Re: PG19 FK fast path: OOB write and missed FK checks during batched

On Mon, Aug 31, 2026 at 5:57 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> On Sat, Aug 29, 2026 at 8:06 PM Ayush Tiwari
> <ayushtiwari(dot)slg01(at)gmail(dot)com> wrote:
> > On Thu, 27 Aug 2026 at 18:11, Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> >> On Wed, Aug 26, 2026 at 10:25 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> >> > On Sat, Aug 22, 2026 at 4:43 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> >> > > Pushed and closed the item.
> >> >
> >> > Ayush Tiwari reported another hole in the per-firing-cycle fix to me
> >> > off-list. Here is his reproducer:
> >> >
> >> > CREATE TABLE pk (id int PRIMARY KEY);
> >> > INSERT INTO pk VALUES (1);
> >> >
> >> > CREATE TABLE fk (
> >> > a int REFERENCES pk (id),
> >> > b int CONSTRAINT fk_deferred REFERENCES pk (id)
> >> > DEFERRABLE INITIALLY DEFERRED);
> >> >
> >> > CREATE FUNCTION check_now() RETURNS trigger LANGUAGE plpgsql AS $$
> >> > BEGIN
> >> > BEGIN
> >> > SET CONSTRAINTS fk_deferred IMMEDIATE;
> >> > EXCEPTION WHEN foreign_key_violation THEN
> >> > RAISE NOTICE 'caught by SET CONSTRAINTS';
> >> > END;
> >> > RETURN NEW;
> >> > END$$;
> >> >
> >> > -- Name sorts after the RI trigger, so column a is already batched.
> >> > CREATE TRIGGER zz_check_now AFTER INSERT ON fk
> >> > FOR EACH ROW EXECUTE FUNCTION check_now();
> >> >
> >> > BEGIN;
> >> > INSERT INTO fk VALUES (1, 999);
> >> >
> >> > On the current master with my fixes from last week, an
> >> > assertion-enabled build produces:
> >> >
> >> > CREATE TABLE
> >> > INSERT 0 1
> >> > CREATE TABLE
> >> > CREATE FUNCTION
> >> > CREATE TRIGGER
> >> > BEGIN
> >> > WARNING: resource was not closed: relation "pk_pkey"
> >> > WARNING: resource was not closed: relation "pk"
> >> > WARNING: resource was not closed: TupleDesc 0xffff812677f0 (16392,-1)
> >> > WARNING: resource was not closed: TupleDesc 0xffff81270500 (16386,-1)
> >> > server closed the connection unexpectedly
> >> > This probably means the server terminated abnormally
> >> > before or while processing the request.
> >> > The connection to the server was lost.
> >> >
> >> > The relevant part of the backtrace is:
> >> >
> >> > #3 ExceptionalCondition (conditionName="false",
> >> > fileName="../src/backend/utils/adt/ri_triggers.c", lineNumber=4543)
> >> > #4 AtEOSubXact_RI (isCommit=true, mySubid=2, parentSubid=1)
> >> > at ../src/backend/utils/adt/ri_triggers.c:4543
> >> > #5 CommitSubTransaction ()
> >> > at ../src/backend/access/transam/xact.c:5247
> >> > #6 ReleaseCurrentSubTransaction ()
> >> > at ../src/backend/access/transam/xact.c:4836
> >> > #7 exec_stmt_block (...)
> >> > at ../src/pl/plpgsql/src/pl_exec.c:1859
> >> >
> >> > SET CONSTRAINTS ... IMMEDIATE starts a nested firing cycle without
> >> > opening a new query level, so keying the cache by constraint OID and
> >> > query depth does not distinguish this cycle from the enclosing one.
> >> >
> >> > I am preparing a patch that uses firing depth, gives the SET
> >> > CONSTRAINTS cycle its own callback list, and removes stale entries on
> >> > either subtransaction commit or abort. I will post it shortly.
> >>
> >> Here is that patch.
> >>
> >> One thing missing from my report: the crash wasn't the worst of it.
> >> The nested cycle's batch was never flushed, so SET CONSTRAINTS ...
> >> IMMEDIATE returned success for a constraint it hadn't checked, and
> >> the orphan row reached commit.
