Was running `python -m pip install ansible` when I got a "segmentation fault".

bt is:

(gdb) bt
#0  0x00000000004a7ec4 in visit_decref () at ../Modules/gcmodule.c:360
#1  0x00000000004a7fa9 in dict_traverse () at ../Objects/dictobject.c:2144
#2  0x00000000004a6f88 in subtract_refs () at ../Modules/gcmodule.c:385
#3  collect.lto_priv () at ../Modules/gcmodule.c:925
#4  0x0000000000500d2e in PyGC_Collect () at ../Modules/gcmodule.c:1440
#5  0x0000000000500184 in Py_Finalize () at ../Python/pythonrun.c:448
#6  0x0000000000525148 in Py_Exit (sts=0) at ../Python/pythonrun.c:1783
#7  0x0000000000522553 in handle_system_exit () at ../Python/pythonrun.c:1151
#8  0x0000000000521e46 in PyErr_PrintEx () at ../Python/pythonrun.c:1161
#9  0x0000000000520dc9 in RunModule.lto_priv.1258 () at ../Modules/main.c:194
#10 0x000000000049de26 in Py_Main () at ../Modules/main.c:587
#11 0x00007fddb03e25f0 in __libc_start_main (main=0x49d710 <main>, argc=6, argv=0x7ffdfcb8cfe8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, 
    stack_end=0x7ffdfcb8cfd8) at libc-start.c:291
#12 0x000000000049d639 in _start ()

Problem looks obvious:
(gdb) p (((PyObject*)(op))->ob_type)
$86202 = (struct _typeobject *) 0x0

But `subtract_refs()` from `Modules/gcmodule.c:385` is iterating a very long chained list, and I'm not sure it's of any help iterating over it.

Can provide core dump if of any help.

Python version: Python 2.7.12
PIP version: pip 8.1.2 from /usr/lib/python2.7/dist-packages (python 2.7)

Looks reproductible:

```
$ python -m pip install --user ansible
Collecting ansible
Collecting jinja2 (from ansible)
  Using cached Jinja2-2.8-py2.py3-none-any.whl
Collecting setuptools (from ansible)
  Using cached setuptools-24.0.3-py2.py3-none-any.whl
Collecting PyYAML (from ansible)
Collecting pycrypto>=2.6 (from ansible)
Collecting paramiko (from ansible)
  Using cached paramiko-2.0.1-py2.py3-none-any.whl
Collecting MarkupSafe (from jinja2->ansible)
Collecting cryptography>=1.1 (from paramiko->ansible)
Collecting pyasn1>=0.1.7 (from paramiko->ansible)
  Using cached pyasn1-0.1.9-py2.py3-none-any.whl
Collecting enum34 (from cryptography>=1.1->paramiko->ansible)
  Using cached enum34-1.1.6-py2-none-any.whl
Collecting idna>=2.0 (from cryptography>=1.1->paramiko->ansible)
  Using cached idna-2.1-py2.py3-none-any.whl
Collecting cffi>=1.4.1 (from cryptography>=1.1->paramiko->ansible)
  Using cached cffi-1.7.0-cp27-cp27mu-manylinux1_x86_64.whl
Collecting ipaddress (from cryptography>=1.1->paramiko->ansible)
  Using cached ipaddress-1.0.16-py27-none-any.whl
Collecting six>=1.4.1 (from cryptography>=1.1->paramiko->ansible)
  Using cached six-1.10.0-py2.py3-none-any.whl
Collecting pycparser (from cffi>=1.4.1->cryptography>=1.1->paramiko->ansible)
Installing collected packages: MarkupSafe, jinja2, setuptools, PyYAML, pycrypto, enum34, idna, pycparser, cffi, ipaddress, six, pyasn1, cryptography, paramiko, ansible
Successfully installed MarkupSafe-0.23 PyYAML-3.11 ansible-2.1.0.0 cffi-1.7.0 cryptography-1.4 enum34-1.1.6 idna-2.1 ipaddress-1.0.16 jinja2-2.8 paramiko-2.0.1 pyasn1-0.1.9 pycparser-2.14 pycrypto-2.6.1 setuptools-24.0.3 six-1.10.0
Segmentation fault (core dumped)
```

