{ config, lib, pkgs, ... }:

with lib;
let
  diskSize = 1536; # MB
  gce = pkgs.google-compute-engine;
in
{
  imports = [ ../profiles/headless.nix ../profiles/qemu-guest.nix ];

  system.build.googleComputeImage = import ../../lib/make-disk-image.nix {
    name = "google-compute-image";
    postVM = ''
      PATH=$PATH:${pkgs.stdenv.lib.makeBinPath [ pkgs.gnutar pkgs.gzip ]}
      pushd $out
      mv $diskImage disk.raw
      tar -Szcf nixos-image-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.raw.tar.gz disk.raw
      rm $out/disk.raw
      popd
    '';
    configFile = <nixpkgs/nixos/modules/virtualisation/google-compute-config.nix>;
    format = "raw";
    inherit diskSize;
    inherit config lib pkgs;
  };

  fileSystems."/" = {
    device = "/dev/disk/by-label/nixos";
    autoResize = true;
  };

  boot.growPartition = true;
  boot.kernelParams = [ "console=ttyS0" "panic=1" "boot.panic_on_fail" ];
  boot.initrd.kernelModules = [ "virtio_scsi" ];
  boot.kernelModules = [ "virtio_pci" "virtio_net" ];

  # Generate a GRUB menu.  Amazon's pv-grub uses this to boot our kernel/initrd.
  boot.loader.grub.device = "/dev/sda";
  boot.loader.timeout = 0;

  # Don't put old configurations in the GRUB menu.  The user has no
  # way to select them anyway.
  boot.loader.grub.configurationLimit = 0;

  # Allow root logins only using the SSH key that the user specified
  # at instance creation time.
  services.openssh.enable = true;
  services.openssh.permitRootLogin = "prohibit-password";
  services.openssh.passwordAuthentication = mkDefault false;

  # Use GCE udev rules for dynamic disk volumes
  services.udev.packages = [ gce ];

  # Force getting the hostname from Google Compute.
  networking.hostName = mkDefault "";

  # Always include cryptsetup so that NixOps can use it.
  environment.systemPackages = [ pkgs.cryptsetup ];

  # Make sure GCE image does not replace host key that NixOps sets
  environment.etc."default/instance_configs.cfg".text = lib.mkDefault ''
    [InstanceSetup]
    set_host_keys = false
  '';

  # Rely on GCP's firewall instead
  networking.firewall.enable = mkDefault false;

  # Configure default metadata hostnames
  networking.extraHosts = ''
    169.254.169.254 metadata.google.internal metadata
  '';

  networking.timeServers = [ "metadata.google.internal" ];

  networking.usePredictableInterfaceNames = false;

  # GC has 1460 MTU
  networking.interfaces.eth0.mtu = 1460;

  # allow the google-accounts-daemon to manage users
  users.mutableUsers = true;
  # and allow users to sudo without password
  security.sudo.enable = true;
  security.sudo.extraConfig = ''
  %google-sudoers ALL=(ALL:ALL) NOPASSWD:ALL
  '';

  # NOTE: google-accounts tries to write to /etc/sudoers.d but the folder doesn't exist
  # FIXME: not such file or directory on dynamic SSH provisioning
  systemd.services.google-accounts-daemon = {
    description = "Google Compute Engine Accounts Daemon";
    # This daemon creates dynamic users
    enable = config.users.mutableUsers;
    after = [
      "network.target"
      "google-instance-setup.service"
      "google-network-setup.service"
    ];
    requires = ["network.target"];
    wantedBy = ["multi-user.target"];
    path = with pkgs; [ shadow ];
    serviceConfig = {
      Type = "simple";
      ExecStart = "${gce}/bin/google_accounts_daemon --debug";
    };
  };

  systemd.services.google-clock-skew-daemon = {
    description = "Google Compute Engine Clock Skew Daemon";
    after = [
      "network.target"
      "google-instance-setup.service"
      "google-network-setup.service"
    ];
    requires = ["network.target"];
    wantedBy = ["multi-user.target"];
    serviceConfig = {
      Type = "simple";
      ExecStart = "${gce}/bin/google_clock_skew_daemon --debug";
    };
  };

