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

with lib;

let

  cfg = config.nix;

  nix = cfg.package;

  makeNixBuildUser = nr:
    { name = "nixbld${toString nr}";
      description = "Nix build user ${toString nr}";

      /* For consistency with the setgid(2), setuid(2), and setgroups(2)
         calls in `libstore/build.cc', don't add any supplementary group
         here except "nixbld".  */
      uid = builtins.add config.ids.uids.nixbld nr;
      group = "nixbld";
      extraGroups = [ "nixbld" ];
    };

  nixbldUsers = map makeNixBuildUser (range 1 cfg.nrBuildUsers);

  nixConf =
    let
      # If we're using a chroot for builds, then provide /bin/sh in
      # the chroot as a bind-mount to bash. This means we also need to
      # include the entire closure of bash.
      sh = pkgs.stdenv.shell;
      binshDeps = pkgs.writeReferencesToFile sh;
    in
      pkgs.runCommand "nix.conf" {extraOptions = cfg.extraOptions; } ''
        extraPaths=$(for i in $(cat ${binshDeps}); do if test -d $i; then echo $i; fi; done)
        cat > $out <<END
        # WARNING: this file is generated from the nix.* options in
        # your NixOS configuration, typically
        # /etc/nixos/configuration.nix.  Do not edit it!
        build-users-group = nixbld
        build-max-jobs = ${toString (cfg.maxJobs)}
        build-cores = ${toString (cfg.buildCores)}
        build-use-chroot = ${if cfg.useChroot then "true" else "false"}
        build-chroot-dirs = ${toString cfg.chrootDirs} /bin/sh=${sh} $(echo $extraPaths)
        binary-caches = ${toString cfg.binaryCaches}
        trusted-binary-caches = ${toString cfg.trustedBinaryCaches}
        binary-cache-public-keys = ${toString cfg.binaryCachePublicKeys}
        ${optionalString cfg.requireSignedBinaryCaches ''
          signed-binary-caches = *
        ''}
        $extraOptions
        END
      '';

in

{

  ###### interface

  options = {

    nix = {

      package = mkOption {
        type = types.package;
        default = pkgs.nix;
        description = ''
          This option specifies the Nix package instance to use throughout the system.
        '';
      };

      maxJobs = mkOption {
        type = types.int;
        default = 1;
        example = 64;
        description = ''
          This option defines the maximum number of jobs that Nix will try
          to build in parallel.  The default is 1.  You should generally
          set it to the number of CPUs in your system (e.g., 2 on an Athlon
          64 X2).
        '';
      };

      buildCores = mkOption {
        type = types.int;
        default = 1;
        example = 64;
        description = ''
          This option defines the maximum number of concurrent tasks during
          one build. It affects, e.g., -j option for make. The default is 1.
          The special value 0 means that the builder should use all
          available CPU cores in the system. Some builds may become
          non-deterministic with this option; use with care! Packages will
          only be affected if enableParallelBuilding is set for them.
        '';
      };

      useChroot = mkOption {
        type = types.bool;
        default = false;
        description = "
          If set, Nix will perform builds in a chroot-environment that it
          will set up automatically for each build.  This prevents
          impurities in builds by disallowing access to dependencies
          outside of the Nix store.
        ";
      };

      chrootDirs = mkOption {
        type = types.listOf types.str;
        default = [];
        example = [ "/dev" "/proc" ];
        description =
          ''
            Directories from the host filesystem to be included
            in the chroot.
          '';
      };

      extraOptions = mkOption {
        type = types.lines;
        default = "";
        example = ''
          gc-keep-outputs = true
          gc-keep-derivations = true
        '';
        description = "Additional text appended to <filename>nix.conf</filename>.";
      };

      distributedBuilds = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether to distribute builds to the machines listed in
          <option>nix.buildMachines</option>.
        '';
      };

      daemonNiceLevel = mkOption {
        type = types.int;
        default = 0;
        description = ''
          Nix daemon process priority. This priority propagates to build processes.
          0 is the default Unix process priority, 20 is the lowest.
        '';
      };

