pgsql: Fix RI fast-path race with REINDEX CONCURRENTLY

Fix RI fast-path race with REINDEX CONCURRENTLY

The RI fast path reads pg_constraint.conindid before taking
RowShareLock on the referenced table. REINDEX CONCURRENTLY can
repoint the constraint and mark the old index dead, or drop it,
between those operations. A backend in that window does not yet
hold a relation lock, so it is not covered by REINDEX CONCURRENTLY's
waits for lockers.

Opening an index that has already been dropped produces "could not open
relation with OID". Opening one that has only been marked dead can
produce wrong answers: the index is no longer maintained or vacuumed,
so a scan can miss a referenced row or follow a stale entry to a reused
heap line pointer.

After locking the referenced table, reload the constraint and use its
current conindid. LockRelationOid() processes invalidation messages
after acquiring the lock, so the reload sees a committed index swap.
If the lock was already held, REINDEX CONCURRENTLY cannot mark the old
index dead or drop it until the transaction releases that lock, so
continuing to use the old conindid is safe.

Do this at both RI fast-path call sites. Add injection-point coverage
for old indexes that have either been dropped or marked dead.

Author: Mihail Nikalayeu <mihailnikalayeu(at)gmail(dot)com>
Discussion: https://postgr.es/m/CADzfLwUJiVuv69uwuF5z4TrMhNkVwQUXW03q+uVNwmYFLtjEhw@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ec51b80b348fe34da15cfb991191ef0bdc4838c6

Modified Files
--------------
src/backend/commands/indexcmds.c | 1 +
src/backend/utils/adt/ri_triggers.c | 20 +++
src/test/modules/injection_points/Makefile | 1 +
.../expected/ri_fastpath_reindex.out | 171 +++++++++++++++++++++
src/test/modules/injection_points/meson.build | 1 +
.../specs/ri_fastpath_reindex.spec | 108 +++++++++++++
6 files changed, 302 insertions(+)