Coverity reports an issue in the callback function:

/Modules/_ssl.c: 2403 ( uninit_use)
   2400            /* remove race condition in this the call back while if removing the
   2401             * callback is in progress */
   2402            PyGILState_Release(gstate);
>>> CID 966640: Uninitialized scalar variable (UNINIT)
>>> Using uninitialized value "ret".
   2403            return ret;
   2404        }
   2405    
   2406        ssl = SSL_get_app_data(s);
   2407        assert(PySSLSocket_Check(ssl));

I don't know which error code should be returned in this case.