Ubuntu Manpage: dpkg-maintscript-helper - works around known dpkg limitations in maintainer scripts

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 [prior-version [package]]

       mv_conffile old-conffile new-conffile [prior-version [package]]

       symlink_to_dir pathname old-target [prior-version [package]]

       dir_to_symlink pathname new-target [prior-version [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 hyphen.

COMMON PARAMETERS

       prior-version
              Defines  the  latest  version  of  the  package  whose  upgrade  should trigger the
              operation. It is  important  to  calculate  prior-version  correctly  so  that  the
              operations  are  correctly  performed  even  if the user rebuilt the package with a
              local version. If prior-version 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).

              For example, for a conffile removed in version 2.0-1 of  a  package,  prior-version
              should  be  set  to  2.0-1~. This will cause the conffile to be removed even if the
              user rebuilt the previous version 1.0-1 as 1.0-1local1.

              If the conffile has not  been  shipped  for  several  versions,  and  you  are  now
              modifying  the  maintainer  scripts  to  clean  up the obsolete file, prior-version
              should be based on the version of the package that you are now preparing,  not  the
              first version of the package that lacked the conffile.

       package
              The  package  name.  When  the  package  is "Multi-Arch: same", this parameter must
              include   the    architecture    qualifier.    If    empty    or    omitted,    the
              DPKG_MAINTSCRIPT_PACKAGE environment variable (as set by dpkg) will be used.

       --     All  the  parameters  of the maintainer scripts have to be forwarded to the program
              after --.

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 prior-version package -- "$@"

conffile is the filename of the conffile to remove.

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 \
old-conffile new-conffile prior-version package -- "$@"

old-conffile and new-conffile are the old and new name of the conffile to rename.

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

SYMLINK AND DIRECTORY SWITCHES

       When upgrading a package, dpkg will not automatically switch a symlink to a  directory  or
       vice-versa.

   Switching a symlink to directory
       If  a symlink is switched to a real directory, you need to make sure before unpacking that
       the symlink is removed. This may seem a simple change to  the  preinst  script  at  first,
       however  that  will  result  in  some problems in case of admin local customization of the
       symlink or when downgrading the package.

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

           dpkg-maintscript-helper symlink_to_dir \
               pathname old-target prior-version package -- "$@"

       pathname  is  the  name of the old symlink (the path will be a directory at the end of the
       installation) and old-target the target of the former symlink at pathname.

       Current implementation: the preinst checks if the symlink exists and points to old-target,
       if  not  then  it's  left  in  place,  otherwise  it's renamed to pathname.dpkg-backup. On
       configuration, the postinst removes pathname.dpkg-backup if pathname.dpkg-backup is  still
       a symlink. On abort-upgrade/abort-install, the postrm renames pathname.dpkg-backup back to
       pathname if required.

   Switching a directory to symlink
       If a real directory is switched to a symlink, you need to make sure before unpacking  that
       the  directory  is  removed. This may seem a simple change to the preinst script at first,
       however that will result in some  problems  in  case  the  directory  contains  conffiles,
       pathnames  owned  by  other  packages,  locally created pathnames, or when downgrading the
       package.

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

           dpkg-maintscript-helper dir_to_symlink \
               pathname new-target prior-version package -- "$@"

       pathname is the name of the of the old directory (the path will be a symlink at the end of
       the installation) and new-target is the target of the new symlink at pathname.

       Current implementation: the preinst checks if  the  directory  exists,  does  not  contain
       conffiles,  pathnames  owned  by other packages, or locally created pathnames, if not then
       it's left in place, otherwise it's renamed to pathname.dpkg-backup, and an  empty  staging
       directory  named  pathname  is  created,  marked with a file so that dpkg can track it. On
       configuration, the postinst  finishes  the  switch  if  pathname.dpkg-backup  is  still  a
       directory  and  pathname  is  the staging directory; it removes the staging directory mark
       file, moves the newly created files inside the staging directory  to  the  symlink  target
       new-target/,  replaces  the  now  empty  staging directory pathname with a symlink to new-
       target, and  removes  pathname.dpkg-backup.  On  abort-upgrade/abort-install,  the  postrm
       renames pathname.dpkg-backup back to pathname 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, for symlink_to_dir and dir_to_symlink it is 1.17.5:

           Pre-Depends: dpkg (>= 1.17.5)

       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