`patch = subprocess` not working in github actions specifically on Windows

[jepler]

Describe the bug
I want to use [run]patch = subprocess on Windows in github actions, but I get a ModuleNotFoundError and coverage is incomplete.

To Reproduce

1. Python 3.x (=3.13.5) installed with actions/setup-python on windows-latest in github actions
2. The minimized test case is at https://github.com/jepler/covsub
3. coverage installed with pip install coverage after pip install wheel. No other non-standard packages required
4. Run the github actions

Expected behavior
The subprocess patch should work in all 3 environments (linux, windows, and macos). However, it fails on Windows:

Error processing line 1 of C:\hostedtoolcache\windows\Python\3.13.5\x64\subcover.pth:

  Traceback (most recent call last):
    File "<frozen site>", line 207, in addpackage
    File "<string>", line 1, in <module>
  ModuleNotFoundError: No module named 'coverage'

Remainder of file ignored
function 1

Furthermore, the overall project coverage is lower than expected, 73% instead of 96%, because the sub-processes are not covered.

(Without --fail-under= the actions would have been green because the subprocesses do return a success value)

Additional context

This is a minimized reproducer. I ran into a problem in the context of an existing project when I tried to adapt it to use patch=subprocess.

I have some vague suspicions that maybe there's a problem with the ordering of .pth files or something like that, but it's difficult to inspect the state of a github actions vm after a failure and I didn't spend much time trying to determine the underlying cause(s).

[nedbat]

I don't understand all the mechanisms at work here. Windows reports two directories from site.getsitepackages(). I was writing to the first one. If I write .pth files into both directories, and be prepared for importing coverage to fail, then it works.

Fixed in commit 958e30e.

[jepler]

Thanks! I just confirmed that in my project's CI patch = subprocess now appears to work properly across all the architectures I test.

[nedbat]

This is now released as part of coverage 7.10.3.