Adding/removing directory entries does not respect directory ownership

[vector-of-bool]

Describe the bug
Adding or removing entries from a directory requires that the user has write permissions to that directory. Trying to modify entries in a directory to which the user cannot write should raise PermissionError.

How To Reproduce
The following test cases should both succeed:

import os
import pytest
import pyfakefs.fake_filesystem

def test_fail_write_dir(fs):
    # Prereq: Create a directory at '/readonly-dir' that is owned by root and has
    # permissions 0o755 (rwxr-xr-x)
    rodir = "/readonly-dir"
    os.makedirs(rodir, exist_ok=True)
    st = os.stat(rodir)
    if st.st_uid != 0:
        os.chown(rodir, 0, 0)
        os.chmod(rodir, 0b111_101_101)
        st = os.stat(rodir)
    # Only modifyable by the root, but readable/traversable by all
    assert st.st_uid == 0
    assert st.st_mode & 0o777 == 0b111_101_101  # == 0o755
    # Try to add a new entry to the readonly subdirectory
    with pytest.raises(PermissionError):
        with open(f"{rodir}/file.txt", "w"):
            pass


def test_fail_fakefs_root_owned_dir():
    fs = pyfakefs.fake_filesystem.FakeFilesystem()
    fake_os = pyfakefs.fake_filesystem.FakeOsModule(fs)
    # Create a regular file
    fs.create_file("file.txt")
    # Create a directory that can only be written-to by root
    fake_os.mkdir("/readonly-dir", mode=0b111_101_101)
    fake_os.chown("/readonly-dir", 0, 0)
    # Creating a new entry should fail
    with pytest.raises(PermissionError):
        fake_os.link("/file.txt", "/readonly-dir/file.txt")

Your environment
Please run the following in the environment where the problem happened and
paste the output.

$ python -c "import platform; print(platform.platform())"
Linux-6.7.6-arch1-1-x86_64-with-glibc2.39
$ python -c "import sys; print('Python', sys.version)"
Python 3.11.7 (main, Jan 29 2024, 16:03:57) [GCC 13.2.1 20230801]
$ python -c "from pyfakefs import __version__; print('pyfakefs', __version__)"
pyfakefs 5.3.5
$ python -c "import pytest; print('pytest', pytest.__version__)"
pytest 7.2.0

---

If I understand correctly, the issue appears to be in FakeDirectory.add_entry, which starts with this guard:

if (
    not helpers.is_root()
    and not self.st_mode & helpers.PERM_WRITE
    and not self.filesystem.is_windows_fs
):
    raise OSError(errno.EACCES, "Permission Denied", self.path)

The check on st_mode checks that the S_IWUSR bit is set, but does not consider the case that st_uid != get_uid().

I'm trying to write code that is robust against "adverse" filesystem conditions, with non-writable directories being a common example.

[mrbean-bremen]

Thanks! pyfakefs currently has only a rudimentary concept of user rights - it is currently mostly concerned with root vs. non-root users, so this (and your other issue) are not surprising.
Thanks for digging into this and providing the tests - I will have a go at this, but that may take a bit.