pgsql: GiST: Invalidate killed items consistently.

GiST: Invalidate killed items consistently.

GiST neglected to invalidate its killedItems[] array on a rescan. As a
result, it was just about possible for the wrong tuples from the wrong
index page to be LP_DEAD-marked on a rescan. The scan mistakenly
believed that the previous rescan's killedItems[] were for this rescan's
curBlkno, causing index corruption.

To fix, bring GiST in line with nbtree and hash: call gistkillitems from
both gistrescan and gistendscan (the existing gistgettuple caller still
handles the common case where we need to LP_DEAD-mark before moving on
to the next page). That way the scan's pending killedItems[] are passed
to gistkillitems while they still describe items from curBlkno. When
gistkillitems runs, it'll invalidate the array in passing (and won't
needlessly miss out on an opportunity to LP_DEAD-mark eligible index
tuples). Back branches just get minimal hardening: we invalidate
killedItems[] at the places where the master branch gets new calls to
gistkillitems (and we invalidate curBlkno and curPageLSN on a rescan).

The test that proved corruption on master didn't result in corruption on
any stable branch, though only because, without commit 9c9ddf109, we'd
clobber curPageLSN without also updating curBlkno -- which accidentally
prevented it. Relying on gistkillitems to not LP_DEAD-mark by passing
it a curBlkno whose curPageLSN was taken from an entirely different page
seems like a very bad idea, which is why this issue is being treated as
a bug affecting all stable branches.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-By: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Discussion: https://postgr.es/m/CAH2-WzmwEThnQf17Ju+t0N9_KJLsEQSXzYrFnaS2=s4KnGGrqw@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bf25c5f832fed181505bbbd46f8ce17a2583b79f

Modified Files
--------------
src/backend/access/gist/gistget.c | 4 ++++
src/backend/access/gist/gistscan.c | 8 ++++++++
2 files changed, 12 insertions(+)