mingw tests fail in CI

[imphil]

Tests running in mingw-env in CI fail now, most likely not due to a change in our code, but due to an environment problem in the CI testing.

Good: https://github.com/cocotb/cocotb/runs/3753710579?check_suite_focus=true (30 Sep 2021 08:32)
Bad: https://github.com/cocotb/cocotb/runs/3898922153?check_suite_focus=true (14 Oct 2021 19:40)

Things I know changed:

• msys2/setup-msys2@v2 updated from 549e904fc53cb840ec72fec29f3e8fc00f579b5b (v2.4.2) to 6e532aa72709f068e400cc0677a3f49162891b0e (v2.5.0)
• The Windows base image changed from 20210920.1 to 20211011.0

Error:

Python path configuration:
  PYTHONHOME = 'D:/a/_temp/msys/msys64/mingw64'
  PYTHONPATH = (not set)
  program name = 'D:/a/_temp/msys64/mingw64/bin/python.exe'
  isolated = 0
  environment = 1
  user site = 1
  import site = 1
  sys._base_executable = 'D:/a/_temp/msys64/mingw64/bin/python.exe'
  sys.base_prefix = 'D:/a/_temp/msys/msys64/mingw64'
  sys.base_exec_prefix = 'D:/a/_temp/msys/msys64/mingw64'
  sys.platlibdir = 'lib'
  sys.executable = 'D:/a/_temp/msys64/mingw64/bin/python.exe'
  sys.prefix = 'D:/a/_temp/msys/msys64/mingw64'
  sys.exec_prefix = 'D:/a/_temp/msys/msys64/mingw64'
  sys.path = [
    'D:/a/_temp/msys/msys64/mingw64/lib/python39.zip',
    'D:/a/_temp/msys/msys64/mingw64/lib/python3.9',
    'D:/a/_temp/msys/msys64/mingw64/lib/python3.9/lib-dynload',
    '',
  ]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'

[imphil]

Looks like it's down to the setup-msys2 version: 2.4.2 works, 2.5.0 doesn't. Diff between the log output of the two versions:
diff.txt

I don't have the PYTHONHOME environment variable for a successful run, but the path looks suspicious:

PYTHONHOME = 'D:/a/_temp/msys/msys64/mingw64'
program name = 'D:/a/_temp/msys64/mingw64/bin/python.exe'

Look at the D:/a/_temp/msys/msys64 vs D:/a/_temp/msys64/mingw64 difference.

setup-msys2 v2.5.0 changed some paths in msys2/setup-msys2#163, which would be the most likely cause for this change in behavior.

[imphil]

Upstream bug report: msys2/setup-msys2#167
Workaround: #2740