Created attachment 113515 [details]
set CairoError to an exception that can be raised

There is a global variable called “CairoError” in python3-cairo, that is passed to PyErr_SetString to raise exceptions for errors returned from the underlying Cairo library. Unfortunately, this is always NULL, which seems to cause the PyErr_SetString calls to be no-ops. This can manifest itself as errors reported from Python to the effect of “null result without error return”.

It turns out an Exception object is created at module initialization time, but never assigned to this variable. This patch fixes the problem.