check if SIGINT handler is callable before triggering it

an apparent Python bug can cause `int not callable` when SIGINT handler is SIG_IGN

It's a bit baffling that this behavior appears to be dependent on pyzmq version
in IPython's own use,
but the bug can be reproduced in a tiny amount of pure Python without zmq or IPython.

IPython/kernel/zmq/parentpoller.py

@@ -1,142 +1,146 b''
-# Standard library imports.
+# Copyright (c) IPython Development Team.
+# Distributed under the terms of the Modified BSD License.
+
 try:
     import ctypes
 except:
     ctypes = None
 import os
 import platform
+import signal
 import time
 try:
     from _thread import interrupt_main  # Py 3
 except ImportError:
     from thread import interrupt_main  # Py 2
 from threading import Thread
 
 from IPython.utils.warn import warn
 
 
 class ParentPollerUnix(Thread):
     """ A Unix-specific daemon thread that terminates the program immediately
     when the parent process no longer exists.
     """
 
     def __init__(self):
         super(ParentPollerUnix, self).__init__()
         self.daemon = True
 
     def run(self):
         # We cannot use os.waitpid because it works only for child processes.
         from errno import EINTR
         while True:
             try:
                 if os.getppid() == 1:
                     os._exit(1)
                 time.sleep(1.0)
             except OSError as e:
                 if e.errno == EINTR:
                     continue
                 raise
 
 
 class ParentPollerWindows(Thread):
     """ A Windows-specific daemon thread that listens for a special event that
     signals an interrupt and, optionally, terminates the program immediately
     when the parent process no longer exists.
     """
 
     def __init__(self, interrupt_handle=None, parent_handle=None):
         """ Create the poller. At least one of the optional parameters must be
         provided.
 
         Parameters
         ----------
         interrupt_handle : HANDLE (int), optional
             If provided, the program will generate a Ctrl+C event when this
             handle is signaled.
 
         parent_handle : HANDLE (int), optional
             If provided, the program will terminate immediately when this
             handle is signaled.
         """
         assert(interrupt_handle or parent_handle)
         super(ParentPollerWindows, self).__init__()
         if ctypes is None:
             raise ImportError("ParentPollerWindows requires ctypes")
         self.daemon = True
         self.interrupt_handle = interrupt_handle
         self.parent_handle = parent_handle
 
     @staticmethod
     def create_interrupt_event():
         """ Create an interrupt event handle.
 
         The parent process should use this static method for creating the
         interrupt event that is passed to the child process. It should store
         this handle and use it with ``send_interrupt`` to interrupt the child
         process.
         """
         # Create a security attributes struct that permits inheritance of the
         # handle by new processes.
         # FIXME: We can clean up this mess by requiring pywin32 for IPython.
         class SECURITY_ATTRIBUTES(ctypes.Structure):
             _fields_ = [ ("nLength", ctypes.c_int),
                          ("lpSecurityDescriptor", ctypes.c_void_p),
                          ("bInheritHandle", ctypes.c_int) ]
         sa = SECURITY_ATTRIBUTES()
         sa_p = ctypes.pointer(sa)
         sa.nLength = ctypes.sizeof(SECURITY_ATTRIBUTES)
         sa.lpSecurityDescriptor = 0
         sa.bInheritHandle = 1
 
         return ctypes.windll.kernel32.CreateEventA(
             sa_p,  # lpEventAttributes
             False, # bManualReset
             False, # bInitialState
             '')    # lpName
 
     @staticmethod
     def send_interrupt(interrupt_handle):
         """ Sends an interrupt event using the specified handle.
         """
         ctypes.windll.kernel32.SetEvent(interrupt_handle)
 
     def run(self):
         """ Run the poll loop. This method never returns.
         """
         try:
             from _winapi import WAIT_OBJECT_0, INFINITE
         except ImportError:
             from _subprocess import WAIT_OBJECT_0, INFINITE
 
         # Build the list of handle to listen on.
         handles = []
         if self.interrupt_handle:
             handles.append(self.interrupt_handle)
         if self.parent_handle:
             handles.append(self.parent_handle)
         arch = platform.architecture()[0]
         c_int = ctypes.c_int64 if arch.startswith('64') else ctypes.c_int
 
         # Listen forever.
         while True:
             result = ctypes.windll.kernel32.WaitForMultipleObjects(
                 len(handles),                            # nCount
                 (c_int * len(handles))(*handles),        # lpHandles
                 False,                                   # bWaitAll
                 INFINITE)                                # dwMilliseconds
 
             if WAIT_OBJECT_0 <= result < len(handles):
                 handle = handles[result - WAIT_OBJECT_0]
 
                 if handle == self.interrupt_handle:
-                    interrupt_main()
+                    if callable(signal.getsignal(signal.SIGINT)):
+                        interrupt_main()
 
                 elif handle == self.parent_handle:
                     os._exit(1)
             elif result < 0:
                 # wait failed, just give up and stop polling.
                 warn("""Parent poll failed.  If the frontend dies,
                 the kernel may be left running.  Please let us know
                 about your system (bitness, Python, etc.) at
                 [e-mail]""")
                 return