Support linux file attributes in 7z archives

[weltkante]

Someone posted an archive over at adamhathcock/sharpcompress#73 which can't be unpacked, neither by sharpcompress nor my own managed-lzma implementation. The official 7z application seems to be able to unpack it.

Should investigate at what point my implementation diverges from the official source.

[weltkante]

Link to archive is http://springrts.com/dl/buildbot/default/develop/102.0.1-19-gfbc1b5b/win32/spring_%7bdevelop%7d102.0.1-19-gfbc1b5b_win32-minimal-portable.7z

[weltkante]

In a first look the metadata section of the 7z file (compressed via lzma) decodes properly (decoded checksum matches the header value) but then parsing the metadata section yields strange file attributes which make no sense; from there on it is probably broken. Right now I'm suspecting a bug in the metadata parsing code.

[weltkante]

Update: the archive has been created on linux, using linux file attributes. I don't parse these properly since they use different constants.

If I ignore the linux file attributes the decoder still throws an exception, so something else is going wrong.

[weltkante]

For the second part its the same issue as #16 just for the lzma decoder instead of lzma2 ... oops

I'm fixing that over at the other issue, leaving this issue to deal with the linux file attributes.

[weltkante]

Further research shows that linux file attributes are not officially supported by 7z archives, but some 3rd party implementations abused the fact that the official codebase is lazy in its error checking and started hiding linux file attributes in unused metadata bits.

Newer versions of the 7z GUI client acknowledge this and try to parse this unofficial extension, but the main 7z codebase still does not support it, it is only handled in the UI layer.

However since it seems to have become a de-facto standard I'll update my implementation to be able to deal with it.

[weltkante]

This should make it possible to read archives using this extension. I'll create a separate issue to track the possible feature request of being able to extract those posix file attributes.

[weltkante]

The fix for this is included in alpha-6