pgsql: Begin RI fast-path index scan under the switched user id

Begin RI fast-path index scan under the switched user id

ri_FastPathCheck() called index_beginscan() before switching to the
referenced relation's owner for the permission check and probe. For
btree this has no consequence, but starting the scan before the user id
switch sets a poor example for code dealing with out-of-tree access
methods, whose beginscan could observe the wrong user id. Move
index_beginscan() after the SetUserIdAndSecContext() call.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260705210533.ee.noahmisch@microsoft.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7c1c2f0b8d837fa49df434ecc4438e7d3476144e

Modified Files
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src/backend/utils/adt/ri_triggers.c | 27 ++++++++++++++++++++-------
1 file changed, 20 insertions(+), 7 deletions(-)