It would be nice to add a unit test to this patch.  I don't think messing with read-only filesystems is feasible in regrtest, but it seems from reading _ctypes_alloc_callback() source that similar behavior can be triggered by passing a failing converters list to a ctypes function constructor.

A nit-pick: it would be easier to verify that all CThunkObject fields are initialized if initialization was done in the same order as declaration:


typedef struct {
    PyObject_VAR_HEAD
    ...
    PyObject *callable;
    PyObject *restype;
    SETFUNC setfunc;
    ffi_type *ffi_restype;
    ffi_type *atypes[1];
} CThunkObject;


In other words, "p->restype = NULL;" should go after "p->callable = NULL;".