A MariaDB stored function which causes a multi-valued insert to partially complete

CREATE TABLE partners (
  user_id INT NOT NULL,
  partner_id INT NOT NULL,
  PRIMARY KEY (user_id),
  INDEX partner_index (partner_id)
);

CREATE FUNCTION add_partner
  (user_identifier INT,
  partner_identifier INT)
  RETURNS BOOLEAN
  MODIFIES SQL DATA
  COMMENT 'True indicates parter added, false one of them is already partnered'
  BEGIN
    -- Catch duplicate key errors from insert
    DECLARE EXIT HANDLER FOR 1062
    BEGIN
      RETURN FALSE;
    END;
    INSERT INTO partners (user_id, partner_id) VALUES (user_identifier, partner_identifier), (partner_identifier, user_identifier);
    RETURN TRUE;
  END;