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mtree: add xattr support #1400

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mtree: add xattr support #1400

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Commits on Jul 18, 2020

  1. Factor out existing base64 encoder/decoder used for xattrs

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    maxcrees committed Jul 18, 2020
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  2. mtree: add xattr support 

    This adds support for optionally writing xattrs to mtree output, and
limited ability to read them from mtree input. The format of the keyword
is the following, one for each xattr present for the entry:

xattr.<name>=<b64 encoded value>

This format is inspired by go-mtree by Vincent Batts, though
there is a possible slight incompatibility: libarchive's existing base64
encoder does not perform padding because it was written to encode xattrs
into PAX attributes which already have a length component. The decoder
should accept padding ok, but the mtree parser may not like the
unescaped '=' characters.

The name is escaped according to existing mtree escaping practices which
is octal encoding of [^!-}] except [ #=\] is also encoded.  go-mtree
performs its own implementation of vis(3) on it with the flags
VIS_WHITE, VIS_OCTAL, and VIS_GLOB. There is some disjointedness here
but it should be compatible in the common case of no escaping of course.

Note that for output this is gated on --options=mtree:xattrs, which I've
left out of the default mtree options.

So far I've tested converting a PAX file directly to mtree with bsdtar
and it seems to work well.

todo:

* when reading an mtree, also fill in xattrs from disk if available -
  this is tricky...
* PAX tar files made by libarchive default to putting both the
  LIBARCHIVE and SCHILY PAX attributes in order to encode xattrs, but
  this causes libarchive to store the same xattr twice internally.
  Therefore if converting directly from PAX to mtree you get duplicated
  xattrs in the output. I don't think there's any easy way to fix this
  other than changing how libarchive keeps track of xattrs internally.
    @maxcrees
    maxcrees committed Jul 18, 2020
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  3. Fix building with cmake

    @maxcrees
    maxcrees committed Jul 18, 2020
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