BUG #19720: pg_trgm GiST index corruption from gtrgm_union() dropping SIGNKEY

The following bug has been logged on the website:

Bug reference: 19720
Logged by: Nate Clark
Email address: natec425(at)gmail(dot)com
PostgreSQL version: 18.6
Operating system: macOS 26 (aarch64)
Description:

Hey all,

I experienced index corruption for a trigram GiST index at work. I believe
it is due to an invalid `TRGM->flag` state.

Currently, `gtrgm_union` sets `result->flag = ALLISTRUE` when it determines
that the signature is all true, but this drops the `SIGNKEY` bit. Downstream
of this, `unionkey` branches on the flag and defaults to the array handling
else branch. This array branch interprets the state as a 0 length array and
produces an empty signature. These two pieces together mean that an insert
in this bad flag state will produce a downlink with only the signature bits
for that new inserted value rather than the true union of the children.

I believe the fix is to change it to `|= ALLISTRUE` (similar to the other
GiST op classes). I've tested this change locally against the following
repro script.

I get the following output against `master` and 18.6 (with some small jitter
in total count):

what | count
-----------------------+-------
tests (total minus 3) | 0
total | 33120

With the `|=` patch I get:

what | count
-----------------------+-------
tests (total minus 3) | 33130
total | 33133

Thanks so much for your time,
Nate Clark

-- repro script
create extension if not exists pg_trgm;
create extension if not exists pageinspect;
drop table if exists t;
create table t (v text);
create index t_idx on t
-- siglen 4 to make it simpler to hit ALLISTRUE
using gist (v gist_trgm_ops(siglen=4));

-- Insert until the root splits as an internal node
-- so we get signature, not array, logic.
do $$
begin
-- insert a null to hit the null handling branch involving gtrgm_union
insert into t values (null);
-- insert an ALLISTRUE input
insert into t select string_agg(i::text, ' ')
from generate_series(1, 100) i;
-- insert until the root splits
loop
insert into t values ('test');
exit when exists (
select from gist_page_items_bytea(get_raw_page('t_idx', 0)) r,
gist_page_opaque_info(get_raw_page('t_idx',
(r.ctid::text::point)[0]::int)) c
where r.itemoffset = 1 and not 'leaf' = any(c.flags));
end loop;
end $$;

-- Insert one row whose trigrams set 0 bits.
-- This causes all children to be unreachable.
insert into t values ('');

select 'total' as what, count(*) from t
union
select 'tests (total minus 3)', count(*) from t where v = 'test';