4.0.0 crashes with TypeError

[felixxm]

We noticed a regression in flake8==4.0.0 bisected to the d1a4043. Running flake8 on the Django repository crashes, e.g.

$ flake8
Traceback (most recent call last):
  File "/.virtualenvs/django-test-3.8/bin/flake8", line 8, in <module>
    sys.exit(main())
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/main/cli.py", line 22, in main
    app.run(argv)
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/main/application.py", line 375, in run
    self._run(argv)
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/main/application.py", line 364, in _run
    self.run_checks()
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/main/application.py", line 271, in run_checks
    self.file_checker_manager.run()
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/checker.py", line 307, in run
    self.run_parallel()
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/checker.py", line 274, in run_parallel
    filename, results, statistics = ret
TypeError: cannot unpack non-iterable NoneType object

flake8 is installed via pip:

$ python -m pip install flake8

$ flake8 --bug-report
{
  "dependencies": [],
  "platform": {
    "python_implementation": "CPython",
    "python_version": "3.8.10",
    "system": "Linux"
  },
  "plugins": [
    {
      "is_local": false,
      "plugin": "assertive",
      "version": "1.3.0"
    },
    {
      "is_local": false,
      "plugin": "mccabe",
      "version": "0.6.1"
    },
    {
      "is_local": false,
      "plugin": "pycodestyle",
      "version": "2.8.0"
    },
    {
      "is_local": false,
      "plugin": "pyflakes",
      "version": "2.4.0"
    }
  ],
  "version": "4.0.0"
}

[felixxm]

It doesn't crash with a single subprocess:

flake8 -j 1

[sigmavirus24]

Do you have a minimal code example of when this happens?

[arporter]

This should do it:

git clone https://github.com/stfc/PSyclone.git
cd PSyclone
flake8 --exit-zero src/psyclone/tests/test_files/dynamo0p3

If I run flake8 on the files in that directory individually then all is well. It only crashes when given the directory.

[felixxm]

Do you have a minimal code example of when this happens?

I narrowed this issue to a single directory in our test suite: tests/test_runner_apps/tagged

$ flake8 tests/test_runner_apps/tagged
...
  File "/.virtualenvs/django-test-3.8/lib/python3.8/site-packages/flake8/checker.py", line 274, in run_parallel
    filename, results, statistics = ret
TypeError: cannot unpack non-iterable NoneType object

[felixxm]

It works when I delete a file with the syntax error which is in it 🤔.

[arporter]

Yes, one of the files in the directory that causes problems for us also has a (deliberate) syntax error in it.

[felixxm]

Many thanks!