The GIL, callbacks, and GPFs

Francois De Serres fdeserres at gmx.net
Wed Jul 6 19:59:48 CEST 2005


Hello,

I'm chasing a GPF in the interpreter when running my extension module.
It's not very elaborated, but uses a system (threaded) callback, and 
therefore the GIL.
Help would be mucho appreciated. Here's the rough picture:

win32_spam.c
------------
/* code here is unit-tested OK */

typedef struct Bag {
    char data[128];
    site_t size;
} Bag;
typedef void (*PCallback)(const Bag * const bag);

Bag bag;
PCallback user_callback = NULL;

/* called by windoz */
static void CALLBACK win32_call_me(void) {
    memcpy(bag.data, somestuff, 100);
    bag.size = 100;
    SPAWN_THREAD(user_callback, & bag);//pseudo-code
}



spam_module.c
-------------
/* most of the code here is pasted from doc */

static PyObject * my_callback = NULL;

static PyObject *
setCallback(PyObject *dummy, PyObject *args) {
    PyObject *result = NULL;
    PyObject *temp;
    if (PyArg_ParseTuple(args, "O:miSetCallback", &temp)) {
        if ((temp != Py_None) && (!PyCallable_Check(temp))) {
            PyErr_SetString(PyExc_TypeError, "parameter must be callable");
            return NULL;
        }
        Py_XINCREF(temp);
        Py_XDECREF(my_callback);
        my_callback = temp;
        Py_INCREF(Py_None);
        result = Py_None;
        
        /* set the actual callback in win32_spam.c */
        user_callback = & external_callback;
    }
    return result;
}

static void external_callback(const Bag * const bag) {     
    if (my_callback && (my_callback != Py_None)) {
        PyObject * arglist = NULL;
        PyObject * result = NULL;
        arglist = Py_BuildValue("(s#)", bag->data, bag->size);
    
        /* do it */
        PyGILState_STATE gil = PyGILState_Ensure();
        result = PyEval_CallObject(my_callback, arglist);            
        PyGILState_Release(gil);
            
        Py_DECREF(arglist);
        Py_DECREF(result);    
    }
}


blowup_spam1.py
-------------
# This throws a GPF on callback.
# Why, since the GIL is acquired by the callback?

import spam.py

def callback(stuff):
    print stuff

if __name__ == '__main__':
    spam.setCallback(callback)
    raw_input()


blowup_spam2.py
-------------
# This throws a GPF as well
# Why, since we're using a threadsafe queue?

import spam
import Queue

q = Queue.Queue()

def callback(stuff):
    q.put(stuff)

if __name__ == '__main__':
    spam.setCallback(callback)
    while True:
        print q.get()



nice_spam.py
-------------
# This works
# Why, since the rest did not?

import spam
import Queue
import threading

q = Queue.Queue()

def callback(stuff):
    q.put(stuff)

def job():
    while True:
        print q.get()

if __name__ == '__main__':
    spam.setCallback(callback)
    t = threading.Thread(job)
    t.start()
    raw_input()



Please point me to what I'm doing wrong...

TIA,
Francois


PS: I've already submitted my issue under "(Win32 API) callback to 
Python, threading hiccups", but I guess it was poorly formulated.



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