Fake os.walk does not resolve symlinks properly

[giladreti]

Describe the bug
os.walk does not reolve the directory before checking if it is a symlink.

    def do_walk(top_dir, top_most=False):
        if not top_most and not followlinks and filesystem.islink(top_dir):
            return
        try:
            top_contents = _classify_directory_contents(filesystem, top_dir)
        except OSError as exc:
            top_contents = None
            if onerror is not None:
                onerror(exc)

        if top_contents is not None:
            if topdown:
                yield top_contents

            for directory in top_contents[1]:
                if not followlinks and filesystem.islink(directory): # here should check filesystem.joinpaths(top_dir, directory)
                    continue
                for contents in do_walk(filesystem.joinpaths(top_dir,
                                                             directory)):
                    yield contents

            if not topdown:
                yield top_contents

How To Reproduce
import os
from pathlib import Path

from pyfakefs.fake_filesystem_unittest import Patcher

def bug():
    with Patcher() as patcher:
        filesystem = patcher.fs
        filesystem.is_windows_fs = filesystem.is_macos = False
        filesystem.reset()

        filesystem.add_real_file(__file__)

        dir_path = Path('/test1/test2')
        dir_path.mkdir(parents=True)

        test_file = dir_path / 'test3'
        test_file.touch()

        print(list(os.walk('/test1')))

        Path('/test2').symlink_to(test_file)

        print(list(os.walk('/test1')))


bug()

Output is:

[('/test1', ['test2'], []), ('/test1\\test2', [], ['test3'])]
[('/test1', ['test2'], [])]

Your enviroment
Please run the following and paste the output.

python -c "import platform; print(platform.platform())"
python -c "import sys; print('Python', sys.version)"
python -c "from pyfakefs.fake_filesystem import __version__; print('pyfakefs', __version__)"

Output is:

C:\Users\gilad\Documents\my_module>python -c "import platform; print(platform.platform())"
Windows-10-10.0.19041-SP0

C:\Users\gilad\Documents\my_module>python -c "import sys; print('Python', sys.version)"
Python 3.8.3 (tags/v3.8.3:6f8c832, May 13 2020, 22:37:02) [MSC v.1924 64 bit (AMD64)]

C:\Users\gilad\Documents\my_module>python -c "from pyfakefs.fake_filesystem import __version__; print('pyfakefs', __version__)"
pyfakefs 4.1.0

[mrbean-bremen]

Good catch! And your proposed fix is also correct.
As an aside--I noticed the inconsistent path separators in your example, and added a fix and a more convenient way to set the file system OS, so thanks for that.
I would appreciate if you could test the master after I add the fix. I will wait for further issues and probably make a new release if none will appear for some time.

[mrbean-bremen]

And done.

[mrbean-bremen]

@giladreti - will you still be testing this? Otherwise, I would make a new release with the changes.

[giladreti]

I couldn't find any more bugs, so it is fine by me :)