Last time I had to make a major change related to signal handling, it was in the asyncio module because of a race conditon which occurred on FreeBSD.

commit fe5649c7b7bf52147480d6b1124a3d8e3597aee3
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Thu Jul 17 22:43:40 2014 +0200

    Python issue #21645, Tulip issue 192: Rewrite signal handling
    
    Since Python 3.3, the C signal handler writes the signal number into the wakeup
    file descriptor and then schedules the Python call using Py_AddPendingCall().
    
    asyncio uses the wakeup file descriptor to wake up the event loop, and relies
    on Py_AddPendingCall() to schedule the final callback with call_soon().
    
    If the C signal handler is called in a thread different than the thread of the
    event loop, the loop is awaken but Py_AddPendingCall() was not called yet. In
    this case, the event loop has nothing to do and go to sleep again.
    Py_AddPendingCall() is called while the event loop is sleeping again and so the
    final callback is not scheduled immediatly.
    
    This patch changes how asyncio handles signals. Instead of relying on
    Py_AddPendingCall() and the wakeup file descriptor, asyncio now only relies on
    the wakeup file descriptor. asyncio reads signal numbers from the wakeup file
    descriptor to call its signal handler.