Subversion Repositories SmartDukaan

	 * The cart row, locked FOR UPDATE, as the first statement of anything that mutates this cart.

	 *

	 * This exists to fix a lock-ordering deadlock, not to guard a read. Two request paths used to

	 * take the same two rows in opposite orders: adding a line INSERTs into user.line, whose

	 * line_cart_id_fk shared-locks the parent cart FIRST and the line second, while validation

	 * mutated the line rows FIRST and wrote cart.total_price second. Run concurrently on one cart

	 * that is a cycle, and InnoDB rolls one side back ("Deadlock found when trying to get lock").

	 * Taking the cart row here, before any line is touched, gives every path the same cart -> line

	 * order and makes the cycle impossible. It also serialises concurrent edits to the SAME cart,

	 * which is what the @Version column on Cart was already trying and failing to express.

	 *

	 * Different carts are different rows, so this costs nothing across partners.

	 */

	public Cart selectByIdForUpdate(int id);

	/**