Re: Odd code around ginScanToDelete

On Tue, Feb 3, 2026 at 6:26 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> While looking at converting more places to UnlockReleaseBuffer(), in the
> course of making UnlockReleaseBuffer() faster than the two separate
> operations, I found this code:
>
> static bool
> ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
> DataPageDeleteStack *parent, OffsetNumber myoff)
> ...
>
> if (!meDelete)
> {
> if (BufferIsValid(me->leftBuffer))
> UnlockReleaseBuffer(me->leftBuffer);
> me->leftBuffer = buffer;
> }
> else
> {
> if (!isRoot)
> LockBuffer(buffer, GIN_UNLOCK);
>
> ReleaseBuffer(buffer);
> }
>
> if (isRoot)
> ReleaseBuffer(buffer);
>
>
> Which sure looks like it'd release buffer twice if isRoot is set? I guess
> that's not reachable, because presumably the root page will always go down the
> !meDelete path. But it sure made me wonder if there's a hard to reach bug.

Yes, it's not possible to have meDelete set for root, because
me->leftBuffer is always InvalidBuffer for the root. So the branch
handling meDelete case should better do Assert(!isRoot).

> This code was introduced in
> commit e14641197a5
> Author: Alexander Korotkov <akorotkov(at)postgresql(dot)org>
> Date: 2019-11-19 23:07:36 +0300
>
> Fix deadlock between ginDeletePage() and ginStepRight()
>
> I didn't trace it further to see if it existed before that in some fashion.

Yes. I think generally this area needs to be reworked to become more
clear, and have vast more comments. It was wrong from my side trying
to fix bugs there without reworking it into something more
appropriate. I'm planning to put work on this during this week.

> There's another oddity here: ginScanToDelete() requires that the root page has
> been locked by the caller already, but will afaict re-read the root page? But
> then have code to avoid locking it again, because that would not have worked?
> Seems odd.

It seems a bit odd for me that caller already have locked buffer, but
passes BlockNumber making us re-read the buffer. But I'm not sure
that's the same as your point. Could you, please, elaborate more on
this?

------
Regards,
Alexander Korotkov
Supabase