# Licensed under GNU Lesser General Public License v3 or later, see COPYING.
# Copyright (c) 2019 Alexander Sosedkin and other contributors, see AUTHORS.

# inspired by https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/etc/setup-etc.pl

etc="${1}"
static="/etc/static"
new_etc="${2}"

function atomic_symlink() {
    local source="${1}"
    local target="${2}"
    local target_tmp="${target}.tmp"

    mkdir -p "$(dirname "${target_tmp}")"
    ln -sf "${source}" "${target_tmp}"
    mv -T "${target_tmp}" "${target}"
}

# Remove dangling symlinks that point to /etc/static.  These are
# configuration files that existed in a previous configuration but not
# in the current one.
function cleanup() {
    local file
    for file in $(find "${etc}" -xtype l); do
        local target="$(readlink "${file}")"
        if [[ ! -L "${target}" ]]; then
            echo "removing obsolete symlink '${file}'..."
            rm "${file}"
        fi
    done
}

# Returns 0 if the argument points to the files in /etc/static.  That
# means either argument is a symlink to a file in /etc/static or a
# directory with all children being static.
function is_static() {
    local path="${1}"

    if [[ -L "${path}" ]]; then
        [[ "$(readlink "${path}")" == ${static}/* ]]
        return
    fi

    if [[ -d "${path}" ]]; then
        local file
        for file in "${path}"/*; do
            is_static "${file}" || return
        done
    fi

    false
}

function link() {
    if [[ ! -d "${new_etc}" ]]; then
        return
    fi

    local name
    for name in $(find "${new_etc}/" -type l | sed -e "s,^${new_etc}/,,"); do
        local target="${etc}/${name}"

        mkdir -p "$(dirname "${target}")"

        if [[ -e "${target}" ]] && ! is_static "${target}"; then
            echo "Linking of ${target} failed. Please remove this file."
        else
            atomic_symlink "${static}/${name}" "${target}"
        fi
    done
}

# On initial build /etc/static is a directory instead of a symlink
if [[ -d "${etc}/static" ]]; then
    rm --recursive "${etc}/static"
fi

# Atomically update /etc/static to point at the etc files of the
# current configuration.
atomic_symlink "${new_etc}" "${etc}/static"

cleanup

link