setuptools' discovery of MSVC fails in tox environment

[StefanD986]

Description

I stumbled upon a hard to debug issue when using tox to kick of builds that produce native artifacts using setuptools and Microsoft Visual Studio.

setuptools relies on the presence of some environment variables to locate vswhere.exe which is used to find the path to the msvc toolchain. setuptools.msvc uses the following environment variables to build the path to vswhere.exe

• ProgramFiles
• ProgramFiles(x86)

vswhere.exe in turn itself relies on some environment variables to locate the msvc toolchain. I tried and it looks like it only requires the ProgramData environment variable

TLDR

Without the presence of these three environment variables

• ProgramFiles
• ProgramFiles(x86)
• ProgramData

the discovery of the msvc toolchain fails. It is hard to debug because setuptools doesn't give much diagnostic output.

Steps to reproduce

0. Create a tox.ini with the following content:

   [tox]
   
   [testenv]
   skip_install=True
   commands =
       python -c "import setuptools;print(setuptools.msvc.msvc14_get_vc_env('amd64'))"

1. Run tox

2. Observe output

Expected behaviour

I expect the tox run to complete sucessfully

Actual behaviour

Run fails with

python run-test: commands[0] | python -c 'import setuptools;print(setuptools.msvc.msvc14_get_vc_env('"'"'amd64'"'"'))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\stefan\PycharmProjects\tox_msvc_bug\.tox\python\lib\site-packages\setuptools\msvc.py", line 314, in msvc14_get_vc_env
    return _msvc14_get_vc_env(plat_spec)
  File "C:\Users\stefan\PycharmProjects\tox_msvc_bug\.tox\python\lib\site-packages\setuptools\msvc.py", line 269, in _msvc14_get_vc_env
    "Unable to find vcvarsall.bat"
distutils.errors.DistutilsPlatformError: Microsoft Visual C++ 14.0 is required. Get it with "Build Tools for Visual Studio": https://visualstudio.microsoft.com/downloads/
ERROR: InvocationError for command 'C:\Users\stefan\PycharmProjects\tox_msvc_bug\.tox\python\Scripts\python.EXE' -c 'import setuptools;print(setuptools.msvc.msvc14_get_vc_env('"'"'amd64'"'"'))

Potential Resolution

There are already a couple of environment variables that are passed through by default. I leave it up to discussion whether this usecase warrants adding the mentioned three variables to the default passthrough on windows platforms.

At the very least it would be nice to document that these environment variables need to be passed through in the tox docs, so that it can be found if users run into this issue and to mention keywords such as cython, vcvarsall.

What do you think?

Additional info

Output of tox -rvv: log.txt

[gaborbernat]

Sadly this is specific to setuptools, and not something we need in general (e.g. for people using flit). As such I think the solution is for setuptools should offer better error.

[StefanD986]

Not only setuptools though. Also vswhere doesn't throw any error message that could help.

[gaborbernat]

What I meant is that it only affectr people calling vswhere 🤷 So it's a tool specific thing, which we generally don't patch within tox.