/usr/share/autopkgtest/setup-commands/ro-apt-update is in autopkgtest 3.20.4.
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 | #!/bin/sh
# This script is is part of autopkgtest
# Copyright (C) 2015 Canonical Ltd.
# Author: Martin Pitt <martin.pitt@ubuntu.com>
#
# --setup-commands script for temporarily making the root file system writable,
# run apt-get update, and make the root fs readable again. This allows tests to
# download/unpack test dependencies on older images whose apt indexes are out
# of date.
set -e
mount -o remount,rw /
apt-get update || (sleep 15; apt-get update)
mount -o remount,ro /
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