(debian-policy.info)Diversions - overriding a package’s version of a file from old Packaging Manual

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19 Diversions - overriding a package’s version of a file (from old Packaging Manual)
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It is possible to have ‘dpkg’ not overwrite a file when it reinstalls
the package it belongs to, and to have it put the file from the package
somewhere else instead.

This can be used locally to override a package’s version of a file, or
by one package to override another’s version (or provide a wrapper for
it).

Before deciding to use a diversion, read Note: Alternative versions of
an interface - update-alternatives (from old Packaging Manual). to
see if you really want a diversion rather than several alternative
versions of a program.

There is a diversion list, which is read by ‘dpkg’, and updated by a
special program ‘dpkg-divert’.  Please see dpkg-divert(8) for full
details of its operation.

When a package wishes to divert a file from another, it should call
‘dpkg-divert’ in its preinst to add the diversion and rename the
existing file.  For example, supposing that a ‘smailwrapper’ package
wishes to install a wrapper around ‘/usr/sbin/smail’:

     dpkg-divert --package smailwrapper --add --rename \
         --divert /usr/sbin/smail.real /usr/sbin/smail

The ‘--package smailwrapper’ ensures that ‘smailwrapper’’s copy of
‘/usr/sbin/smail’ can bypass the diversion and get installed as the true
version.  It’s safe to add the diversion unconditionally on upgrades
since it will be left unchanged if it already exists, but ‘dpkg-divert’
will display a message.  To suppress that message, make the command
conditional on the version from which the package is being upgraded:

     if [ upgrade != "$1 || dpkg --compare-versions "$2" lt 1.0-2; then
         dpkg-divert --package smailwrapper --add --rename \
             --divert /usr/sbin/smail.real /usr/sbin/smail
     fi

where ‘1.0-2’ is the version at which the diversion was first added to
the package.  Running the command during abort-upgrade is pointless but
harmless.

The postrm has to do the reverse:

     if [ remove = "$1" -o abort-install = "$1" -o disappear = "$1; then
         dpkg-divert --package smailwrapper --remove --rename \
             --divert /usr/sbin/smail.real /usr/sbin/smail
     fi

If the diversion was added at a particular version, the postrm should
also handle the failure case of upgrading from an older version (unless
the older version is so old that direct upgrades are no longer
supported):

     if [ abort-upgrade = "$1 && dpkg --compare-versions "$2" lt 1.0-2; then
         dpkg-divert --package smailwrapper --remove --rename \
             --divert /usr/sbin/smail.real /usr/sbin/smail
     fi

where ‘1.0-2’ is the version at which the diversion was first added to
the package.  The postrm should not remove the diversion on upgrades
both because there’s no reason to remove the diversion only to
immediately re-add it and since the postrm of the old package is run
after unpacking so the removal of the diversion will fail.

Do not attempt to divert a file which is vitally important for the
system’s operation - when using ‘dpkg-divert’ there is a time, after it
has been diverted but before ‘dpkg’ has installed the new version, when
the file does not exist.

Do not attempt to divert a conffile, as ‘dpkg’ does not handle it well.