pgsql: test_slru: Fix LWLock tranche allocation in EXEC_BACKEND builds.

test_slru: Fix LWLock tranche allocation in EXEC_BACKEND builds.

Currently, test_slru's shmem_startup_hook unconditionally generates
new LWLock tranche IDs. This is fine on non-EXEC_BACKEND builds,
where only the postmaster executes this hook, but on EXEC_BACKEND
builds, every backend executes it, too. To fix, only generate the
tranche IDs in the postmaster process by checking the
IsUnderPostmaster variable.

This is arguably a bug fix and could be back-patched, but since the
damage is limited to some extra unused tranche IDs in a test
module, I'm not going to bother.

Reported-by: Sami Imseih <samimseih(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAA5RZ0vaAuonaf12CeDddQJu5xKL%2B6xVyS%2B_q1%2BcH%3D33JXV82w%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/530cfa8eb50ca5a2151dfc50a6a5999ec8aff148

Modified Files
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src/test/modules/test_slru/test_slru.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)