/usr/share/ltsp/ltsp-update-image-functions is in ltsp-server 5.5.4-4.
This file is owned by root:root, with mode 0o644.
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  | # Debian-specific function overrides for ltsp-update-image
lock_package_management() {
    test -f /var/lib/dpkg/lock || return 0
    if lsof -t /var/lib/dpkg/lock; then
        warn "A package management process is active, waiting for it to finish..."
        warn "Press Ctrl+C to abort"
        while lsof -t /var/lib/dpkg/lock; do
            sleep 10
        done
    fi
    tail -F /var/lib/dpkg/lock &
    lockpid=$!
}
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