Clang RealtimeSanitizer support

[messmerd]

The discussion in free-audio/clap#448 gave me the idea of marking real-time functions with a macro, say CLAP_REALTIME, rather than marking them as real-time in a comment.

By default, CLAP_REALTIME would be empty and do nothing (besides being documentation), but when compiled with Clang's new RealtimeSanitizer enabled in C or C++, it could provide runtime checking of CLAP API functions marked as real-time:

#if defined(__has_feature) && __has_feature(realtime_sanitizer)
#   define CLAP_REALTIME [[clang::nonblocking]]
#else
#   define CLAP_REALTIME
#endif

At least that's what I hoped we could do, but unfortunately it appears unlikely that [[clang::nonblocking]] can be applied to function pointers like those the CLAP API uses, though I haven't confirmed this yet. I was wrong, it actually is possible to use on function pointers.

Fortunately it should be possible to apply the RealtimeSanitizer here since this library uses regular functions instead of function pointers. clap-helpers is probably more fitting for this sort of thing anyway given the focus on enforcing correctness through the use of checking levels and misbehavior handlers. Putting them in the CLAP API will allow C23 programs to benefit from RtSan too.

Does this sound like a good idea?

[abique]

Hi, this is very interesting.
I didn't know clang did support such a thing.

Do you want to work on a PR?

[abique]

Maybe this can go in clap/private/macros.h in the clap repository?

[messmerd]

@abique I just got confirmation that RtSan does actually work with function pointers too. realtime-sanitizer/rtsan#42

This is great news since it will allow us to use it in the CLAP API rather than just these clap-helpers.

I didn't know clang did support such a thing.

It hasn't actually been released yet, but it will be available in Clang 20 which should release in March.

Do you want to work on a PR?

Sure, I will work on a PR once I get the chance.

[abique]

It's good to have the PR ready, so we can checkout the branch and test.
But I'll wait for clang-20 to be released to actually test it before merging.