pgsql: Fix race in SSI interaction with gin fast path.

Fix race in SSI interaction with gin fast path.

The ginfast.c code previously checked for conflicts in before locking
the relevant buffer, leaving a window where a RW conflict could be
missed. Re-order.

There was also a place where buffer ID and block number were confused
while trying to predicate-lock a page, noted by visual inspection.

Back-patch to all supported releases. Fixes one more problem discovered
with the reproducer from bug #17949, in this case when Dmitry tried
other index types.

Reported-by: Artem Anisimov <artem(dot)anisimov(dot)255(at)gmail(dot)com>
Reported-by: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Discussion: https://postgr.es/m/17949-a0f17035294a55e2%40postgresql.org

Branch
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REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/255a925d333be9a3d6314e2fe698063eb8e753a6

Modified Files
--------------
src/backend/access/gin/ginfast.c | 9 +++++++--
src/backend/access/gin/ginget.c | 4 +++-
2 files changed, 10 insertions(+), 3 deletions(-)