Just want to note that this is only for the fuzzing configuration

Right, this just seems like "it could be more performant", not a specific bug, if I'm reading your description correctly, which isn't super relevant given its only used in fuzzing and presumably after initial startup things are "warm" and just stay DONE.

Ah, missed that it's only in fuzzing. Not looking immediately suspicious could have some benefit for reviewers too but if nobody wants to bother feel free to close.

We could add a comment, I guess? But, really, std should expose Once even if we don't have the ability to park a thread - the code here is basically the same as the std Once, with the only difference being that we don't park threads and instead busy-loop to avoid using std.

Once in std is actually quite involved. It uses an intrusive linked list to track waiting threads. I think it'd be pretty complicated to make usable without std.

Yes, that's my point, it does the same thing plus parks threads (with relevant tracking to wake them up again).

The code difference is so large that I'm not sure it'd be reasonable to abstract. And I generally love abstractions.

Anyway I think just changing swap to compare_exchange is better than a comment. :)

I agree with changing swap to compare_exchange.

Philosophically I agree with Matt that "it doesn't matter" and also that it's somehow std's fault ;) but there's a straightforward solution so let's just take it.