> >>
> >> I'm not proposing to commit this yet. I'm assessing whether the
> >> batching layer holds up under a test harness and will give the RMT my
> >> assessment by Monday on whether to keep it in v19. If the batching
> >> layer is reverted from v19, this patch is master-only, assuming it's
> >> ok for master to keep the layer in its current form. If it stays, it
> >> belongs in both branches. I'm happy to commit and backpatch sooner if
> >> that's preferred for the open item.
> >
> >
> > Thanks for the patch. I'm aware that you are still testing this but wanted to
> > add that I do see a problem when ALTER TABLE ... ADD FORIEGN KEY
> > is run from an AFTER trigger.
>
> Thanks for the report.
>
> > I tried such a case in a non-cassert build, with RLS forcing the existing rows
> > to be checked one at a time. The command appeared to succeed and the
> > constraint was marked valid, even though the table still contained an orphan
> > row. I also saw warnings about relation and TupleDesc resources not being
> > closed.
> >
> > The reproducer I used was:
> >
> > CREATE ROLE fp_alter_role;
> > CREATE TABLE fp_alter_pk (id int PRIMARY KEY);
> > INSERT INTO fp_alter_pk VALUES (1);
> > ALTER TABLE fp_alter_pk ENABLE ROW LEVEL SECURITY;
> > CREATE POLICY fp_alter_pk_all ON fp_alter_pk USING (true);
> > GRANT REFERENCES, SELECT ON fp_alter_pk TO fp_alter_role;
> >
> > CREATE TABLE fp_alter_fk (a int);
> > INSERT INTO fp_alter_fk VALUES (1), (999);
> > ALTER TABLE fp_alter_fk OWNER TO fp_alter_role;
> >
> > CREATE TABLE fp_alter_outer (a int);
> > ALTER TABLE fp_alter_outer OWNER TO fp_alter_role;
> >
> > CREATE FUNCTION fp_alter_from_trigger() RETURNS trigger
> > LANGUAGE plpgsql AS $$
> > BEGIN
> > BEGIN
> > EXECUTE 'ALTER TABLE fp_alter_fk ADD CONSTRAINT '
> > 'fp_alter_bad_fk FOREIGN KEY (a) '
> > 'REFERENCES fp_alter_pk(id)';
> > EXCEPTION WHEN others THEN
> > RAISE;
> > END;
> > RETURN NEW;
> > END
> > $$;
> >
> > CREATE TRIGGER fp_alter_trg
> > AFTER INSERT ON fp_alter_outer
> > FOR EACH ROW EXECUTE FUNCTION fp_alter_from_trigger();
> >
> > SET ROLE fp_alter_role;
> > INSERT INTO fp_alter_outer VALUES (1);
> > RESET ROLE;
> >
> > SELECT conname, convalidated
> > FROM pg_constraint
> > WHERE conname = 'fp_alter_bad_fk';
> > TABLE fp_alter_fk;
> >
> > Could the validation calls be joining the outer trigger's batch because
> > AfterTriggerIsActive() is true, and then be removed by AtEOSubXact_RI() before
> > they are checked? The validation path sets trig.tgoid to InvalidOid, so would
> > it make sense to use that to keep these calls on the non-batched path?
>
> Your diagnosis is correct; tgoid would work. Though, I'd rather make
> the caller state that explicitly instead of gleaning it from
> trigger.c-internal state, which is what I should have done originally.
> Add a allow_batch parameter to RI_FKey_check() and new validation
> function called from ALTER TABLE code, instead of RI_FKey_check_ins(),
> which calls RI_FKey_check() with 'false' for allow_batch. Attached
> 0001 does that and also contains your test case. 0002 unchanged.

Added a separate open item for this, so there are two for 0001 and 0002, resp.

Fixed by 0001 (just added):
RI fast-path batching wrongly used by ALTER TABLE inside a trigger
Commit: b7b27eb41a5
Owner: Amit Langote

Fixed by 0002:
RI fast-path batching fails during nested SET CONSTRAINTS
Commit: 6fc2a486417d
Owner: Amit Langote

--
Thanks, Amit Langote