pgsql: Give RI fast-path cached FmgrInfos their own memory context

Give RI fast-path cached FmgrInfos their own memory context

ri_populate_fastpath_metadata() copies the cast and equality FmgrInfos
into the cached FastPathMeta with fn_mcxt set to TopMemoryContext, the
context active at the copy. fn_mcxt is scratch space for the called
function: record_eq(), and the record I/O functions generally, allocate
their per-call cache there and keep a pointer to it in fn_extra.

Because that scratch is in TopMemoryContext, it outlives the metadata.
When the metadata is discarded on invalidation, whatever the cast and
equality functions cached is left behind, with nothing pointing at it.
Each subsequent repopulation allocates afresh, so a session that
repeatedly invalidates a foreign key constraint grows TopMemoryContext
without bound.

Give the metadata its own context for the FmgrInfos' fn_mcxt and delete
it along with the metadata, so the cached scratch is freed with the
FmgrInfos that point at it. Since the preceding commit defers release
of the metadata to AtEOXact_RI(), the context is deleted there rather
than in InvalidateConstraintCacheCallBack().

The context deliberately is not reset while the metadata is in use.
fn_extra points into fn_mcxt, so resetting it would leave those
pointers dangling; the next call would find fn_extra non-NULL and read
freed memory. fmgr_info_copy() zeroing fn_extra in the copy is the
same invariant seen from the other side. Nothing accumulates in the
context during use in any case: record_eq() and friends allocate only
when fn_extra is NULL and reuse the cache afterwards.

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
Discussion: https://postgr.es/m/CA+HiwqFFB6vzx8v3t2=rbNYyxMistLf5kkJfqzJ81nadFyLrxA@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/590b8e89a53a4637b07cf5f4ccb290d4c29ec3b7

Modified Files
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src/backend/utils/adt/ri_triggers.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)