Re: Declarative packaging (Was: Re: Intended MBF: maintainer scripts not using strict mode) [and 1 more messages]

[Ian Jackson <ijackson@chiark.greenend.org.uk>]

(adding debian-dpkg)


I think it would be worth stepping back a moment and thinking about
why we don't have declarative machinery for things like this.  (And
also about why we have ucf as well as dpkg conffile prompts.)

* There is a potential difficulty with sequencing of the
  install/removal-time actions requested by declarative configuration:
  some of them may need to be interleaved with maintscript actions or
  dpkg actions.

  Figuring out exactly how this should be done needs to be done
  separately for each proposed new declarative mechanism.  It's
  nontrivial work.

* New declarative schemes can be implemented in dpkg.  But that means
  implementing them in C, and is awkward.  It also involves engaging
  with the necessarily-conservative dpkg upstream.  Both of these
  factors make experimentation awkward.

* Alternatively, we could try to design some kind of extension
  mechanism.  The design troubles and possible pitfalls here are
  obvious...


I suggest the following approach:

Create an extension mechanism which allows dpkg's primary C code to
retain responsibility for sequencing (and which prevents out-of-tree
extensions from causing havoc).

Something like a set of ordered tables in dpkg's C code which contains
information about declarative extensions.  Each kind of maintscript
would have a "before" table and an "after" table.

When running a maintainer script, dpkg would walk the "before" and
"after" tables before and after running the script.  For each table
entry NAME
   - if the .deb contains no control member NAME, skip the entry
   - look for /usr/lib/dpkg/declarative/NAME
   - if ENOENT, skip the entry
   - otherwise, execute
      /usr/lib/dpkg/declarative/NAME < /var/lib/dpkg/info/P.NAME
     with env vars set to various helpful values.
   - if it fails, treat it like a maintscript failure.
   - the extension is permitted to write to
      /var/lib/dpkg/declarative/NAME/

This scheme allows an declarative extension to be implemented in a
separate package, with dpkg's cooperation.  The extensions' users need
to (pre-)Depend on the extension and the relevant dpkg version.


Michael Biebl writes ("Re: Declarative packaging (Was: Re: Intended MBF: maintainer scripts not using strict mode)"):
> systemd provides a facility called systemd-sysusers which allows to
> describe system user accounts declaratively. Maybe we could leverage that.

Not unless it can be decoupled from the rest of systemd, of course.

Michael Biebl writes ("Re: Declarative packaging (Was: Re: Intended MBF: maintainer scripts not using strict mode)"):
> To avoid any confusion: The dh_sysuser helper has no relationship with
> systemd-sysuser.

Thanks for the clarification.

> They work quite differently from what I can see. dh_sysuser is simply a
> helper which automates generating maintainer scripts code calling adduser.

That is good, because it means that if we invent a new declarative
format to be used in .debs, dh_sysuser could be changed to take
advantage of it.  So we could get the benefits (reduced fragility,
better management, etc. etc.) of declarative configuration without
needing to go through all the source packages up-front.

Ian.