To: Debian developers list <debian-devel@pixar.com>
Subject: Re: dpkg maintainer script calls and arguments
In-Reply-To: <1506077@toto.iv>
FCC: ~/mail/Outbound
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In order to support easier upgrades where important packages get split
into several pieces, I have implemented the following scheme, which I
described on debian-private a while ago.  It is enabled by the use of
`--auto-deconfigure', or `-B', and the dselect method scripts in
0.93.76 have been changed to supply this option.

] I plan to make it possible to `deconfigure' packages at installation
] time, in order to keep dependency invariants satisfied.  In a sane
] installation this will mean that everything will work right, even when
] (for example) an important package is split into two pieces or
] packages.
] 
] Basically, suppose that package A is being split into A1 and A2, and B
] and C depend on A1 and A2 respectively.  If you try to install A1 dpkg
] will consider removing A (because of the conflict between A and each
] of A1 and A2), but then C's dependency is not satisifed, and if you
] try to install A2 B's dependency wouldn't be satisfied.
] 
] At the moment dpkg will simply refuse to do it.  You have to say
] --force-depends, or remove either B or C.
] 
] I'm going to arrange that dpkg will automatically deconfigure B or C,
] as appropriate, and try to reconfigure it later.
] 
] So, if you do `dpkg -i A1.deb A2.deb' all will be well; if you do
] `dpkg -i A1.deb' you'll get A1 installed and configured correctly, but
] error messages about C being broken, and in order to fix C you'll have
] to install A2 as well, or return to A (dpkg will remove A1).
] 
] All of this will appear very automatic to people who use dselect.
] People who do things manually will have a slightly more complicated
] task, as dpkg won't remove A (in the scenario above) unless it has
] been selected for deinstallation using dselect or dpkg --remove (which
] would fail because of the dependencies from B and C).

This means that maintainer scripts can get called in two new ways:
  <prerm(C)> deconfigure in-favour <A1> <version> removing <A> <v.>
  <postinst(C)> abort-deconfigure in-favour <A1> <version> removing <A> <v.>
using the example package names above.

The first call happens before the prerm script of the package which is
being removed (A) is called; the second happens if an error occurs and
dpkg wants to back out of the installation.

If the installation of both A1 and A2 is successful dpkg will then
call both
  <postinst(B)> configure
  <postinst(C)> configure
as usual.

Some time ago I posted a message documenting all the maintainer script
calls and their arguments.  Below is a revised version of that
message.  I shall upload it as maintainer-script-args.txt, and it
should go in project/standards.  The top half of this message will go
in auto-deconfiguration.txt.

Ian.