  systemd.services.google-instance-setup = {
    description = "Google Compute Engine Instance Setup";
    after = ["local-fs.target" "network-online.target" "network.target" "rsyslog.service"];
    before = ["sshd.service"];
    wants = ["local-fs.target" "network-online.target" "network.target"];
    wantedBy = [ "sshd.service" "multi-user.target" ];
    path = with pkgs; [ ethtool openssh ];
    serviceConfig = {
      ExecStart = "${gce}/bin/google_instance_setup --debug";
      Type = "oneshot";
    };
  };

  systemd.services.google-network-daemon = {
    description = "Google Compute Engine Network Daemon";
    after = ["local-fs.target" "network-online.target" "network.target" "rsyslog.service" "google-instance-setup.service"];
    wants = ["local-fs.target" "network-online.target" "network.target"];
    requires = ["network.target"];
    partOf = ["network.target"];
    wantedBy = [ "multi-user.target" ];
    path = with pkgs; [ iproute ];
    serviceConfig = {
      ExecStart = "${gce}/bin/google_network_daemon --debug";
    };
  };

  systemd.services.google-shutdown-scripts = {
    description = "Google Compute Engine Shutdown Scripts";
    after = [
      "local-fs.target"
      "network-online.target"
      "network.target"
      "rsyslog.service"
      "systemd-resolved.service"
      "google-instance-setup.service"
      "google-network-daemon.service"
    ];
    wants = [ "local-fs.target" "network-online.target" "network.target"];
    wantedBy = [ "multi-user.target" ];
    serviceConfig = {
      ExecStart = "${pkgs.coreutils}/bin/true";
      ExecStop = "${gce}/bin/google_metadata_script_runner --debug --script-type shutdown";
      Type = "oneshot";
      RemainAfterExit = true;
      TimeoutStopSec = 0;
    };
  };

  systemd.services.google-startup-scripts = {
    description = "Google Compute Engine Startup Scripts";
    after = [
      "local-fs.target"
      "network-online.target"
      "network.target"
      "rsyslog.service"
      "google-instance-setup.service"
      "google-network-daemon.service"
    ];
    wants = ["local-fs.target" "network-online.target" "network.target"];
    wantedBy = [ "multi-user.target" ];
    serviceConfig = {
      ExecStart = "${gce}/bin/google_metadata_script_runner --debug --script-type startup";
      KillMode = "process";
      Type = "oneshot";
    };
  };

  # TODO: remove this
  systemd.services.fetch-ssh-keys =
    { description = "Fetch host keys and authorized_keys for root user";

      wantedBy = [ "sshd.service" ];
      before = [ "sshd.service" ];
      after = [ "network-online.target" ];
      wants = [ "network-online.target" ];

      script = let wget = "${pkgs.wget}/bin/wget --retry-connrefused -t 15 --waitretry=10 --header='Metadata-Flavor: Google'";
                   mktemp = "mktemp --tmpdir=/run"; in
        ''
          # When dealing with cryptographic keys, we want to keep things private.
          umask 077
          # Don't download the SSH key if it has already been downloaded
          echo "Obtaining SSH keys..."
          mkdir -m 0700 -p /root/.ssh
          AUTH_KEYS=$(${mktemp})
          ${wget} -O $AUTH_KEYS http://metadata.google.internal/computeMetadata/v1/instance/attributes/sshKeys
          if [ -s $AUTH_KEYS ]; then

            # Read in key one by one, split in case Google decided
            # to append metadata (it does sometimes) and add to
            # authorized_keys if not already present.
            touch /root/.ssh/authorized_keys
            NEW_KEYS=$(${mktemp})
            # Yes this is a nix escape of two single quotes.
            while IFS=''' read -r line || [[ -n "$line" ]]; do
              keyLine=$(echo -n "$line" | cut -d ':' -f2)
              IFS=' ' read -r -a array <<< "$keyLine"
              if [ ''${#array[@]} -ge 3 ]; then
                echo ''${array[@]:0:3} >> $NEW_KEYS
                echo "Added ''${array[@]:2} to authorized_keys"
              fi
            done < $AUTH_KEYS
            mv $NEW_KEYS /root/.ssh/authorized_keys
            chmod 600 /root/.ssh/authorized_keys
            rm -f $KEY_PUB
          else
            echo "Downloading http://metadata.google.internal/computeMetadata/v1/project/attributes/sshKeys failed."
            false
          fi
          rm -f $AUTH_KEYS
          SSH_HOST_KEYS_DIR=$(${mktemp} -d)
          ${wget} -O $SSH_HOST_KEYS_DIR/ssh_host_ed25519_key http://metadata.google.internal/computeMetadata/v1/instance/attributes/ssh_host_ed25519_key
          ${wget} -O $SSH_HOST_KEYS_DIR/ssh_host_ed25519_key.pub http://metadata.google.internal/computeMetadata/v1/instance/attributes/ssh_host_ed25519_key_pub
          if [ -s $SSH_HOST_KEYS_DIR/ssh_host_ed25519_key -a -s $SSH_HOST_KEYS_DIR/ssh_host_ed25519_key.pub ]; then
              mv -f $SSH_HOST_KEYS_DIR/ssh_host_ed25519_key* /etc/ssh/
              chmod 600 /etc/ssh/ssh_host_ed25519_key
              chmod 644 /etc/ssh/ssh_host_ed25519_key.pub
          else
              echo "Setup of ssh host keys from http://metadata.google.internal/computeMetadata/v1/instance/attributes/ failed."
              false
          fi
          rm -rf $SSH_HOST_KEYS_DIR
        '';
      serviceConfig.Type = "oneshot";
      serviceConfig.RemainAfterExit = true;
      serviceConfig.StandardError = "journal+console";
      serviceConfig.StandardOutput = "journal+console";
    };

