Fix several sources of undefined behavior in type casters

[oremanj]

• Some cast operators can raise exceptions, but many container casters would call the cast operator in their noexcept from_python function. Fix by adding a new can_cast<T>() caster method that returns false if operator cast_op<T>() would throw an exception. Note that implementing this properly for unique_ptr required some changes to the nb_type_relinquish_ownership API, mostly in the creation of a new nb_type_restore_ownership function to reverse its effects. Fixes #497.

• In nb::cast and nb::try_cast, we were previously deallocating the cleanup list before initializing the result object, which is generally UB when implicit conversions are involved.

• Avoid creating dangling pointers/references in container type casters, using a new flags_for_local_caster() function that casters call when they're delegating to an inner caster. There are two ways this would happen in the past:

  • Pointers to bound types created using implicit conversions would dangle once the cleanup_list was destroyed, which is problematic for nb::cast(). This is resolved by removing convert from the inner caster's flags if the manual flag is set.
  • Pointers to custom-caster types usually refer into the type caster object, so they would dangle if the caster were stored on the stack, which is a common idiom for containers. This is resolved by static_asserting in flags_for_local_caster that we're not casting to T* unless T is a bound type.

[oremanj]

I'm not sure yet what's going on with the failing tests - all of them are because the destruction of one of the Example objects is being delayed past the end of the test. It happens on different builds each time I run, but consistently 2-3 of them. Haven't been able to reproduce it locally yet. Adding an extra collect() didn't help.

[wjakob]

Hi @oremanj, just to keep you posted: I'm currently swamped with a deadline (in ~2 weeks), and then I'm moving to another country. I won't have time to debug the test issue in a near timeframe.

[wjakob]

I looked into the GC issue and was eventually able to reproduce it on a windows machine. As far as flakes go, it seems relatively benign -- some instances not being collected by the time they're supposed to, but then they do get deleted later on.

It's weird, as if something was wrong with the GC. Or somebody else is holding further references. Perhaps it's a combination of the GC and the extra rewriting that Pytest does to capture exceptions and warnings with extra contexts to provide readable output. But then it's weird that this only seems to happen on Windows.

The issue goes away simply by refactoring the big test into smaller ones. So I will leave it at this for now.

[wjakob]

This PR was merged out of band in commit c36584e577686500842e0cf1ac04f45608bd73de. (I officially merged the PR but will force-push the tweaked version. Just in case it plays a role for the bean counting somewhere ;))

Thanks for your work on this, it's a great improvement.