2 deletable.
> > - Crash with concurrent VACUUM, leaving 2 half-dead after having visited 1-2.
> >
> > I didn't try to reproduce that, and something may well prevent it.
> 
> FWIW a couple of factors prevent it (in the absence of corruption). These are:
> 
> 1. Only VACUUM can delete pages, and in general the only possible
> source of half-dead pages is an unfortunately timed crash/error within
> VACUUM. Each interrupted VACUUM can leave behind at most one half-dead
> page.

Agreed.

> 2. One thing that makes VACUUM back out of deleting an empty page is
> the presence of a half-dead right sibling leaf page left behind by
> some VACUUM that was interrupted at some point in the past -- see
> _bt_rightsib_halfdeadflag() for details.
> 
> Obviously, factors 1 and 2 together make three consecutive half-dead
> sibling pages impossible.

Can't it still happen if the sequence of unfortunately timed crashes causes
deletions from left to right?  Take this example, expanding the one above.
Half-kill 4, crash, half-kill 3, crash, half-kill 2 in:

 *                  1
 *           /    / | \    \
 *         4 <-> 3 <-> 2 <-> 1

(That's not to say it has ever happened outside of a test.)