pgsql: Fix race in SSI interaction with empty btrees.

Fix race in SSI interaction with empty btrees.

When predicate-locking btrees, we have a special case for completely
empty btrees, since there is no page to lock. This was racy, because,
without buffer lock held, a matching key could be inserted between the
_bt_search() and the PredicateLockRelation() calls.

Fix, by rechecking _bt_search() after taking the relation-level SIREAD
lock, if using SERIALIZABLE isolation and an empty btree is discovered.

Back-patch to all supported releases. Fixes one aspect of bug #17949.

Reported-by: Artem Anisimov <artem(dot)anisimov(dot)255(at)gmail(dot)com>
Reviewed-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
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ae6d536ed0dcb5e29126975d4a07eb308fdc5cfa

Modified Files
--------------
src/backend/access/nbtree/nbtsearch.c | 39 +++++++++++++++++++++++------------
1 file changed, 26 insertions(+), 13 deletions(-)