Handling errors from signal handlers in callbacks

Created originally on Bitbucket by denis (Denis Bilenko)

Consider the following script:

#!python

pypycore ~/work/gevent $ cat cffi_signal_problem.py 
import signal
from cffi import FFI

ffi = FFI()
ffi.cdef('''
void call_callback(void*);
''')

C = ffi.verify('''
#include <unistd.h>
void call_callback(void(*cb)(void));
void call_callback(void(*cb)(void)) {
    sleep(2);
    cb();
}
''', libraries=[])


def mycallback():
    try:
        print 'hello world'
    except Exception, ex:
        print 'caught error: %s' % ex

cb = ffi.callback("void(*)()", mycallback)

def alarm_handler(*args):
    1 // 0

signal.signal(signal.SIGALRM, alarm_handler)
signal.alarm(1)
C.call_callback(cb)

It will never print "caught error", instead it'll print this:

pypycore ~/work/gevent $ pypy cffi_signal_problem.py 
From callback <function mycallback at 0x00007f4475e15100>:
Traceback (most recent call last):
  File "cffi_signal_problem.py", line 20, in mycallback
    try:
  File "cffi_signal_problem.py", line 38, in alarm_handler
    1 // 0
ZeroDivisionError: integer division by zero

I would like in gevent to be able to handle such errors (have my handle_error() function called) as I'd like those to be re-raised in main greenlet, as it happens with builtin blocking functions:

pypycore ~/work/gevent $ cat time_signal.py 
import signal
import time

def alarm_handler(*args):
    1 // 0

signal.signal(signal.SIGALRM, alarm_handler)
signal.alarm(1)
try:
    time.sleep(2)
except Exception, ex:
    print 'caught error: %s' % ex


pypycore ~/work/gevent $ pypy time_signal.py 
caught error: integer division by zero

However, I cannot currently find a way to make it work.

In CPython version of gevent I do

    PyErr_CheckSignals();
    if (PyErr_Occurred()) gevent_handle_error(loop, Py_None);

Would be nice to have special support for this in CFFI / PyPy, in a form of a special callback that will be called with exception converted to value for those exception that were set before entering the callback.