twisted.internet.test.test_process.*.test_openFileDescriptors fails in nixpkgs

[tjni]

Describe the incorrect behavior you saw

When the nixpkgs package manager builds and tests twisted, the following two tests fails:

twisted.internet.test.test_process.ProcessTestsBuilder_AsyncioSelectorReactorTests.test_openFileDescriptors
twisted.internet.test.test_process.ProcessTestsBuilder_SelectReactorTests.test_openFileDescriptors

Describe how to cause this behavior

It happens all of the time (but the only machine I have verified this on is my M1 Mac), but I only know how to reproduce it inside of nixpkgs at the moment. You can do so by installing the nix package manager and running:

nix build nixpkgs/staging-next#python310Packages.twisted --extra-experimental-features "nix-command flakes"

However, this is only reproducible currently on the staging or staging-next branches, which contain changes to core packages, and, due to how nix works, hundreds of packages require being rebuilt taking several hours. (Once changes move to the nixpkgs-unstable branch, they will be built and cached in a binary cache.)

Testing environment

❯ sw_vers                                                                                                                             took 26s
ProductName:    macOS
ProductVersion: 12.6
BuildVersion:   21G115

❯ twist --version
22.8.0

Additional context

The root cause is that executing the source defined for that test results in:

Traceback (most recent call last):
  File "<string>", line 4, in <module>
  File "/nix/store/pvfxk0gw5nb276gqhb2xj5w24w67zwgi-python3.10-twisted-22.8.0/lib/python3.10/site-packages/twisted/__init__.py", line 10, in <module>
    from twisted._version import __version__ as version
  File "/nix/store/pvfxk0gw5nb276gqhb2xj5w24w67zwgi-python3.10-twisted-22.8.0/lib/python3.10/site-packages/twisted/_version.py", line 8, in <module>
    from incremental import Version\nModuleNotFoundError: No module named \'incremental\'

Indeed, there is nothing else in that site-packages directory except for twisted. My guess is that the build in nixpkgs adds test dependencies specified to PYTHONPATH instead of using virtual environments.

From looking at the code, I think that a solution (which I have verified fixes the issue in nixpkgs) is to emulate the other tests in that file and pass properEnvs to the spawned process. I will create a PR illustrating this and attach it to this issue, but please feel free to reject it if you think it isn't the right way to resolve this.