Linux/POSIX properties persistence

[adreed-msft]

This PR implements a new flag, --preserve-posix-properties that allows a user to persist the results of a statx(2) (or stat(2) if statx is unavailable) syscall on upload.

Download is not implemented in this PR. S2S is handled by metadata, and in S2S --preserve-posix-properties acts equivalently to --include-directory-stubs.

[shalinijoshi19]

Hey @adreed-msft ! Thanks for putting this up - question - does this also include the metadata side-car'ing to Az Blobs in particular?

[linuxsmiths]

LG mostly. Few comments.

[linuxsmiths]

better to align the types with statx struct.
so numLinks should be uint32
and mode should be uint16

[adreed-msft]

I disagree. This struct (and interface) is OS-agnostic, but POSIX specific. If we decide to support some other POSIX-based OS like MacOS or FreeBSD in the future for the POSIX property persistence feature, this could cause problems if we adhere to Linux-specific data typing.

[linuxsmiths]

I meant those sizes are POSIX specific not Linux specific ("stat()" is a POSIX call and these sizes are not specific to statx()). So any POSIX OS should have the same sizes.

[shalinijoshi19]

The PR remains in draft state, as download is partially broken in some aspects (attributes fail to even be written, etc.), there is no automated testing of the feature yet, and the build fails on other OSes due to certain function signatures..

This PR implements a new flag, --preserve-posix-properties that allows a user to persist the results of a statx(2) (or stat(2) if statx is unavailable) syscall.

To work around cases where the representative device ID may not be related on the destination system, --preserve-posix-properties has two modes: FullFidelity, and SpecialFilesAndProperties, where FullFidelity persists even files representing devices, and SpecialFilesAndProperties skips over these files.

Do update the description wiht this version of the PR pls! thanks

[zezha-msft]

lgtm