  # Settings taken from https://github.com/GoogleCloudPlatform/compute-image-packages/blob/master/google_config/sysctl/11-gce-network-security.conf
  boot.kernel.sysctl = {
    # Turn on SYN-flood protections.  Starting with 2.6.26, there is no loss
    # of TCP functionality/features under normal conditions.  When flood
    # protections kick in under high unanswered-SYN load, the system
    # should remain more stable, with a trade off of some loss of TCP
    # functionality/features (e.g. TCP Window scaling).
    "net.ipv4.tcp_syncookies" = mkDefault "1";

    # ignores source-routed packets
    "net.ipv4.conf.all.accept_source_route" = mkDefault "0";

    # ignores source-routed packets
    "net.ipv4.conf.default.accept_source_route" = mkDefault "0";

    # ignores ICMP redirects
    "net.ipv4.conf.all.accept_redirects" = mkDefault "0";

    # ignores ICMP redirects
    "net.ipv4.conf.default.accept_redirects" = mkDefault "0";

    # ignores ICMP redirects from non-GW hosts
    "net.ipv4.conf.all.secure_redirects" = mkDefault "1";

    # ignores ICMP redirects from non-GW hosts
    "net.ipv4.conf.default.secure_redirects" = mkDefault "1";

    # don't allow traffic between networks or act as a router
    "net.ipv4.ip_forward" = mkDefault "0";

    # don't allow traffic between networks or act as a router
    "net.ipv4.conf.all.send_redirects" = mkDefault "0";

    # don't allow traffic between networks or act as a router
    "net.ipv4.conf.default.send_redirects" = mkDefault "0";

    # reverse path filtering - IP spoofing protection
    "net.ipv4.conf.all.rp_filter" = mkDefault "1";

    # reverse path filtering - IP spoofing protection
    "net.ipv4.conf.default.rp_filter" = mkDefault "1";

    # ignores ICMP broadcasts to avoid participating in Smurf attacks
    "net.ipv4.icmp_echo_ignore_broadcasts" = mkDefault "1";

    # ignores bad ICMP errors
    "net.ipv4.icmp_ignore_bogus_error_responses" = mkDefault "1";

    # logs spoofed, source-routed, and redirect packets
    "net.ipv4.conf.all.log_martians" = mkDefault "1";

    # log spoofed, source-routed, and redirect packets
    "net.ipv4.conf.default.log_martians" = mkDefault "1";

    # implements RFC 1337 fix
    "net.ipv4.tcp_rfc1337" = mkDefault "1";

    # randomizes addresses of mmap base, heap, stack and VDSO page
    "kernel.randomize_va_space" = mkDefault "2";

    # Reboot the machine soon after a kernel panic.
    "kernel.panic" = mkDefault "10";

    ## Not part of the original config

    # provides protection from ToCToU races
    "fs.protected_hardlinks" = mkDefault "1";

    # provides protection from ToCToU races
    "fs.protected_symlinks" = mkDefault "1";

    # makes locating kernel addresses more difficult
    "kernel.kptr_restrict" = mkDefault "1";

    # set ptrace protections
    "kernel.yama.ptrace_scope" = mkOverride 500 "1";

    # set perf only available to root
    "kernel.perf_event_paranoid" = mkDefault "2";

  };

}