Confusing error - OSError: AF_UNIX path too long - when default temporary directory is too long

[adamtheturtle]

Describe the bug A clear and concise description of what the bug is.

When black --diff is run with a long temporary directory, a confusing OSError is raised.

To Reproduce Steps to reproduce the behavior:

1. Create a directory with >= 2 Python files
2. Set the default temporary directory for tempfile.mkstemp() to something long (e.g. by using export TMPDIR=$TMPDIR$TMPDIR$TMPDIR; mkdir -p $TMPDIR).
3. Run Black on it with the --diff option
4. See error

Process SyncManager-1:
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/[email protected]/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/opt/homebrew/Cellar/[email protected]/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/homebrew/Cellar/[email protected]/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/managers.py", line 582, in _run_server
    server = cls._Server(registry, address, authkey, serializer)
  File "/opt/homebrew/Cellar/[email protected]/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/managers.py", line 155, in __init__
    self.listener = Listener(address=address, backlog=16)
  File "/opt/homebrew/Cellar/[email protected]/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/connection.py", line 453, in __init__
    self._listener = SocketListener(address, family, backlog)
  File "/opt/homebrew/Cellar/[email protected]/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/connection.py", line 596, in __init__
    self._socket.bind(address)
OSError: AF_UNIX path too long

Aborted!

Expected behavior A clear and concise description of what you expected to happen.

I expected to see the diff suggested by Black.

Else, I would prefer to see a clear error.

Environment (please complete the following information):

• Version: 20.8b1
• OS and Python version: macOS and Linux. Python 3.9.4

Does this bug also happen on master? To answer this, you have two options:

Yes.

Additional context Add any other context about the problem here.

The reason for this bug is that multiprocessing.Manager() creates a socket.
The socket default address depends on the default temporary directory.
Sockets have a maximum address length.

In my use case I am running on CI where I do not control the temporary directory. I therefore have to omit --diff.

[austinbutler]

Found a few ways others have fixed this in the past:
https://github.com/agronholm/anyio/pull/42/files#diff-f0c1c7497545019b8489fa8bc06be6cdR208
aio-libs/aiohttp#3832
https://github.com/Clinical-Genomics/BALSAMIC/pull/550/files