Constant rerun_c::THREAD_LIFE_TRACKER

pub const THREAD_LIFE_TRACKER: LocalKey<TrivialTypeWithDrop>;

It can happen that we end up inside of rr_recording_stream_free during a thread shutdown. This happens either when:

• the application shuts down, causing the destructor of globally defined recordings to be invoked -> Not an issue, we likely already destroyed the recording list.
• the user stored their C++ recording in a thread local variable, and then shut down the thread. -> More problematic, since we can’t access RECORDING_STREAMS now, meaning we leak the recording. (we can’t access it because we use channels internally which in turn use thread-local storage) In either case we have a problem, since destroying a recording bottoms out to some thread-local storage access inside of channels, causing a crash!

So how do we figure out that our thread is shutting down? As of writing std::thread::current() panics if there’s nothing on std::sys_common::thread_info::current_thread(). Unfortunately, std::sys_common is a private implementation detail! So instead, we try accessing a thread local variable and see if that’s still possible. If not, then we assume that the thread is shutting down.

Just any thread local variable will not do though! We need something that is guaranteed to be dropped with the thread shutting down. A simple integer value won’t do that, Box works but seems wasteful, so we use a trivial type with a drop implementation.