dpkg/doc/disappear-replace.txt

From ian Tue Apr 18 23:30:04 1995 
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To: Debian developers list <debian-devel@pixar.com>
Subject: Handling of base packages

I propose to implement the following scheme to enable obsolete base
packages to disappear, and to allow files in the base packages to move
between one package and another.

1. When a package is installed, and contains files that are already
marked as belonging to some other package, the older package will have
the files that have been overwritten removed from its file list.

2. When a package ceases to contain any files due to the action of
point 1 above, its postrm script is run with the argument `disappear'
(in place of `remove', `purge' or whatever).  It will then be moved
into the `purge ok not-installed' state, so that it will cease to
appear in dpkg and dselect lists.  Its conffiles will be ignored,
*not* purged.  The prerm will *not* be run as the packaging system
doesn't know what files are in a package until it unpacks it.

This will all happen during the `unpack' phase of the replacing
package.

3. If a base system package which is being installed conflicts with
another base system package which is currently installed on the
system, the currently installed one will be removed first (the prerm
will be run with `replace <package> <version>' as arguments, then the
package will be removed, then the postrm will be run, likewise with
`replace').  If the replacement fails the removal will be aborted,
involving running the old packages' scripts with `abort-replace'.

4. Base system packages may not be removed except under 2. or 3.
above.  (There will be a --force-remove-base flag to allow foolhardy
users to go ahead anyway.)

Ian.