[BUG] wrong FK constraint name when colliding name on ATTACH

Hi,

While studying and hacking on the parenting constraint issue, I found an
incoherent piece of code leading to badly chosen fk name. If a constraint
name collision is detected, while choosing a new name for the constraint,
the code uses fkconstraint->fk_attrs which is not yet populated:

/* No dice. Set up to create our own constraint */
fkconstraint = makeNode(Constraint);
if (ConstraintNameIsUsed(CONSTRAINT_RELATION,
RelationGetRelid(partRel),
NameStr(constrForm->conname)))
fkconstraint->conname =
ChooseConstraintName(RelationGetRelationName(partRel),
ChooseForeignKeyConstraintNameAddition(
fkconstraint->fk_attrs), // <= WOO000OPS
"fkey",
RelationGetNamespace(partRel), NIL);
else
fkconstraint->conname = pstrdup(NameStr(constrForm->conname));
fkconstraint->fk_upd_action = constrForm->confupdtype;
fkconstraint->fk_del_action = constrForm->confdeltype;
fkconstraint->deferrable = constrForm->condeferrable;
fkconstraint->initdeferred = constrForm->condeferred;
fkconstraint->fk_matchtype = constrForm->confmatchtype;
for (int i = 0; i < numfks; i++)
{
Form_pg_attribute att;

att = TupleDescAttr(RelationGetDescr(partRel),
mapped_conkey[i] - 1);
fkconstraint->fk_attrs = lappend(fkconstraint->fk_attrs, // <= POPULATING
makeString(NameStr(att->attname)));
}

The following SQL script showcase the bad constraint name:

DROP TABLE IF EXISTS parent, child1;

CREATE TABLE parent (
id bigint NOT NULL default 1,
no_part smallint NOT NULL,
id_abc bigint,
CONSTRAINT dummy_constr FOREIGN KEY (id_abc, no_part)
REFERENCES parent(id, no_part) ON UPDATE RESTRICT ON DELETE RESTRICT,
PRIMARY KEY (id, no_part)
)
PARTITION BY LIST (no_part);

CREATE TABLE child1 (
id bigint NOT NULL default 1,
no_part smallint NOT NULL,
id_abc bigint,
PRIMARY KEY (id, no_part),
CONSTRAINT dummy_constr CHECK ((no_part = 1))
);

ALTER TABLE parent ATTACH PARTITION child1 FOR VALUES IN ('1');

SELECT conname
FROM pg_constraint
WHERE conrelid = 'child1'::regclass
AND contype = 'f';

DROP TABLE
CREATE TABLE
CREATE TABLE
ALTER TABLE

conname
--------------
child1__fkey
(1 row)

The resulting constraint name "child1__fkey" is missing the attributes name the
original code wanted to add. The expected name is "child1_id_abc_no_part_fkey".

Find in attachment a simple fix, moving the name assignation after the
FK attributes are populated.

Regards,