DOC: add note about why reading the socket is complicated

tacaswell · tacaswell · commit 49e1cceec8ad · 2021-09-30T15:39:45.000-04:00
diff --git a/lib/matplotlib/backends/qt_compat.py b/lib/matplotlib/backends/qt_compat.py
@@ -228,6 +228,12 @@ def _maybe_allow_interrupt(qapp):
             rsock.fileno(), _enum('QtCore.QSocketNotifier.Type').Read
         )
 
+        # We do not actually care about this value other than running some
+        # Python code to ensure that the interpreter has a chance to handle the
+        # signal in Python land.  We also need to drain the socket because it
+        # will be written to as part of the wakeup!  There are some cases where
+        # this may fire too soon / more than once on Windows so we should be
+        # forgiving about reading an empty socket.
         rsock.setblocking(False)
         # Clear the socket to re-arm the notifier.
         @sn.activated.connect
