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forkserver throws a TypeError if default SIGINT handler is not used #159418
Description
Bug report
If a profiler or other program sets a SIGINT signal handler, forkserver eventually throws a TypeError. forkserver became the default start method in 3.14, causing code that was previously fine to fail.
Reproducer
Small setup to reproduce the bug below. Call the first program below foreign_sigint.c
#define _GNU_SOURCE
#include <signal.h>
static void myhandler(int sig) { (void) sig; }
__attribute__((constructor))
static void install_handler(void)
{
struct sigaction sa;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
sa.sa_handler = myhandler;
sigaction(SIGINT, &sa, NULL);
}Then call the python script below repro.py:
import multiprocessing, signal, sys
def work(x):
return abs(x)
if __name__ == "__main__":
print("python:", sys.version.split()[0])
print("default start method:", multiprocessing.get_start_method())
print("getsignal(SIGINT) ->", signal.getsignal(signal.SIGINT))
p = multiprocessing.Process(target=work, args=(-1,))
p.start()
p.join()
print("worker exitcode:", p.exitcode)Then we can reproduce the bug with the following compilation/execution below.
$ gcc -shared -fPIC -o foreign_sigint.so foreign_sigint.c
$ python3 repro.py
python: 3.14.8
default start method: forkserver
getsignal(SIGINT) -> <built-in function default_int_handler>
worker exitcode: 0
$ LD_PRELOAD=./foreign_sigint.so python3 repro.py
python: 3.14.8
default start method: forkserver
getsignal(SIGINT) -> None
Traceback (most recent call last):
File ".../multiprocessing/forkserver.py", line 353, in main
code = _serve_one(child_r, fds,
unused_fds,
old_handlers)
File ".../multiprocessing/forkserver.py", line 383, in _serve_one
signal.signal(sig, val)
~~~~~~~~~~~~~^^^^^^^^^^
File ".../signal.py", line 58, in signal
handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
TypeError: signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object
worker exitcode: 1CPython versions tested on:
3.14
Operating systems tested on:
Linux
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