file(1) / magic(5) database vs shared-mime-info database

[Bastien Nocera]

On Fri, 2018-02-23 at 23:39 -0800, Kip Warner wrote:
> Hey list,
> 
> Unless I'm misunderstanding things, it seems to me as though typical
> GNU typical distros seem to ship two different systems for
> determining
> a file's magic. 
> 
> There is the older file(1) / libmagic / magic(5) mechanism. One can
> test it via running file(1) on a particular file and it will identify
> it and can also provide the MIME type.
> 
> The second way is via /usr/share/mime/packages/<mypackage>.xml
> mechanism as described by the fd.o shared-mime-info-spec. Typical
> desktop shells like Gnome's Nautilus, Xfce's Thunar, pcmanfm, etc.,
> seem to identify files via this way.
> 
> The amount of redundant work in specifying file magic patterns for
> both
> systems is substantial considering how many different types of files
> there are out there in the wild. I am assuming, but I don't know this
> for certain, but the latter's
> /usr/share/mime/packages/freedesktop.org.xml might have been
> initially
> generated from file(1)'s upstream magic source database (ships as
> libmagic-mgc on my distro).
> 
> Ironically neither one of these two mechanisms seem to communicate
> with
> each other. If the second mechanism is intended to superannuate the
> first, would it not make sense to provide an fd.o replacement for
> file(1) which queries the system's shared-mime-info database instead?
> I
> would think this should be trivial to implement.

No, it wouldn't make sense. They have very different use cases and
restrictions. In shared-mime-info:
- mime-types don't have to have a magic associated
- and mime-types have globs associated
- the magic length is limited to avoid seeking through huge files
- descriptions are translated, acronyms can be split-off and expanded
- mime-types have inheritance

There's probably others, but that's already a good chunk of the
problems we'd encounter if we used file's database.