{ pkgs, lib, config, ... }:
with lib;
let
    fromAddress = config.email.fromAddress;
    fromIdentity = config.email.fromIdentity;
    toAddress = config.email.adminEmail;
    cfg = config.system.autoUpgrade;
in
{
    options.system.autoUpgrade = {
        sendEmail = mkOption {
            type = types.bool;
            default = false;
            description = mdDoc ''
                Whether to send a status email after an upgrade.
            '';
        };
    };

    config = mkIf cfg.enable {
        systemd.services.nixos-upgrade.script = mkOverride 50 (
            let 
                nixos-rebuild = "${config.system.build.nixos-rebuild}/bin/nixos-rebuild";
                date = "${pkgs.coreutils}/bin/date";
                readlink = "${pkgs.coreutils}/bin/readlink";
                grep = "${pkgs.gnugrep}/bin/grep";
                shutdown = "${config.systemd.package}/bin/shutdown";
                sendmail = "${pkgs.system-sendmail}/bin/sendmail";
                upgradeFlag = optional (cfg.channel == null) "--upgrade";
            in ''
                set -o pipefail
                
                upgrade() {
                    ${nixos-rebuild} ${cfg.operation} ${toString (cfg.flags ++ upgradeFlag)}
                }
                
                indent()
                {
                    while read -r line ; do
                        echo "  $line"
                    done <<< "$1"
                }
                
                start_time="$(${date})"
                reboot_allowed="no"
                do_reboot="no"
                exit_code=0
                
                ${optionalString cfg.allowReboot ''
                reboot_allowed="yes"
                
                ${optionalString (cfg.rebootWindow != null) ''
                current_time="$(${date} +%H:%M)"
                lower="${cfg.rebootWindow.lower}"
                upper="${cfg.rebootWindow.upper}"
                if [[ "''${lower}" < "''${upper}" ]]; then
                    if [[ "''${current_time}" > "''${lower}" ]] && \
                        [[ "''${current_time}" < "''${upper}" ]]; then
                        reboot_allowed="yes"
                    else
                        reboot_allowed="no"
                    fi
                else
                    # lower > upper, so we are crossing midnight (e.g. lower=23h, upper=6h)
                    # we want to reboot if cur > 23h or cur < 6h
                    if [[ "''${current_time}" < "''${upper}" ]] || \
                        [[ "''${current_time}" > "''${lower}" ]]; then
                        reboot_allowed="yes"
                    else
                        reboot_allowed="no"
                    fi
                fi
                ''}
                ''}

                output_file="$(mktemp)"
                upgrade 2>&1 | tee "$output_file" || exit_code=$?
                upgrade_output="$(cat "$output_file")"
                rm -f "$output_file"
                
                send_email=no
                email_subject="Upgrade succeeded"
                email_body="The system upgrade started at $start_time has succeeded."
                if [ "$exit_code" -ne 0 ] ; then
                    send_email=yes
                    email_subject="Upgrade failed (exit code $exit_code)"
                    email_body="The system upgrade started at $start_time has failed with exit code $exit_code."
                fi
                
                possible_warnings="$(${grep} -e "^trace:" <<<"$upgrade_output" || true)"
                if [ "$possible_warnings" != "" ] ; then
                    send_email=yes
                    email_subject="$email_subject with warnings"
                    email_body="$(cat <<-EOF
                $email_body
        
        
                These trace messages and warnings were encountered:
                ---------------------------------------------------
                $possible_warnings
                EOF
                    )"
                fi
                
                booted_version="$(${readlink} /run/booted-system/{initrd,kernel,kernel-modules})"
                built_version="$(${readlink} /nix/var/nix/profiles/system/{initrd,kernel,kernel-modules})"
                
                if [ "$booted_version" != "$built_version" ] ; then
                    version_comparison="$(cat <<-EOF
                The booted kernel version
                $(indent "$booted_version")
                does not match currently active
                $(indent "$built_version")
                .
                EOF
                    )"
                    echo "$version_comparison"
                    send_email=yes
                    email_subject="$email_subject, reboot required"
                    email_body="$(cat <<-EOF
                $email_body
        
        
                A reboot is required, because:
                ------------------------------
                $version_comparison
                EOF
                    )"
                    if [ "$reboot_allowed" = "yes" ] && [ $exit_code -eq 0 ] ; then
                        email_body="$(printf "%s\n%s" "$email_body" "The system will reboot now.")"
                        do_reboot="yes"
                    fi
                fi
                
                ${optionalString cfg.sendEmail ''
                if [ "$send_email" = "yes" ] ; then
                    ${sendmail} -t -X - <<-EOF
                To: ${toAddress}
                From: ${fromIdentity}
                Subject: $email_subject
                Content-Transfer-Encoding: 8bit
                Content-Type: text/plain; charset=UTF-8
                X-Priority: 3
        
                $email_body
        
        
                Full upgrade output:
                --------------------
                $upgrade_output
                EOF
                fi
                ''}
                
                if [ "$do_reboot" = "yes" ] ; then
                    echo "Rebooting system."
                    ${shutdown} -r +1
                fi
                
                exit $exit_code
            ''
        );
    };
}