Ref: https://bugs.ruby-lang.org/issues/18339 Design: - This tries to minimize the overhead when no hook is registered. It should only incur an extra unsynchronized boolean check. - The hook list is protected with a read-write lock as to cause contention when some hooks are registered. - The hooks MUST be thread safe, and MUST NOT call into Ruby as they are executed outside the GVL. - It's simply a noop on Windows. API: ``` rb_internal_thread_event_hook_t * rb_internal_thread_add_event_hook(rb_internal_thread_event_callback callback, rb_event_flag_t internal_event, void *user_data); bool rb_internal_thread_remove_event_hook(rb_internal_thread_event_hook_t * hook); ``` You can subscribe to 3 events: - READY: called right before attempting to acquire the GVL - RESUMED: called right after successfully acquiring the GVL - SUSPENDED: called right after releasing the GVL. The hooks MUST be threadsafe, as they are executed outside of the GVL, they also MUST NOT call any Ruby API.