Thanks for filing this PR to test things out. As I predicted, the CI on Windows failed, even though your commit was trivial.
The first thing I notice is this:

https://dev.azure.com/twistedmatrix/twisted/_build/results?buildId=1970&view=logs&j=7f8b683a-20ec-5925-1eb0-0f9b67db6dcc&t=75248844-3620-5e60-359d-192232fba6b9

This:

python -c "import os; print(os.environ)"

fails with:

2020-06-21T07:24:15.5571943Z Traceback (most recent call last):
2020-06-21T07:24:15.5580812Z   File "<string>", line 1, in <module>
2020-06-21T07:24:15.5581991Z   File "C:\hostedtoolcache\windows\Python\3.6.8\x64\lib\encodings\cp1252.py", line 19, in encode
2020-06-21T07:24:15.5583012Z     return codecs.charmap_encode(input,self.errors,encoding_table)[0]
2020-06-21T07:24:15.5584308Z UnicodeEncodeError: 'charmap' codec can't encode character '\u0142' in position 2585: character maps to <undefined>
2020-06-21T07:24:15.6292586Z 64

Looks like the default encoding for the console is CP1252, and not UTF-8.

@hawkowl posted a good writeup of this here: https://twistedmatrix.com/pipermail/twisted-python/2020-June/065140.html

I need to figure out a way to force the Azure Pipeline's Window console to be UTF-8. I'm not sure how to do that.

@hawkowl yes I think you are right. On the Azure pipeline side, the docs for configuring an environment variable is here:

https://docs.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch

and on the Python side, the docs for those variables is here:

https://docs.python.org/3/using/cmdline.html#envvar-PYTHONIOENCODING

So if we can set the right value in the Azure pipeline yaml, we might be able to get this to work.

We might need to CHCP 65001 to force the codepage to UTF-8,
in addition to setting the PYTHONIOENCODING and PYTHONUTF8 environment variables.

https://ss64.com/nt/chcp.html

@hawkowl so I set PYTHONUTF8=1 in the azure yaml for windows, and I got the Python 3.7 and 3.8 builds to work. Looks like PYTHONUTF8 is only on Python 3.7+

In this log: https://dev.azure.com/twistedmatrix/twisted/_build/results?buildId=1975&view=logs&j=a9f5be88-9f44-53f4-896b-687d3085a015&t=2d2e9c85-c051-5b0a-8878-a82a75551fb6

I see an environment variable is getting set:

BUILD_SOURCEVERSIONAUTHOR Michał Górny

This confirm's my suspicion that Michał's non-ASCII name was getting exported as an environment variable somehow on the Windows build.

@hawkowl do you know what the correct incantation of PYTHONIOENCODING is for Python 3.5 and 3.6, but won't interfere with the PYTHONUTF8 variable on Python 3.7+?

@hawkowl OK, I managed to set PYTHONIOENCODING=utf-8:surrogateescape in the Azure pipeline, and things got further along.

However on Python 3.5 and 3.6 some tests are failing:

https://dev.azure.com/twistedmatrix/twisted/_build/results?buildId=1982&view=logs&j=b37a2146-32d6-5808-6b10-83380cf11296&t=f9c15f0d-fc29-520b-b028-e1b618593e2c&l=16499

Traceback (most recent call last):
  File "d:\a\1\s\build\alldeps-withcov-windows\lib\site-packages\twisted\test\test_process.py", line 2260, in test_stdinReader_bytesArgs
    d = self._test_stdinReader(pyExe, args, env, path)
  File "d:\a\1\s\build\alldeps-withcov-windows\lib\site-packages\twisted\test\test_process.py", line 2237, in _test_stdinReader
    reactor.spawnProcess(p, pyExe, args, env, path)
  File "d:\a\1\s\build\alldeps-withcov-windows\lib\site-packages\twisted\internet\posixbase.py", line 337, in spawnProcess
    args, env = self._checkProcessArgs(args, env)
  File "d:\a\1\s\build\alldeps-withcov-windows\lib\site-packages\twisted\internet\base.py", line 1014, in _checkProcessArgs
    "non-string value: {}".format(val))
builtins.TypeError: Environment contains a non-string value: Michał Górny

twisted.test.test_process.Win32ProcessTests.test_stdinReader_bytesArgs

In twisted/internet/base.py there is a function _checkProcessArgs() which
validates all arguments against sys.getfilesystemencoding().

It looks like on Python 3.5 and 3.6, setting PYTHONIOENCODING=utf-8:surrogateescape does not cause sys.getfilesystemencoding() to change.

My recommendation is to nuke the _checkProcessArgs() function from twisted/internet/base.py.

This was added 12 years ago, and is a leftover from Python 2.

In Python 3, os.environ is unicode on all platforms.

So I deleted _checkProcessArgs() and one accompanying test, and managed to get all the tests to pass on all platforms, with this environment variable set:

BUILD_SOURCEVERSIONAUTHOR Michał Górny

@hawkowl I can submit a separate PR to eliminate _checkProcessArgs(), but I don't know if you think that is the way to go in terms of solving this problem.

@mgorny thanks for helping me test this under your ID. I submitted a full PR here: #1302

I'm closing this PR as it appears to have been superseded by #1302. If I'm mistaken please feel free to re-open. Thank you all!

#1302 is not merged yet. We are leaving this PR open, to test when someone with non-ASCII characters in their name, specifically Michał Górny, files a PR.