[BUG] FK broken after DETACHing referencing part

Hi,

Considering two partitionned tables with a FK between them:

DROP TABLE IF EXISTS p, c, c_1 CASCADE;

----------------------------------
-- Parent table + partition + data
CREATE TABLE p (
id bigint PRIMARY KEY
)
PARTITION BY list (id);

CREATE TABLE p_1 PARTITION OF p FOR VALUES IN (1);

INSERT INTO p VALUES (1);

------------------------------------
-- Child table + partition + data
CREATE TABLE c (
id bigint PRIMARY KEY,
p_id bigint NOT NULL,
FOREIGN KEY (p_id) REFERENCES p (id)
)
PARTITION BY list (id);

CREATE TABLE c_1 PARTITION OF c FOR VALUES IN (1);

INSERT INTO c VALUES (1,1);

After DETACHing the "c_1" partition, current implementation make sure it
keeps the FK herited from its previous top table "c":

ALTER TABLE c DETACH PARTITION c_1;
\d c_1
-- outputs:
-- [...]
-- Foreign-key constraints:
-- "c_p_id_fkey" FOREIGN KEY (p_id) REFERENCES p(id)

However, because the referenced side is partionned, this FK is half backed, with
only the referencing (insert/update on c_1) side enforced, but not the
referenced side (update/delete on p):

INSERT INTO c_1 VALUES (2,2); -- fails as EXPECTED
-- ERROR: insert or update on table "child_1" violates foreign key [...]

DELETE FROM p; -- should actually fail
-- DELETE 1

SELECT * FROM c_1;
-- id | parent_id
-- ----+-----------
-- 1 | 1
-- (1 row)

SELECT * FROM p;
-- id
-- ----
-- (0 rows)

When detaching "c_1", current implementation adds two triggers to enforce
UPDATE/DELETE on "p" are restricted if "c_1" keeps referencing the
related rows... But it forgets to add them on partitions of "p_1", where the
triggers should actually fire.

To make it clear, the FK c_1 -> p constraint and triggers after DETACHING c_1
are:

SELECT c.oid AS conid, c.conname, c.conparentid AS conparent,
r2.relname AS pkrel,
t.tgrelid::regclass AS tgrel,
p.proname
FROM pg_constraint c
JOIN pg_class r ON c.conrelid = r.oid
JOIN pg_class r2 ON c.confrelid = r2.oid
JOIN pg_trigger t ON t.tgconstraint = c.oid
JOIN pg_proc p ON p.oid = t.tgfoid
WHERE r.relname = 'c_1' AND r2.relname LIKE 'p%'
ORDER BY r.relname, c.conname, t.tgrelid::regclass::text, p.proname;

-- conid | conname | conparent | pkrel | tgrel | proname
-- -------+-------------+-----------+-------+-------+----------------------
-- 18454 | c_p_id_fkey | 0 | p | c_1 | RI_FKey_check_ins
-- 18454 | c_p_id_fkey | 0 | p | c_1 | RI_FKey_check_upd
-- 18454 | c_p_id_fkey | 0 | p | p | RI_FKey_noaction_del
-- 18454 | c_p_id_fkey | 0 | p | p | RI_FKey_noaction_upd

Where they should be:

-- conid | conname | conparent | pkrel | tgrel | proname
-- -------+--------------+-----------+-------+-------+----------------------
-- 18454 | c_p_id_fkey | 0 | p | c_1 | RI_FKey_check_ins
-- 18454 | c_p_id_fkey | 0 | p | c_1 | RI_FKey_check_upd
-- 18454 | c_p_id_fkey | 0 | p | p | RI_FKey_noaction_del
-- 18454 | c_p_id_fkey | 0 | p | p | RI_FKey_noaction_upd
-- NEW!! | c_p_id_fkey1 | 18454 | p_1 | p_1 | RI_FKey_noaction_del
-- NEW!! | c_p_id_fkey1 | 18454 | p_1 | p_1 | RI_FKey_noaction_upd

I poked around DetachPartitionFinalize() to try to find a way to fix this, but
it looks like it would duplicate a bunch of code from other code path (eg.
from CloneFkReferenced).

Instead of tweaking existing FK, keeping old constraint name (wouldn't
"c_1_p_id_fkey" be better after detach?) and duplicating some code around, what
about cleaning up the FK constraints from the detached table and
recreating a cleaner one using the known code path ATAddForeignKeyConstraint() ?

Thanks for reading me down to here!

++