pgsql: Fix possible crash when RI fast-path metadata is invalidated mid

Fix possible crash when RI fast-path metadata is invalidated mid-check

ri_FastPathCheck() read riinfo->fpmeta again after
ri_CheckFunctionPermissions(), which looks up catalog entries and so
can process invalidation messages. If one of them reaches
InvalidateConstraintCacheCallBack() for the constraint, as a pg_amop
change or a cache reset would, the callback detaches the metadata and
sets riinfo->fpmeta to NULL, which build_index_scankeys() then
dereferences.

The callback already defers freeing detached metadata until
AtEOXact_RI(), and its comment assumes callers keep their own pointer
to it, so use a local pointer instead of riinfo->fpmeta.

There is no test, as hitting this needs an invalidation to arrive
during those catalog lookups.

Discussion: https://postgr.es/m/CA+HiwqFaipkMsZ8XP-9sMk21h0Q7rih=NpJQXsBAaD39VtnrOA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e73e0196d30198e4579656881497f057788bb53d

Modified Files
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src/backend/utils/adt/ri_triggers.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)