option to not override signal handler

Inizia in ipykernel/kernelbase.py, in pre_handler_hook, e traccia come dispatch_shell lo invoca. Determina come un’applicazione che incorpora il kernel potrebbe scegliere di non sostituire il proprio gestore SIGINT, quindi verifica che il gestore personalizzato rimanga intatto mentre la normale gestione dei segnali del kernel continua a funzionare.

See also https://github.com/jupyter/qtconsole/issues/490

Hey there,

how can I embed qtconsole into an application without it overriding my own application's custom sigint handler?

Assume you have code that does

    std::signal(SIGINT, &handleSigInt);
    ...
    // do not initialize signal handlers
    const auto initsigs = 0;
    Py_InitializeEx(initsigs);

Then it is embedding qtconsole eventually, but at that point the sigint handler will be unset:

Thread 1 "QiTissue" hit Breakpoint 3, __interceptor_sigaction (signum=2, act=0x7fffffff8a60, oldact=0x7fffffff8b00)
    at /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_signal_interceptors.inc:55
55      in /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_signal_interceptors.inc
(gdb) bt
#0  __interceptor_sigaction(int, __sanitizer::__sanitizer_sigaction const*, __sanitizer::__sanitizer_sigaction*)
    (signum=2, act=0x7fffffff8a60, oldact=0x7fffffff8b00)
    at /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_signal_interceptors.inc:55
#1  0x00007fff5c055f0f in PyOS_setsig (sig=sig@entry=2, handler=handler@entry=0x7fff5c0b6fec <signal_handler>)
    at Python/pylifecycle.c:2536
#2  0x00007fff5c0b849f in signal_signal_impl
    (module=<optimized out>, handler=<built-in method default_int_handler of module object at remote 0x7fff51eeaa40>, signalnum=2) at ./Modules/signalmodule.c:505
#3  signal_signal (module=<optimized out>, args=<optimized out>, nargs=<optimized out>)
    at ./Modules/clinic/signalmodule.c.h:141
#4  0x00007fff5bf7bbf8 in cfunction_vectorcall_FASTCALL
    (func=<built-in method signal of module object at remote 0x7fff51eeaa40>, args=0x7fff58a24a88, nargsf=<optimized out>, kwnames=<optimized out>) at Objects/methodobject.c:426

or in python terms:

(gdb) py-bt
Traceback (most recent call first):
  <built-in method signal of module object at remote 0x7fff51eeaa40>
  File "/usr/lib/python3.9/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
  File "/home/milian/.local/lib/python3.9/site-packages/ipykernel/kernelbase.py", line 280, in pre_handler_hook
    self.saved_sigint_handler = signal(SIGINT, default_int_handler)
  File "/home/milian/.local/lib/python3.9/site-packages/ipykernel/kernelbase.py", line 257, in dispatch_shell
    self.pre_handler_hook()

the kernelbase.py code seems to reset the signal handler unconditionally to some default which is highly unexpected to me:

    def pre_handler_hook(self):
        """Hook to execute before calling message handler"""
        # ensure default_int_handler during handler call
        self.saved_sigint_handler = signal(SIGINT, default_int_handler)

Is there a way to prevent this from happening?