pgsql: Add tests for cross-session temp table access

Add tests for cross-session temp table access

Add a TAP test in src/test/modules/test_misc that documents what
happens when one session attempts to read or modify another session's
temporary table. This commit only adds tests; it does not change
backend behavior, so the assertions reflect current behavior:

- SELECT, UPDATE, DELETE, MERGE, COPY on a table without an index
silently succeed with no error and zero rows / zero affected rows.
These commands run through the read-stream path, which currently
bypasses the RELATION_IS_OTHER_TEMP() check. This is the
underlying bug to be fixed in a follow-up.
- INSERT errors with "cannot access temporary tables of other
sessions" because hio.c calls ReadBufferExtended() to find a page
with free space and is caught by the existing check there.
- Index scan errors via the same existing check, reached through
nbtree -> ReadBuffer -> ReadBufferExtended.
- TRUNCATE / ALTER TABLE / ALTER INDEX / CLUSTER fail with their
command-specific error messages.
- VACUUM is silently skipped to avoid noise during database-wide
VACUUM (vacuum_rel() returns without warning).
- DROP TABLE is intentionally allowed: DROP does not touch the
table's contents, and autovacuum relies on this to clean up
temp relations orphaned by a crashed backend.
- ALTER FUNCTION / DROP FUNCTION on an owner-created function over
its own temp row type work as catalog operations -- they don't
read the underlying data.
- CREATE FUNCTION from a separate session, using another session's
temp row type as an argument, is allowed but emits a NOTICE: the
function is moved into the creator's pg_temp namespace with an
auto-dependency on the borrowed type, so it disappears together
with the session that created it.
- A bare DROP TABLE on a temp table that has a cross-session
dependent function fails with a catalog-level dependency error.
- LOCK TABLE in ACCESS SHARE mode on another session's temp table
succeeds and properly blocks the owner's session-exit cleanup
(which acquires AccessExclusiveLock via findDependentObjects).
This exercises the same LockRelationOid path used by autovacuum
when cleaning up orphaned temp relations.
- When the owner session ends, the normal session-exit cleanup
cascades through DEPENDENCY_NORMAL and removes both the temp
objects and any cross-session functions that depended on them.

Also, document the contract for RELATION_IS_OTHER_TEMP() so that
future buffer-access entry points enforce the same rule.

Backpatch this through PostgreSQL 17, where b7b0f3f27241 introduces a code
path bypassing this check.

Author: Jim Jones <jim(dot)jones(at)uni-muenster(dot)de>
Author: Daniil Davydov <3danissimo(at)gmail(dot)com>
Co-authored-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Soumya S Murali <soumyamurali(dot)work(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAJDiXghdFcZ8%3Dnh4G69te7iRr3Q0uFyXxb3ZdG09_GTNZXwH0g%40mail.gmail.com
Backpatch-through: 17

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1fee0e857e330d148cf14b8d6027d02111841cf8

Modified Files
--------------
src/include/utils/rel.h | 14 ++
src/test/modules/test_misc/meson.build | 1 +
.../test_misc/t/013_temp_obj_multisession.pl | 267 +++++++++++++++++++++
3 files changed, 282 insertions(+)