[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,