Weird bug? with CFUNCTYPE and glfw

Hi folks,

I found a weird bug (or at least I don't understand whats happening)
when writing some Python 3 code using GLFW 3.0.1. Unfortunately I was
not able to reproduce the error with a selfwritten small C library, it
only occours when using glfw on linux (testet on arch 64 bit).

I reduced the problem to the following minimal code:
#!/usr/bin/env python3

import ctypes as c

class GLFWwindow(c.Structure):
    pass
GLFWwindow_p = c.POINTER(GLFWwindow)
GLFWkeyfun = c.CFUNCTYPE(None, GLFWwindow_p, c.c_int, c.c_int, c.c_int,
c.c_int)

lib = c.CDLL("libglfw.so")
lib.glfwCreateWindow.restype = GLFWwindow_p

lib.glfwInit()
win = lib.glfwCreateWindow(800, 600, "Title".encode(), None, None)
lib.glfwSetKeyCallback(win, GLFWkeyfun(lambda w, key, scancode, action,
mods: print(key, action)))

#def cb(*args):
#    print(args)

while True:
    lib.glfwPollEvents()

The window is created successfully, but when I press a key I get the
following errors in the console:

Traceback (most recent call last):
  File "_ctypes/callbacks.c", line 260, in 'calling callback function'
TypeError: 'LP_GLFWwindow' object is not callable
Traceback (most recent call last):
  File "_ctypes/callbacks.c", line 260, in 'calling callback function'
TypeError: 'LP_GLFWwindow' object is not callable

(this block for each press and release). I don't know why it tries to
call a LP_GLFWwindow object...
But if I now uncomment the cb function (and not! changing any other
thing inside the code) the output becomes even more weird:

(<__main__.LP_GLFWwindow object at 0x7ff51e03edd0>, 87, 25, 1, 0)
(<__main__.LP_GLFWwindow object at 0x7ff51e03edd0>, 87, 25, 0, 0)

But I have never registered cb as a callback to glfwSetKeyCallback.

So I tried changing to Python 2.7.5. The behaviour changes to this:

Output without the cb-function:

> 87 1
> [1]    1795 segmentation fault (core dumped)  ./test2.py

and with cb-function:

> (<__main__.LP_GLFWwindow object at 0x7f72fab86680>, 87, 25, 1, 0)
> [1]    1758 segmentation fault (core dumped)  ./test2.py

The code header changed to:

#!/usr/bin/env python2
from __future__ import print_function

the rest of the code is exactly the same.

I have the following idea that may explain this:
Python is freeing the lambda function, because GLFWkeyfun does not
properly increment the reference counter on it. After this it creates
the cb function, and stores it on the same memory where the lambda lived
before, so this one gets called instead. But I don't know much about
Python or ctypes internals so this is just speculation.

I googled a lot but found nothing on this. I am using Python 3.3.2 and
GLFW 3.0.1 both in the recent version from the arch repos (64 bit). You
can find the glfw documentation at [1]. I'm not even sure if this is a
bug or just missusage of the ctypes module or maybe even a bug inside
gcc or glfw. If you need any other information (including hardware or
system information) or require me to test stuff, please tell me.

Cheers

References:
[1] http://www.glfw.org/docs/3.0/group__input.html



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