Re: 64-bit XIDs in deleted nbtree pages

On Wed, Feb 10, 2021 at 7:10 PM Peter Geoghegan <pg@bowt.ie> wrote:
> Attached is v3 of the index. I'll describe the changes I made in more
> detail in my response to your points below.

I forget to mention that v3 adds several assertions like this one:

Assert(!_bt_page_recyclable(BufferGetPage(buf)));

These appear at a few key points inside generic routines like
_bt_getbuf(). The overall effect is that every nbtree buffer access
(with the exception of buffer accesses by VACUUM) will make sure that
the page that they're about to access is not recyclable (a page that
an index scan lands on might be half-dead or deleted, but it had
better not be recyclable).

This can probably catch problems with recycling pages too early, such
as the problem fixed by commit d3abbbeb back in 2012. Any similar bugs
in this area that may appear in the future can be expected to be very
subtle, for a few reasons. For one, a page can be recyclable but not
yet entered into the FSM by VACUUM for a long time. (I could go on.)

The assertions dramatically improve our chances of catching problems
like that early.

-- 
Peter Geoghegan