Comment 39 for bug 1341420

What you describe is fundamental to how nova works right now. We speculate in the scheduler, and if we race between two, we handle it with a reschedule. Nova specifically states that scheduling every last resource is out of scope. When trying to do that (which is often the use case for ironic) you're likely to hit this race as you run out of capacity:

https://github.com/openstack/nova/blob/master/doc/source/project_scope.rst#iaas-not-batch-processing

In the next few cycles we plan to move the claim process to the placement engine, which will eliminate most of these race-to-claim type issues, and in that situation things will be better for this kind of arrangement.

Until that point, this is not a bug though, because it's specifically how nova is designed to work.