dpkg/debian/dpkg.prerm

#!/bin/sh -e
# This script can be called in the following ways:
#
# Before the package is removed:
#      <prerm> remove
#
# Before an upgrade:
#      <old-prerm> upgrade <new-version>
# if that fails:
#      <new-prerm> failed-upgrade <old-version>
#
#
# Before package is deconfigured while dependency is replaced due to conflict:
#      <prerm> deconfigure in-favour <new-package> <version>
#              removing <old-package> <version>
#
# Before the package is replaced due to conflict:
#      <prerm> remove in-favour <new-package> <version>

ensure_no_triggers_noawait()
{
    admindir=${DPKG_ADMINDIR:-/var/lib/dpkg}
    pkgadmindir=$admindir/info

    trig_noawait=$(find "$pkgadmindir" -name "*.triggers" -type f | \
                   xargs -r grep -El "^(interest|activate)-(no)?await" | \
		   sed -e 's,^.*/\([^/.:]\+\)[^/]\+$,\1,')

    # Abort if we cannot possibly downgrade
    if [ -n "$trig_noawait" ]; then
        cat <<- MSG
	dpkg: error: You have packages using the "interest-noawait" and/or
	"activate-noawait" trigger directives but the dpkg version that
	you're trying to downgrade to doesn't support them. Aborting
	downgrade.

	List of affected packages:

	$trig_noawait
	MSG
        exit 1
    fi

    bad_triggers_files=$(find "$admindir/triggers" -type f | \
                         xargs -r grep -l "/noawait$" || true)
    if [ -n "$bad_triggers_files" ]; then
        cat <<- MSG
	dpkg: error: Some internal trigger files unexpectedly reference
	packages tagged with "/noawait" while their corresponding
	infodb files doesn't seem to contain any "interest-noawait"
	directive. Aborting the downgrade as those tags are not supported
	by the version you're trying to downgrade to.

	List of internal trigger files that are affected:

	$bad_triggers_files
	MSG
	exit 1
    fi
}

case "$1" in
    upgrade)
        # Allow the downgrade only if no package is using the
	# (interest|activate)-noawait trigger directives
        if dpkg --compare-versions "$2" lt 1.16.1~; then
            ensure_no_triggers_noawait
        fi
        ;;

    remove|failed-upgrade|deconfigure)
        ;;

    *)
        echo "$0 called with unknown argument \`$1'" 1>&2
        exit 1
        ;;
esac

#DEBHELPER#
exit 0