      daemonIONiceLevel = mkOption {
        type = types.int;
        default = 0;
        description = ''
          Nix daemon process I/O priority. This priority propagates to build processes.
          0 is the default Unix process I/O priority, 7 is the lowest.
        '';
      };

      buildMachines = mkOption {
        type = types.listOf types.attrs;
        default = [];
        example = [
          { hostName = "voila.labs.cs.uu.nl";
            sshUser = "nix";
            sshKey = "/root/.ssh/id_buildfarm";
            system = "powerpc-darwin";
            maxJobs = 1;
          }
          { hostName = "linux64.example.org";
            sshUser = "buildfarm";
            sshKey = "/root/.ssh/id_buildfarm";
            system = "x86_64-linux";
            maxJobs = 2;
            supportedFeatures = "kvm";
            mandatoryFeatures = "perf";
          }
        ];
        description = ''
          This option lists the machines to be used if distributed
          builds are enabled (see
          <option>nix.distributedBuilds</option>).  Nix will perform
          derivations on those machines via SSH by copying the inputs
          to the Nix store on the remote machine, starting the build,
          then copying the output back to the local Nix store.  Each
          element of the list should be an attribute set containing
          the machine's host name (<varname>hostname</varname>), the
          user name to be used for the SSH connection
          (<varname>sshUser</varname>), the Nix system type
          (<varname>system</varname>, e.g.,
          <literal>"i686-linux"</literal>), the maximum number of
          jobs to be run in parallel on that machine
          (<varname>maxJobs</varname>), the path to the SSH private
          key to be used to connect (<varname>sshKey</varname>), a
          list of supported features of the machine
          (<varname>supportedFeatures</varname>) and a list of
          mandatory features of the machine
          (<varname>mandatoryFeatures</varname>). The SSH private key
          should not have a passphrase, and the corresponding public
          key should be added to
          <filename>~<replaceable>sshUser</replaceable>/authorized_keys</filename>
          on the remote machine.
        '';
      };

      # Environment variables for running Nix.
      envVars = mkOption {
        type = types.attrs;
        internal = true;
        default = {};
        description = "Environment variables used by Nix.";
      };

      nrBuildUsers = mkOption {
        type = types.int;
        description = ''
          Number of <literal>nixbld</literal> user accounts created to
          perform secure concurrent builds.  If you receive an error
          message saying that “all build users are currently in use”,
          you should increase this value.
        '';
      };

      readOnlyStore = mkOption {
        type = types.bool;
        default = true;
        description = ''
          If set, NixOS will enforce the immutability of the Nix store
          by making <filename>/nix/store</filename> a read-only bind
          mount.  Nix will automatically make the store writable when
          needed.
        '';
      };

      binaryCaches = mkOption {
        type = types.listOf types.str;
        default = [ https://cache.nixos.org/ ];
        description = ''
          List of binary cache URLs used to obtain pre-built binaries
          of Nix packages.
        '';
      };

      trustedBinaryCaches = mkOption {
        type = types.listOf types.str;
        default = [ ];
        example = [ http://hydra.nixos.org/ ];
        description = ''
          List of binary cache URLs that non-root users can use (in
          addition to those specified using
          <option>nix.binaryCaches</option> by passing
          <literal>--option binary-caches</literal> to Nix commands.
        '';
      };

      requireSignedBinaryCaches = mkOption {
        type = types.bool;
        default = false;
        description = ''
          If enabled, Nix will only download binaries from binary
          caches if they are cryptographically signed with any of the
          keys listed in
          <option>nix.binaryCachePublicKeys</option>. If disabled (the
          default), signatures are neither required nor checked, so
          it's strongly recommended that you use only trustworthy
          caches and https to prevent man-in-the-middle attacks.
        '';
      };

      binaryCachePublicKeys = mkOption {
        type = types.listOf types.str;
        example = [ "hydra.nixos.org-1:CNHJZBh9K4tP3EKF6FkkgeVYsS3ohTl+oS0Qa8bezVs=" ];
        description = ''
          List of public keys used to sign binary caches. If
          <option>nix.requireSignedBinaryCaches</option> is enabled,
          then Nix will use a binary from a binary cache if and only
          if it is signed by <emphasis>any</emphasis> of the keys
          listed here. By default, only the key for
          <uri>cache.nixos.org</uri> is included.
        '';
      };

