Allow listing an empty drive root

[gaoflow]

Summary

• treat an empty top-level Drive listing as an empty directory instead of a missing path
• keep FileNotFoundError for missing non-root paths
• add unit tests for both paths

Tests

• .venv/bin/python -m pytest -q
• .venv/bin/pre-commit run --files gdrive_fsspec/core.py tests/test_core.py

Fixes #59

AI assistance was used under my direction.

[martindurant]

What happens if the drive in question doesn't exist at all? I would expect FileNotFound rather than an empty listing.

[gaoflow]

Good point. I updated the branch so the empty-root case and missing-root case are distinct.

Current behavior after 01ede5e:

• ls("") on an existing but empty default/root folder still returns [].
• ls("") with a custom root_file_id that exists but has no children still returns [].
• ls("") with a custom root_file_id whose listing is empty and whose id cannot be resolved now raises FileNotFoundError instead of caching/returning an empty listing.

I also rebased the branch onto the latest master, which should resolve the dirty merge state.

Verification:

.venv/bin/python -m pytest tests/test_core.py -q
# 31 passed, 2 deselected in 0.06s

.venv/bin/python -m pytest -q
# 31 passed, 2 deselected in 0.06s

ruff check gdrive_fsspec/core.py tests/test_core.py
# All checks passed!

ruff format --check gdrive_fsspec/core.py tests/test_core.py
# 2 files already formatted

git diff --check origin/master...HEAD
# no output

[martindurant]

Just one question

[martindurant]

Basically the same as self._list_directory_by_id ?

[gaoflow]

Not quite — _list_directory_by_id lists the children of a directory (files.list with a parents filter) and returns [] for a missing id, which is exactly the ambiguity this disambiguates. _file_id_exists does a metadata-only files.get (fields=id) to ask whether the id itself resolves. A missing custom root then raises FileNotFoundError while an empty real root still lists [].

[martindurant]

This gets rechecked on every call. That is ... OK, the cache doesn't have a way to say "tried but not found".

[gaoflow]

Right — agreed. The recheck only runs in the narrow case (empty listing of a custom root), and the dircache indeed has no way to record a negative, so it's the cheapest correct option.