dpkg-maintscript-helper(1)

NAME
dpkg-maintscript-helper - works around known dpkg limitations in maintainer scripts

SYNOPSIS
dpkg-maintscript-helper command [parameter...] -- maint-script-parameter...

COMMANDS AND PARAMETERS

rm_conffile conffile [lastversion [package]]

mv_conffile oldconffile newconffile [lastversion [package]]

DESCRIPTION

This program is designed to be run within maintainer scripts to achieve some tasks that dpkg can't (yet) handle natively either because of design decisions or due to current limitations.

Many of those tasks require coordinated actions from several maintainer scripts (preinst, postinst, prerm, postrm). To avoid mistakes the same call simply needs to be put in all scripts and the program will automatically adapt its behaviour based on the environment variable DPKG_MAINTSCRIPT_NAME and on the maintainer scripts arguments that you have to forward after a double dash.

CONFFILE RELATED TASKS

When upgrading a package, dpkg will not automatically remove a conffile (a configuration file for which dpkg should preserve user changes) if it is not present in the newer version. There are two principal reasons for this; the first is that the conffile could've been dropped by accident and the next version could restore it, users wouldn't want their changes thrown away. The second is to allow packages to transition files from a dpkg-maintained conffile to a file maintained by the package's maintainer scripts, usually with a tool like debconf or ucf.

This means that if a package is intended to rename or remove a conffile, it must explicitly do so and dpkg-maintscript-helper can be used to implement graceful deletion and moving of conffiles within maintainer scripts.

  REMOVING A CONFFILE

If a conffile is completely removed, it should be removed from disk, unless the user has modified it. If there are local modifications, they should be preserved. If the package upgrades aborts, the newly obsolete conffile should not disappear.

All of this is implemented by putting the following shell snippet in the preinst, postinst and postrm maintainer scripts:

    dpkg-maintscript-helper rm_conffile \
        conffile lastversion package -- "$@"

conffile is the filename of the conffile to remove. lastversion is the last version of the package that contained the conffile (or the last version of the package that did not take care to remove the obsolete conffile if this was not immediately implemented). If lastversion is empty or omitted, then the operation is tried on every upgrade. package is the package name, it's optional as it will default to $DPKG_MAINTSCRIPT_PACKAGE (this variable is set by dpkg to the name of the package acted upon). All the parameters of the maintainer scripts have to be forwarded to the program after "--".

Current implementation: in the preinst, it checks if the conffile was modified and renames it either to conffile.dpkg-remove (if not modified) or to conffile.dpkg-backup (if modified). In the postinst, the latter file is renamed to conffile.dpkg-bak and kept for reference as it contains user modifications but the former will be removed. If the package upgrade aborts, the postrm reinstalls the original conffile. During purge, the postrm will also delete the .dpkg-bak file kept up to now.

  RENAMING A CONFFILE

If a conffile is moved from one location to another, you need to make sure you move across any changes the user has made. This may seem a simple change to the preinst script at first, however that will result in the user being prompted by dpkg to approve the conffile edits even though they are not responsible of them.

Graceful renaming can be implemented by putting the following shell snippet in the preinst, postinst and postrm maintainer scripts:

    dpkg-maintscript-helper mv_conffile \
        oldconffile newconffile lastversion package -- "$@"

oldconffile and newconffile are the old and new name of the conffile to rename. lastversion is the last version of the package that contained the conffile with the old name. If lastversion is empty or omitted, then the operation is tried on every upgrade (note: it's safer to give the version and have the operation tried only once). package is the package name, it's optional as it will default to $DPKG_MAINTSCRIPT_PACKAGE (this variable is set by dpkg to the name of the package acted upon). All the parameters of the maintainer scripts have to be forwarded to the program after "--".

Current implementation: the preinst checks if the conffile has been modified, if yes it's left on place otherwise it's renamed to oldconffile.dpkg-remove. On configuration, the postinst removes oldconffile.dpkg-remove and renames oldconffile to newconffile if oldconffile is still available. On abort-upgrade/abort-install, the postrm renames oldconffile.dpkg-remove back to oldconffile if required.

INTEGRATION IN PACKAGES

Given that dpkg-maintscript-helper is used in the preinst, using it unconditionally requires a pre-dependency to ensure that the required version of dpkg has been unpacked before. The required version depends on the command used, for rm_conffile and mv_conffile it is 1.15.7.2:

    Pre-Depends: dpkg (>= 1.15.7.2)

But in many cases the operation done by the program is not critical for the package, and instead of using a pre-dependency we can call the program only if we know that the required command is supported by the currently installed dpkg:

    if dpkg-maintscript-helper supports command; then
        dpkg-maintscript-helper command ...
    fi