Comment 1 for bug 1294788

Although it would be much slower, there's an argument for doing the checks one copy at a time; it would lessen the chance of a later copy being performed that depends on the first copy. In the absence of being able to do a transactional set of copies, proposed-migration *tends* to perform the copies roughly in dependency order.

Of course, if there's a large set of circularly-dependent copies, we're screwed either way. So maybe it isn't worth the slowness given that this type of failure is rare.