    };

  };


  ###### implementation

  config = {

    nix.binaryCachePublicKeys = [ "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=" ];

    environment.etc."nix/nix.conf".source = nixConf;

    # List of machines for distributed Nix builds in the format
    # expected by build-remote.pl.
    environment.etc."nix/machines" =
      { enable = cfg.buildMachines != [];
        text =
          concatMapStrings (machine:
            "${machine.sshUser}@${machine.hostName} "
            + (if machine ? system then machine.system else concatStringsSep "," machine.systems)
            + " ${machine.sshKey} ${toString machine.maxJobs} "
            + (if machine ? speedFactor then toString machine.speedFactor else "1" )
            + " "
            + (if machine ? supportedFeatures then concatStringsSep "," machine.supportedFeatures else "" )
            + " "
            + (if machine ? mandatoryFeatures then concatStringsSep "," machine.mandatoryFeatures else "" )
            + "\n"
          ) cfg.buildMachines;
      };

    systemd.packages = [ nix ];

    systemd.sockets.nix-daemon.wantedBy = [ "sockets.target" ];

    systemd.services.nix-daemon =
      { path = [ nix pkgs.openssl pkgs.utillinux pkgs.openssh ]
          ++ optionals cfg.distributedBuilds [ pkgs.gzip ];

        environment = cfg.envVars
          // { CURL_CA_BUNDLE = "/etc/ssl/certs/ca-bundle.crt"; }
          // config.networking.proxy.envVars;

        serviceConfig =
          { Nice = cfg.daemonNiceLevel;
            IOSchedulingPriority = cfg.daemonIONiceLevel;
            LimitNOFILE = 4096;
          };

        restartTriggers = [ nixConf ];
      };

    nix.envVars =
      { NIX_CONF_DIR = "/etc/nix";

        # Enable the copy-from-other-stores substituter, which allows
        # builds to be sped up by copying build results from remote
        # Nix stores.  To do this, mount the remote file system on a
        # subdirectory of /run/nix/remote-stores.
        NIX_OTHER_STORES = "/run/nix/remote-stores/*/nix";
      }

      // optionalAttrs cfg.distributedBuilds {
        NIX_BUILD_HOOK = "${nix}/libexec/nix/build-remote.pl";
        NIX_REMOTE_SYSTEMS = "/etc/nix/machines";
        NIX_CURRENT_LOAD = "/run/nix/current-load";
      };

    # Set up the environment variables for running Nix.
    environment.sessionVariables = cfg.envVars;

    environment.extraInit =
      ''
        # Set up secure multi-user builds: non-root users build through the
        # Nix daemon.
        if [ "$USER" != root -o ! -w /nix/var/nix/db ]; then
            export NIX_REMOTE=daemon
        fi
      '';

    nix.nrBuildUsers = mkDefault (lib.max 10 cfg.maxJobs);

    users.extraUsers = nixbldUsers;

    services.xserver.displayManager.hiddenUsers = map ({ name, ... }: name) nixbldUsers;

    system.activationScripts.nix = stringAfter [ "etc" "users" ]
      ''
        # Nix initialisation.
        mkdir -m 0755 -p \
          /nix/var/nix/gcroots \
          /nix/var/nix/temproots \
          /nix/var/nix/manifests \
          /nix/var/nix/userpool \
          /nix/var/nix/profiles \
          /nix/var/nix/db \
          /nix/var/log/nix/drvs \
          /nix/var/nix/channel-cache
        mkdir -m 1777 -p \
          /nix/var/nix/gcroots/per-user \
          /nix/var/nix/profiles/per-user \
          /nix/var/nix/gcroots/tmp

        ln -sf /nix/var/nix/profiles /nix/var/nix/gcroots/
        ln -sf /nix/var/nix/manifests /nix/var/nix/gcroots/
      '';

  };

}