Improve LayerFilesystem iterdir and scandir

[JSCU-CNI]

This PR prevents NotADirectoryError DirectoryFilesystemEntry exceptions from crashing plugins when iterating LayerFilesystem entries using recurse, iterdir or scandir.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 77.69%. Comparing base (334e6a4) to head (14758e6).

Files with missing lines	Patch %	Lines
dissect/target/plugins/filesystem/walkfs.py	33.33%	4 Missing ⚠️
dissect/target/filesystem.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #973      +/-   ##
==========================================
- Coverage   77.71%   77.69%   -0.02%     
==========================================
  Files         326      326              
  Lines       28559    28569      +10     
==========================================
+ Hits        22194    22198       +4     
- Misses       6365     6371       +6     

Flag	Coverage Δ
unittests	77.69% <40.00%> (-0.02%)	⬇️

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[Schamper]

Can you add unit tests?

[JSCU-CNI]

Upon further debugging we discovered the root cause for these NotADirectoryError exceptions.

The Overlay2Filesystem uses the DirectoryFilesystem to map layer folders inside a target to a filesystem. The DirectoryFilesystem does not seem well suited for mapping TargetPath folders as it seems to originally have been created for mapping folders originating from the host system. This is where the problem starts.

When a TargetPath layer inside Overlay2Filesystem is mapped, at some point when iterating over the filesystem, DirectoryFilesystemEntry resolves an entry to a file entry on the host system using os.*. For privileged paths this will cause a PermissionDenied exception, which propagates to a FilesystemError and causes is_dir to be false.

This is undesired behaviour to say the least, so we replaced DirectoryFilesystem with a VirtualFilesystem and created a map_vdir method for VirtualFilesystem. Perhaps a proper VirtualDirectoryFilesystem implementation should be created at some point, but this seems like a valid solution for the meantime.

[Schamper]

@JSCU-CNI can you still add a unit test that showcases the originally faulty behavior so that I can try some other ways to fix this?