I tried installing only a few packages to see if one in particular causes the bug and yes, cffi may be the one:

```
$ python -m pip install --user cffi
Collecting cffi
  Using cached cffi-1.7.0-cp27-cp27mu-manylinux1_x86_64.whl
Collecting pycparser (from cffi)
Installing collected packages: pycparser, cffi
Successfully installed cffi-1.7.0 pycparser-2.14
Segmentation fault (core dumped)
```

But not pycparser alone:

```
$ python -m pip install --user pycparser
Collecting pycparser
Installing collected packages: pycparser
Successfully installed pycparser-2.14
```

Try installing cffi from source rather than using the pre-compiled wheel:


python -m pip uninstall cffi
python -m pip install -v --no-use-wheel --user cffi

If that works, there could be an incompatibility between the Python 2.7.12 you are using and the C extensions in the wheel.  What platform are you on and what is the source of the Python 2.7?

@Ned

Despite the segfault, cffi installs well, the segfault occurs during a garbage collect, very late, inside Py_Exit according to the backtrace, so it has no impact.

> What platform are you on and what is the source of the Python 2.7

Debian stretch, python (and sources) 2.7.12-1 from Debian testing packages, pip 8.1.2 from Debian packages too, version 8.1.2-2.

It would still be interesting to know whether you see the same behavior with building from source.  If a cffi extension module is invoked during the install process, it might screw things up.  Otherwise, unless you can reproduce the problem with a vanilla Python 2.7.12 built from scratch, you'll probably have to pursue this with the Debian folks.

@ned Oh ok. I just tried, with `--no-use-wheel`: no segfault.

Which makes me think that --no-use-wheel should be the default in pip ...

As a Linux user I'm *very* uneasy about this whole binary wheel thing.

Thanks for trying!  Perhaps the first thing to do is to check with the cffi project; perhaps Armin will recognize something.  I'm going to mark this issue as "closed" but feel free to re-open it if a problem with Python turns up.

CFFI issue, like for the record: https://bitbucket.org/cffi/cffi/issues/272/segfault-while-installing-via-pip

At cffi they thinks the bug may be from cpython or pip, as

```
$ python -m pip install --user cffi
Collecting cffi
  Using cached cffi-1.7.0-cp27-cp27mu-manylinux1_x86_64.whl
Collecting pycparser (from cffi)
Installing collected packages: pycparser, cffi
Successfully installed cffi-1.7.0 pycparser-2.14
Segmentation fault (core dumped)
```

may not import cffi.
c.f.: https://bitbucket.org/cffi/cffi/issues/272/segfault-while-installing-via-pip#comment-29256302

IMO, there are too many possible scenarios here where a segfault could be caused by the Debian patches, by pip, by wheel, or by cffi - too many to assume a CPython problem.

FWIW, I am unable to reproduce the segfault on a pretty current Debian testing system using a vanilla built-from-source Python 2.7.12 and the currently available wheel on PyPI and, for that matter, I'm unable to reproduce using a current Debian python2.7.12-1/pip 8.1.2-2/wheel 0.29.0-1.  Unless someone can provide a reproducible test case that points to a Python problem using a vanilla Python 2.7.12, the issue will have to remain closed as we just don't have the resources to try to isolate every problem like this.  Sorry!

In case someone reaches this bug report via search engine:

apt-get remove python-cffi-backend

fixed this problem for me.

If you're here looking for a solution, deleting previously installed deps helped me. In my case this was:

$ rm -rf ~/.local/lib/python2.7/site-packages/  # path may differ depending where you install these
$ pip install cffi

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.

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

I asked him on IRC. No, he cannot reproduce the bug anymore.

I tried again with the same version of pip and cffi on cpython 2.7.16 (no 13, I have no VM atm to compile a 2.7.13) but no segfault this time.