The curious case of O_DIRECTORY|O_CREAT

> Not if they have different metadata (permissions or ownership, which could arise with multiple processes accessing /tmp, say).

A can fstat it after opening it, if A cares.

> Also, is there any guarantee B even left it empty and didn't also start populating it with things that would conflict with A's plans for it?

If the permissions on the original dir allow it, then B can do this anyway. If not, then see previous reply.