Re: BUG #15896: pg_upgrade from 10-or-earlier: TRAP: FailedAssertion(»!(metad->btm_version >= 3)«

On Fri, Jul 5, 2019 at 8:49 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> > TRAP: FailedAssertion(»!(metad->btm_version >= 3)«, Datei:
> > »/build/postgresql-12-3URvLF/postgresql-12-12~beta2/build/../src/backend/access/nbtree/nbtpage.c«,
> > Zeile: 665)
>
> Seems that _bt_getrootheight is too optimistic about the metapage
> version it'll find.  I suppose this could be handled by just not caching
> the metapage if it is of the old version ... or maybe by calling
> _bt_upgrademetapage().

The problem here predates v12 -- the call to _bt_cachemetadata() was
added to _bt_getrootheight() by commit 0a64b45152b, which went into
v11. My commit dd299df8189 added a new assertion that fails, but
that's just a symptom -- I changed the code in  _bt_getrootheight() to
use a BTMetaPageData pointer to shared memory (i.e. a pointer to the
authoritative version), rather than using the newly-out-of-sync cached
version. It shouldn't be out-of-sync at all.

_bt_getrootheight() is mostly just something that exists for the
planner, so it has no business calling _bt_cachemetadata(), which will
"upgrade" the cached metadata image from version 2 to version 3 if it
happens to be on version 2. How can it be okay to upgrade the cached
version without also upgrading the on-disk/shared_buffers version?
This bug was hiding in plain sight.

-- 
Peter Geoghegan