tox: detox worked, `tox -p auto` hangs with `trial -j` tests

I’ve been happily running my test suite in detox for the past year or so. Today I upgraded to tox with the --parallel option, and suddenly my unit-tests environment hangs, despite mypy, lint, and integration-tests (which uses the same test runner and similar options) all passing and exiting nicely.

Here are some hopefully relevant details:

• The test runner in question is trial -j 8, so the subprocess has subprocesses of its own, which may be confounding things.

• unit-tests is the longest running environment.

• When the tests hang, I see a --installpkg process as well as all of my trial worker processes hanging. As such, I tried --parallel--safe-build just in case, but it didn’t change the behavior at all.

If submitting a BUG please provide:

• Minimal reproducible example or detailed description, assign “bug”

Sorry to say that I can’t produce a minimal reproducer; thus far I’ve only managed to produce this on a proprietary test suite.

• OS and pip list output

macOS 10.14.3

Package    Version
---------- -------
filelock   3.0.10
pip        18.1
pluggy     0.9.0
py         1.8.0
setuptools 40.5.0
six        1.12.0
toml       0.10.0
tox        3.7.0
virtualenv 16.4.3
wheel      0.32.2