Run TLS destructors at process exit on all platforms

[surban]

This calls TLS destructors for the thread that initiates a process exit. This is done by registering a process-wide
exit callback.

Previously UNIX platforms other than Linux and Apple did not destruct TLS variables on the thread that initiated the process exit (either by returning from main or calling std::process::exit).

This also adds a test to verify this behavior.

@joboet Can we run CI tests for all available platforms for this?

r? joboet

[bors]

☔ The latest upstream changes (presumably #134108) made this pull request unmergeable. Please resolve the merge conflicts.

[joboet]

Sorry for the delay, I've been quite busy recently.

At a first glance, the changes look mostly good. There is a problem with the TLS destructor registration however: the TLS key can be used as soon as it is initialized, so it is possible that another thread completes before the destructor is registered. Always registering the destructor is not an option either, since that might lead to cycles in the destructor list. Windows uses INIT_ONCE synchronization to resolve this issue, but replicating that might prove difficult on platforms without a native Once-like interface (Once itself cannot be used in the TLS code since it needs TLS itself). I think this is best solved by keeping a thread-local destructor list similar to the one used on platforms with native TLS, but using the keys (or perhaps references to the LazyKeys) instead of pointers to values. Or perhaps you can think of another way?

[surban]

There is a problem with the TLS destructor registration however: the TLS key can be used as soon as it is initialized, so it is possible that another thread completes before the destructor is registered.

You are right.

I think this is best solved by keeping a thread-local destructor list similar to the one used on platforms with native TLS, but using the keys (or perhaps references to the LazyKeys) instead of pointers to values.

I implemented it using references to LazyKeys.

[surban]

I now have the problem that I need to register the LazyKey with the thread local list of destructors for every thread that uses it. Thus I need to call the register_dtor function once per thread that calls LazyKey::force.

Do you have an idea how I could achieve that without registering a second TLS key per LazyKey that keeps track whether it has been added to the destructors list? An option might be to store a sentinel value (for example 1 or 2) as the TLS value to indicate that the LazyKey been registered. However, I am unsure if this is acceptable practice. What do you think?

[surban]

I now have the problem that I need to register the LazyKey with the thread local list of destructors for every thread that uses it. Thus I need to call the register_dtor function once per thread that calls LazyKey::force.

I solved this by modifying os::Storage::get so that it checks if the value is being initialized for a particular thread. If so, it calls a function to add the TLS key to the thread-local destructor list. I hope this approach is okay.

This should be ready for review now.