While inspecting rx-runloop.hpp I noticed there might be a potential deadlock in the code:

Last two lines appear to be flipped, guard.unlock() is a no-op if it's at the end of a scope. During notify_earlier_wakeup it could call back into the runloop and take another lock which would hang since these locks aren't recursive.

/***/           if (need_earlier_wakeup_notification) st->notify_earlier_wakeup(when);
/***/           guard.unlock(); // So we can't get attempt to recursively lock the state