The reason I did it this way was to keep a bit of separation between the action flags and the current-state flags. In fact, if I'm changing those numbers, it might even be better to start PD_LOCKED at 0x10 or even 0x100.

If by "jid=8 is not started yet" you mean that nothing at all has been done with jid=8, then yes it would be picked. If you mean it's in the process of creation and not ready yet, then jid=9 would be picked, so no problem.

get_next_prid is based on the code that used to be inside kern_jail_set, moved into its function because in one iteration of this code I called it in two different places. Even though it's now only called in one, I'm keeping it in its own function because I should have been better with my code organization all along.

Currently, lastid sets a limit on the total number of jails, current or dying. With the potential renumbering of dying jails, lastid still sets a limit on the total number of jails, current and dying. So I don't expect any more of an allocator problem than we have now.

Generally, I see two scenarios:
1: well-behaved users who take whatever jid is given to them. get_next_prid will only ever be called for new jails, and at least for the first million jail creations will take the common case of not even looking through the list for a free jid.
2: users who want to specify things by jid, and may re-start jails that are in the dying state. get_next_prid will only ever be called to renumber the dying jails to an unused ID, and once the counter gets past the range of static jids that are being used, it too will usually just pick the next jid without having to traverse the list.
True, there could be a mixed case. But even then, it's little worse than the current case where adding a new jail will either check that a jid is available or look for a new one, only as it may have to do both for a dying jail replacement.

JID partitioning would either cut down the number of available active jails, limit the number of dying jails (which probably wouldn't be an issue), or convert dying jails to jids greater than JAIL_MAX.

This isn't about relying on the jid staying under a certain number when allocated dynamically; that works exactly the same as it has before.

This is about you having 50 jails with static-JID definitions like:
1 { ip4.addr 10.0.0.1; }
2 { ip4.addr 10.0.0.2; }

Currently, jail 1 dies with a TCP keepalive or other such thing that keeps it in dying state, and you can "create" it again with the same jid using "-d" or "allow.dying", which really just brings it back to life.

This doesn't change much from the user perspective: you can still re-create jail 1 when it's dying (with or without the now-unused allow.dying), but you'll get a new jail while the old one gets shoved off to jid 51 or whatever. Either way, the user sees "jail -c 1" working as expected.

The difference, and the reason for the patch, is that now the new jail 1 is always in a consistent newly-created state, without potential problems from something in the old jail state and jail.conf.

Part of this is to ease potential security problems (I don't have any examples cleaner is better). Part is with a guarantee that dead jails stay dead, more aggressive action can be taken to clean up dying jails. Notably, the potential advantages are the same for other scenarios of making dying jails totally invisible to userspace, or of not allowing static jids.

OK, it took a while, but I think get it now :-).

Yes, that would happen if you started jails 1-41, and 1 died, and then you restarted it (explicitly setting it to 1), and then you started what you thought would be 42-50 which would turn out to be 43-51.

The dying jails don't get renumbered when they're dying. That only happens when a newly created jail wants to use the jid that a dying one currently occupies. So this problem scenario only happens when jails are implicitly numbered at first, and then explicitly numbered to their old jids on re-creation, and that re-creation can happen before all the jails are first created. Yes, in that scenario, the jids may not be as expected, and counting down the renumbered dying jails from the top would solve the issue.