pgsql: Fix backend state after a failed after-startup shmem request

Fix backend state after a failed after-startup shmem request

RegisterShmemCallbacks() left the backend in a bad state, if an error
occurred in the callbacks or if an allocation failed. Firstly,
'shmem_request_state' was left in wrong state, causing a subsequent
call to RegisterShmemCallbacks() to wrongly take the postmaster
startup codepath or assertion failures in some other functions.
Secondly, the 'pending_shmem_requests' list was not properly cleaned
up, causing a subsequent RegisterShmemCallbacks() to try to process
the stale, already-freed requests.

To fix, add a PG_TRY() block to clean those things up on error.

Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/CAJTYsWVRRWH48=PcuAo_2Y4Ap6M0QRmzxgUfFkNRtdWK74LjBQ@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/92d44b200e12c7ecbaee57d37fcd5e8803811358

Modified Files
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src/backend/storage/ipc/shmem.c | 74 +++++++++++++++-------
.../modules/test_shmem/t/001_late_shmem_alloc.pl | 28 ++++++++
src/test/modules/test_shmem/test_shmem.c | 28 +++++++-
3 files changed, 105 insertions(+), 25 deletions(-)