pgsql: Refuse RI fast-path row locks in read-only transactions

Refuse RI fast-path row locks in read-only transactions

The SPI path checks a foreign key with SELECT ... FOR KEY SHARE, which
ExecCheckXactReadOnly() refuses in a read-only transaction unless the
referenced table is temporary. The fast path locks the referenced row
without that check, so for example a deferred check that fires at
COMMIT after SET TRANSACTION READ ONLY succeeded, having locked a row
in a read-only transaction, where the SPI path fails with "cannot
execute SELECT FOR KEY SHARE in a read-only transaction".

Fix the fast path to refuse it the same way so the behavior matches
with checks going through SPI.

Discussion: https://postgr.es/m/CA+HiwqG79XK1oObdZ2AwT660CeJ6s3Mn4LrFPCme-k4L2rF_ag@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/c993d9f7c8d4bcada0d5c0bd6ea32416d956fc8f

Modified Files
--------------
src/backend/utils/adt/ri_triggers.c | 10 ++++++++++
src/test/regress/expected/foreign_key.out | 31 +++++++++++++++++++++++++++++++
src/test/regress/sql/foreign_key.sql | 21 +++++++++++++++++++++
3 files changed, 62 insertions(+)