Julien: I'm working on enhancement of debug traces on visit_decref(). Are you still able to reproduce the crash in 2019?

I failed to reproduce the bug in a virtual environment with:

* Python 2.7.14
* pip 8.1.2
* cffi 1.7.0

$ virtualenv -p python2 venv
Running virtualenv with interpreter /usr/bin/python2
New python executable in /home/haypo/venv/bin/python2
Also creating executable in /home/haypo/venv/bin/python
Installing setuptools, pkg_resources, pip, wheel...done.

$ venv/bin/python -m pip install pip==8.1.2
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support
Collecting pip==8.1.2
  Using cached https://files.pythonhosted.org/packages/9c/32/004ce0852e0a127f07f358b715015763273799bd798956fa930814b60f39/pip-8.1.2-py2.py3-none-any.whl
Installing collected packages: pip
  Found existing installation: pip 19.2.3
    Uninstalling pip-19.2.3:
      Successfully uninstalled pip-19.2.3
Successfully installed pip-8.1.2

$ venv/bin/python -m pip install cffi==1.7.0
Collecting cffi==1.7.0
  Using cached https://files.pythonhosted.org/packages/46/26/2413bf923b1de47d9e9cf48f248dec1a16027a7773452315f34746253c35/cffi-1.7.0-cp27-cp27mu-manylinux1_x86_64.whl
Collecting pycparser (from cffi==1.7.0)
Installing collected packages: pycparser, cffi
Successfully installed cffi-1.7.0 pycparser-2.19
You are using pip version 8.1.2, however version 19.2.3 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.