/*

# Dropbear SSHd Configuration

OpenSSH adds ~35MB closure size. Let's try `dropbear` instead!


## Implementation

```nix
#*/# end of MarkDown, beginning of NixOS module:
dirname: inputs: { config, pkgs, lib, ... }: let inherit (inputs.self) lib; in let
    prefix = inputs.config.prefix;
    cfg = config.${prefix}.services.dropbear;
in {

    options.${prefix} = { services.dropbear = {
        enable = lib.mkEnableOption "dropbear SSH daemon";
        socketActivation = lib.mkEnableOption "socket activation mode for dropbear";
        rootKeys = lib.mkOption { description = "Literal lines to write to »/root/.ssh/authorized_keys«"; default = [ ]; type = lib.types.listOf lib.types.str; };
        hostKeys = lib.mkOption { description = "Location of the host key(s) to use. If empty, then a key(s) will be generated at »/etc/dropbear/dropbear_(ecdsa/rsa)_host_key« on first access to the server."; default = [ ]; type = lib.types.listOf lib.types.path; };
    }; };

    config = let
        defaultArgs = lib.concatStringsSep "" [
            "${pkgs.dropbear}/bin/dropbear"
            " -p 22" # listen on TCP/22
            (if cfg.hostKeys == [ ] then (
                " -R" # generate host keys on connection
            ) else lib.concatMapStrings (path: (
                " -r ${path}"
            )) cfg.hostKeys)
        ];

    in lib.mkIf cfg.enable (lib.mkMerge [ ({
        environment.systemPackages = (with pkgs; [ dropbear ]);

        networking.firewall.allowedTCPPorts = [ 22 ];

    }) (lib.mkIf (!cfg.socketActivation) {

        systemd.services."dropbear" = {
            description = "dropbear SSH server (listening)";
            wantedBy = [ "multi-user.target" ]; after = [ "network.target" ];
            serviceConfig.PreExec = lib.mkIf (cfg.hostKeys == [ ]) "${pkgs.coreutils}/bin/mkdir -p /etc/dropbear/";
            serviceConfig.ExecStart = defaultArgs + " -F -E"; # don't fork, use stderr
            #serviceConfig.PIDFile = "/var/run/dropbear.pid"; serviceConfig.Type = "forking"; after = [ "network.target" ]; # alternative to »-E -F« (?)
        };

    }) (lib.mkIf (cfg.socketActivation) {

        # This did not work: dropbear errors out with "socket operation on non-socket".

        systemd.sockets.dropbear = { # start a »dropbear@.service« on any number of TCP connections on port 22
            conflicts = [ "dropbear.service" ];
            listenStreams = [ "22" ];
            socketConfig.Accept = true;
            wantedBy = [ "sockets.target" ]; # (isn't this implicit?)
        };
        systemd.services."dropbear@" = {
            description = "dropbear SSH server (per-connection)";
            after = [ "syslog.target" ];
            serviceConfig.PreExec = lib.mkIf (cfg.hostKeys == [ ]) "${pkgs.coreutils}/bin/mkdir -p /etc/dropbear/"; # or before socket?
            serviceConfig.ExecStart = lib.concatStringsSep "" [
                "-"  # for the most part ignore exit != 0
                defaultArgs
                " -i" # handle a single connection on stdio
            ];
        };

    }) (lib.mkIf (cfg.rootKeys != [ ]) {

        # TODO: This is suboptimal when the system gets activated more than once. Could use a »tmpfiles« rule, or simply »>« (instead of »>>« here).
        system.activationScripts.root-authorized_keys = ''
            mkdir -pm 700 /root/.ssh/
            [ -e /root/.ssh/authorized_keys ] || install -m 600 -T /dev/null /root/.ssh/authorized_keys
            chmod 600 /root/.ssh/authorized_keys
            ${lib.concatMapStringsSep "\n" (key: "printf %s ${lib.escapeShellArg key} >>/root/.ssh/authorized_keys") cfg.rootKeys}
        '';

    }) ]);

}