/usr/lib/dpkg/methods/disk/update is in dselect 1.17.27.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102  | #!/bin/sh
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
set -e
vardir="$1"
method=$2
option=$3
cd "$vardir/methods/disk"
. ./shvar.$option
if [ -z "$p_main_packages" ] && [ -z "$p_ctb_packages" ] && \
   [ -z "$p_nf_packages" ] && [ -z "$p_nus_packages " ] && \
   [ -z "$p_lcl_packages" ]
then
	echo '
No Packages files available, cannot update available packages list.
Hit RETURN to continue.  '
	read response
	exit 0
fi
xit=1
trap '
	rm -f packages-main packages-ctb packages-nf packages-nus packages-lcl
	if [ -n "$umount" ]
	then
		umount "$umount" >/dev/null 2>&1
	fi
	exit $xit
' 0
if [ -n "$p_blockdev" ]
then
	umount="$p_mountpoint"
	mount -rt "$p_fstype" -o nosuid,nodev "$p_blockdev" "$p_mountpoint"
fi
if [ -n "$p_nfs" ]
then
	umount="$p_mountpoint"
	mount -rt nfs "$p_nfs" -o nosuid,nodev "$p_mountpoint"
fi
updatetype=update
for f in main ctb nf nus lcl
do
	eval 'this_packages=$p_'$f'_packages'
	case "$this_packages" in
	'')
		continue
		;;
	scan)
		eval 'this_binary=$p_'$f'_binary'
		if [ -z "$this_binary" ]; then continue; fi
		if [ "$updatetype" = update ]
		then
			dpkg --admindir $vardir --clear-avail
			updatetype=merge
		fi
		echo Running dpkg --record-avail -R "$p_mountpoint$this_binary"
		dpkg --admindir $vardir --record-avail -R "$p_mountpoint$this_binary"
		;;
	*)
		packagesfile="$p_mountpoint$this_packages"
		case "$packagesfile" in
		*.gz | *.Z | *.GZ | *.z)
			echo -n "Uncompressing $packagesfile ... "
			zcat <"$packagesfile" >packages-$f
			echo done.
			dpkg --admindir $vardir --$updatetype-avail packages-$f
			updatetype=merge
			;;
		'')
			;;
		*)
			dpkg --admindir $vardir --$updatetype-avail "$packagesfile"
			updatetype=merge
			;;
		esac
		;;
	esac
done
echo -n 'Update OK.  Hit RETURN.  '
read response
xit=0
 |