SystemError: <built-in method callback_decorator ...> returned NULL without setting an error

[codingcatgirl]

This following exception happens on gentoo when calling surface.write_to_png() after using rsvg to render an svg.

• python 3.6
• cairo 1.14.8
• cairocffi 0.8.0, installed from pip

the code works on other operating systems (tested on arch), so i guess it's some dependency problem. what could it be?

Traceback (most recent call last):
  […]
  File "/opt/c3nav/instances/34c3/c3nav/src/c3nav/mapdata/utils/svg.py", line 92, in get_png
    f.write(surface.write_to_png())
  File "/opt/c3nav/instances/34c3/env/lib/python3.6/site-packages/cairocffi/surfaces.py", line 593, in write_to_png
    write_func = _make_write_func(target)
  File "/opt/c3nav/instances/34c3/env/lib/python3.6/site-packages/cairocffi/surfaces.py", line 42, in _make_write_func
    @ffi.callback("cairo_write_func_t", error=constants.STATUS_WRITE_ERROR)
SystemError: <built-in method callback_decorator of tuple object at 0x676b04d14d18> returned NULL without setting an error

the exception can be triggered easily in a python shell:

>>> import cairocffi
>>> surface = cairocffi.SVGSurface(None, 10, 10)
>>> print(surface.write_to_png())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/c3nav/instances/34c3/env/lib/python3.6/site-packages/cairocffi/surfaces.py", line 593, in write_to_png
    write_func = _make_write_func(target)
  File "/opt/c3nav/instances/34c3/env/lib/python3.6/site-packages/cairocffi/surfaces.py", line 42, in _make_write_func
    @ffi.callback("cairo_write_func_t", error=constants.STATUS_WRITE_ERROR)
SystemError: <built-in method callback_decorator of tuple object at 0x752d9f20fc78> returned NULL without setting an error
>>> surface = cairocffi.SVGSurface(None, 10, 10)

[codingcatgirl]

I found the source. The Problem is that your code uses old-style callbacks, which do not work on hardened systems. The solution would be to convert your callbacks to new-style callbacks. I would offer a pull request, but as cairocffi uses cffi in ABI mode, it is not possible to use them. So i guess i'll have to un-harden the server i want to run it on or look for other cairo bindings. 😞