Clean up Qt socket notifier to avoid spurious interrupt handler calls

Closes matplotlib#29688

Objects without a parent are not necessarily cleaned up in `PyQt5/6`
when their reference count reaches zero, and must be explicitly cleaned
up with `deleteLater()`

This prevents the notifier firing after the signal handling was supposed
to have been reset to its previous state.

Rather than have both `bakend_bases._allow_interrupt()` and
`backend_qt._allow_interrupt_qt()` hold a reference to the notifier, we
pass it to  the backend-specific `handle_signint()` function for
cleanup.

Note the approach to cleaning up the notifier with `.deleteLater()`
followed by `sendPostedEvents()` is the documented workaround for when
immediate deletion is desired:

https://doc.qt.io/qt-6/qobject.html#deleteLater

This ensures the object is still deleted up even if the same event loop
does not run again.

Author: chrisjbillington

lib/matplotlib/backend_bases.py

@@ -1619,7 +1619,8 @@ def _allow_interrupt(prepare_notifier, handle_sigint):
     If SIGINT was indeed caught, after exiting the on_signal() function the
     interpreter reacts to the signal according to the handler function which
     had been set up by a signal.signal() call; here, we arrange to call the
-    backend-specific *handle_sigint* function.  Finally, we call the old SIGINT
+    backend-specific *handle_sigint* function, passing the notifier object
+    as returned by prepare_notifier().  Finally, we call the old SIGINT
     handler with the same arguments that were given to our custom handler.
 
     We do this only if the old handler for SIGINT was not None, which means
@@ -1629,7 +1630,7 @@ def _allow_interrupt(prepare_notifier, handle_sigint):
     Parameters
     ----------
     prepare_notifier : Callable[[socket.socket], object]
-    handle_sigint : Callable[[], object]
+    handle_sigint : Callable[[object], object]
     """
 
     old_sigint_handler = signal.getsignal(signal.SIGINT)
@@ -1645,9 +1646,10 @@ def _allow_interrupt(prepare_notifier, handle_sigint):
     notifier = prepare_notifier(rsock)
 
     def save_args_and_handle_sigint(*args):
-        nonlocal handler_args
+        nonlocal handler_args, notifier
         handler_args = args
-        handle_sigint()
+        handle_sigint(notifier)
+        notifier = None
 
     signal.signal(signal.SIGINT, save_args_and_handle_sigint)
     try:

lib/matplotlib/backends/backend_qt.py

@@ -169,9 +169,14 @@ def _may_clear_sock():
                 # be forgiving about reading an empty socket.
                 pass
 
-        return sn  # Actually keep the notifier alive.
-
-    def handle_sigint():
+        # We return the QSocketNotifier so that the caller holds a reference, and we
+        # also explicitly clean it up in handle_sigint(). Without doing both, deletion
+        # of the socket notifier can happen prematurely or not at all.
+        return sn
+
+    def handle_sigint(sn):
+        sn.deleteLater()
+        QtCore.QCoreApplication.sendPostedEvents(sn, QtCore.QEvent.Type.DeferredDelete)
         if hasattr(qapp_or_eventloop, 'closeAllWindows'):
             qapp_or_eventloop.closeAllWindows()
         qapp_or_eventloop.quit()

src/_macosx.m

@@ -258,7 +258,7 @@ static void lazy_init(void) {
 }
 
 static PyObject*
-stop(PyObject* self)
+stop(PyObject* self, PyObject* _ /* ignored */)
 {
     stopWithEvent();
     Py_RETURN_NONE;
@@ -1863,7 +1863,7 @@ - (void)flagsChanged:(NSEvent *)event
             "written on the file descriptor given as argument.")},
         {"stop",
          (PyCFunction)stop,
-         METH_NOARGS,
+         METH_VARARGS,
          PyDoc_STR("Stop the NSApp.")},
         {"show",
          (PyCFunction)show,