pathlib exception when symlinks are involved

[tg2648]

Describe the bug

Hello! First of all, thank you for this invaluable tool.

I first ran into this issue on Windows when using black to format code located on a mapped network drive.

This line in get_sources() doesn't ignore symlinks pointing outside of root, which has symlinks resolved by find_project_root():

black/src/black/__init__.py

Line 687 in a0e270d

root_relative_path = path.absolute().relative_to(root).as_posix()

To Reproduce on Linux

mkdir test_black
ln -s test_black test_black_sym
cd test_black
git init
echo "print('hello')" > app.py
black ~/test_black_sym/app.py

git init is needed so that find_project_root() doesn't return the root of the file system.

The resulting error is:

Traceback (most recent call last):
  <some omitted>
  File "/home/tg2648/black/src/black/__init__.py", line 600, in main
    sources = get_sources(
              ^^^^^^^^^^^^
  File "/home/tg2648/black/src/black/__init__.py", line 687, in get_sources
    root_relative_path = path.absolute().relative_to(root).as_posix()
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tg2648/.pyenv/versions/3.11.0/lib/python3.11/pathlib.py", line 730, in relative_to
    raise ValueError("{!r} is not in the subpath of {!r}"
ValueError: '/home/tg2648/test_black_sym/app.py' is not in the subpath of '/home/tg2648/test_black' OR one path is relative and the other is absolute.

Environment

• Black's version: 23.11.0
• OS and Python version: Windows and Linux / Python 3.11.0

Ideas

normalize_path_maybe_ignore(), which is used later in get_sources(), also calculates the root relative path but catches the ValueError exception:

black/src/black/files.py

Lines 262 to 269 in 46be1f8

	try:
	root_relative_path = normalized_path.relative_to(root).as_posix()
	except ValueError:
	if report:
	report.path_ignored(
	path, f"is a symbolic link that points outside {root}"
	)
	return None

Maybe one solution is to extract these lines into a function like get_root_relative_path and then use it in get_sources() and normalize_path_maybe_ignore(). I can submit a PR if this sounds reasonable.

[faph]

Subscribing as we are affected by this too.