pgsql: Fix possible use after free in expand_partitioned_rtentry()

Fix possible use after free in expand_partitioned_rtentry()

It's possible that if the only live partition is concurrently dropped
and try_table_open() fails, that the bms_del_member() will pfree the
live_parts Bitmapset. Since the bms_del_member() call does not assign
the result back to the live_parts local variable, the while loop could
segfault as that variable would still reference the pfree'd Bitmapset.

Backpatch to 15. 52f3de874 was backpatched to 14, but there's no
bms_del_member() there due to live_parts not yet existing in RelOptInfo in
that version. Technically there's no bug in version 15 as
bms_del_member() didn't pfree when the set became empty prior to
00b41463c (from v16). Applied to v15 anyway to keep the code similar and
to avoid the bad coding pattern.

Author: Bernd Reiß <bd_reiss(at)gmx(dot)at>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/6b88f27a-c45c-4826-8e37-d61a04d90182@gmx.at
Backpatch-through: 15

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8286bce0a06a3975bc7d292b5c5f972fc31a9a57

Modified Files
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src/backend/optimizer/util/inherit.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)