Trace a better error message on installation failure due to invalid .data files in wheels

[chrahunt]

This explicitly handles erroneous files in the .data directory of wheels.

Before:

ERROR: Exception:
Traceback (most recent call last):
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/cli/base_command.py", line 216, in _main
    status = self.run(options, args)
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/cli/req_command.py", line 182, in wrapper
    return func(self, options, args)
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 421, in run
    pycompile=options.compile,
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/req/__init__.py", line 90, in install_given_reqs
    pycompile=pycompile,
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/req/req_install.py", line 831, in install
    requested=self.user_supplied,
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/operations/install/wheel.py", line 830, in install_wheel
    requested=requested,
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/operations/install/wheel.py", line 658, in _install_wheel
    for file in files:
  File "/tmp/venv/lib/python3.6/site-packages/pip/_internal/operations/install/wheel.py", line 587, in make_data_scheme_file
    _, scheme_key, dest_subpath = normed_path.split(os.path.sep, 2)
ValueError: not enough values to unpack (expected 3, got 2)

After:

ERROR: For req: python-apt==0.0.0. Unexpected file in /home/chris/.cache/pip/wheels/ff/46/14/4b2fa60308f0a0cf20345e549cb154c557f117417af56cf5f7/python_apt-0.0.0-cp38-cp38-linux_x86_64.whl: 'python_apt-0.0.0.data/purelib'. .data directory contents should be named like: '<scheme key>/<path>'.

While updating this error message, I noticed the very next line was subject to throwing a similarly confusing error. That has also been updated.

Fixes #8654.

[uranusjr]

Are files in the .data root disallowed? PEP 427 does not seem to explicitly say it.

[chrahunt]

I could see it interpreted either way, which means it could probably use a clarifying update.

With regards to the pip behavior, our current and past implementations disallowed it in general. Example with pip 20.1:

$ ./.tox/py38/bin/python
Python 3.8.0 (default, Jan  4 2020, 14:08:19)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from tests.lib.wheel import make_wheel
>>> make_wheel("simple", "0.1.0", extra_data_files={"abc": "hello.txt"}).save_to_dir("/tmp/user/1000/tmp.E0kpIwbLtL")
'/tmp/user/1000/tmp.E0kpIwbLtL/simple-0.1.0-py2.py3-none-any.whl'
>>>

Then in that directory with a pip 20.1:

$ ./env/bin/python -m pip install '/tmp/user/1000/tmp.E0kpIwbLtL/simple-0.1.0-py2.py3-none-any.whl'
Processing ./simple-0.1.0-py2.py3-none-any.whl
Installing collected packages: simple
ERROR: Exception:
Traceback (most recent call last):
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/cli/base_command.py", line 188, in _main
    status = self.run(options, args)
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/cli/req_command.py", line 185, in wrapper
    return func(self, options, args)
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/commands/install.py", line 398, in run
    installed = install_given_reqs(
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/req/__init__.py", line 67, in install_given_reqs
    requirement.install(
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/req/req_install.py", line 804, in install
    install_wheel(
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/operations/install/wheel.py", line 622, in install_wheel
    install_unpacked_wheel(
  File "/tmp/user/1000/tmp.E0kpIwbLtL/env/lib/python3.8/site-packages/pip/_internal/operations/install/wheel.py", line 452, in install_unpacked_wheel
    dest = getattr(scheme, subdir)
AttributeError: 'Scheme' object has no attribute 'abc'
WARNING: You are using pip version 20.1; however, version 20.2 is available.
You should consider upgrading via the '/tmp/user/1000/tmp.E0kpIwbLtL/env/bin/python -m pip install --upgrade pip' command.

When the file happened to be named the same as a scheme key (which is the case in #8654), we silently ignored it.

[uranusjr]

cc @dholth for wheel spec inputs.

[uranusjr]

The implementation looks good to me, just a minor comment but I wouldn’t mind if this is merged without it.

[cjp256]

LGTM, thank you for addressing this so quickly 👍

[eamanu]

LGTM :)

[pradyunsg]

Looks like everyone agrees that this is good to go, so I'll click merge. git will blame Chris if things break anyway. :P