[basedir-spec] XDG_DATA_HOME and reboot-preserved read-write state?

[Sebastian Pipping]

Hi!


I am writing to you because I cannot find an answer about something in
basedir-spec (latest, 0.7 [1]) that seems well within its scope,
yet under-specified and controversial.  If you can help me with the
answer, I may be able to help you with the next version of the spec.

I think it's fair to understand the XDG basedir-spec as the user-
specific equivalent of some core FHS [2] locations, _roughly_:

  FHS         XDG variable     XDG default
  =================================================
  /var/cache  XDG_CACHE_HOME   $HOME/.cache
  /etc        XDG_CONFIG_HOME  $HOME/.config
  /usr/share  XDG_DATA_HOME    $HOME/.local/share
  /var/lock   XDG_RUNTIME_DIR  /run/user/$USER/ ?
  /var/run    XDG_RUNTIME_DIR  /run/user/$USER/ ?

Now there are some key locations of FHS that are _not_ in that list,
for instance:

  FHS location  Rough rules about content
  =================================================
  /var/lib      State information; persistent across reboots
  /var/log      Various logs
  /var/spool    Queue of tasks waiting to be processed
  /var/tmp      Temporary files to be preserved between reboots


_Maybe_ it's okay that these are not in XDG, _maybe_ that's a feature,
but the spec doesn't seem to say.

More specifically, the spec does not seem to say whether XDG_DATA_HOME
($HOME/.local/share) should

  (A) have the same read-only/write-once semantic that we know from
      /usr/share and /usr/local/share

or

  (B) be the home to all sorts of long-lived read-write state data
      (like you would expect from /var/lib for when things are less
      user-specific).

The common use of the word "share" in $HOME/.local/share and /usr/share
seems to support (A) but then where does all the long-lived _read-write_
state data go?  (The spec is clear that XDG_RUNTIME_DIR is not the
answer here (..).)

If (B) then why are apps expected to look for files in XDG_DATA_HOME and
all of XDG_DATA_DIRS if the defaults for XDG_DATA_DIRS are not writable
to unprivileged users?


I'm not the first [3] to wonder about this, yet I only see ideas,
guessing and discussions, no bulletproof answers.  That's the job of a
spec, right?

So I am hoping for a version 0.8 of the spec that says

 - where read-write state files that should survive reboots should go,
   and

 - how much of a catch-all pot XDG_DATA_HOME should be or not be,
   and

 - if ~/.local/var/lib/ is something that XDG is concerned about
   and has different ideas for.

That would be awesome for future discussions and decision making in free
software projects.  Many thanks in advance!

Best



Sebastian


[1]
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
[2] https://refspecs.linuxfoundation.org/fhs.shtml
[3] https://wiki.debian.org/XDGBaseDirectorySpecification#state