{
  description = "A Nix-based continuous build system";

  epoch = 201909;

  outputs = { self, nixpkgs, nix }:
    let

      version = "${builtins.readFile ./version}.${builtins.substring 0 8 self.lastModified}.${self.shortRev}";

      # FIXME: use nix overlay?
      nix' = nix.hydraJobs.build.x86_64-linux // {
        perl-bindings = nix.hydraJobs.perlBindings.x86_64-linux;
      };

      pkgs = import nixpkgs {
        system = "x86_64-linux";
        overlays = [ self.overlay ];
      };

      # NixOS configuration used for VM tests.
      hydraServer =
        { config, pkgs, ... }:
        { imports = [ self.nixosModules.hydra ];

          virtualisation.memorySize = 1024;
          virtualisation.writableStore = true;

          services.hydra-dev.enable = true;
          services.hydra-dev.hydraURL = "http://hydra.example.org";
          services.hydra-dev.notificationSender = "admin@hydra.example.org";

          services.postgresql.enable = true;
          services.postgresql.package = pkgs.postgresql95;

          environment.systemPackages = [ pkgs.perlPackages.LWP pkgs.perlPackages.JSON ];

          # The following is to work around the following error from hydra-server:
          #   [error] Caught exception in engine "Cannot determine local time zone"
          time.timeZone = "UTC";

          nix = {
            # The following is to work around: https://github.com/NixOS/hydra/pull/432
            buildMachines = [
              { hostName = "localhost";
                system = "x86_64-linux";
              }
            ];
            # Without this nix tries to fetch packages from the default
            # cache.nixos.org which is not reachable from this sandboxed NixOS test.
            binaryCaches = [];
          };
        };

    in rec {

      # A Nixpkgs overlay that provides a 'hydra' package.
      overlay = final: prev: {

        hydra = with final; let

          perlDeps = buildEnv {
            name = "hydra-perl-deps";
            paths = with perlPackages;
              [ ModulePluggable
                CatalystActionREST
                CatalystAuthenticationStoreDBIxClass
                CatalystDevel
                CatalystDispatchTypeRegex
                CatalystPluginAccessLog
                CatalystPluginAuthorizationRoles
                CatalystPluginCaptcha
                CatalystPluginSessionStateCookie
                CatalystPluginSessionStoreFastMmap
                CatalystPluginStackTrace
                CatalystPluginUnicodeEncoding
                CatalystTraitForRequestProxyBase
                CatalystViewDownload
                CatalystViewJSON
                CatalystViewTT
                CatalystXScriptServerStarman
                CatalystXRoleApplicator
                CryptRandPasswd
                DBDPg
                DBDSQLite
                DataDump
                DateTime
                DigestSHA1
                EmailMIME
                EmailSender
                FileSlurp
                IOCompress
                IPCRun
                JSON
                JSONAny
                JSONXS
                LWP
                LWPProtocolHttps
                NetAmazonS3
                NetStatsd
                PadWalker
                Readonly
                SQLSplitStatement
                SetScalar
                Starman
                SysHostnameLong
                TermSizeAny
                TestMore
                TextDiff
                TextTable
                XMLSimple
                nix'
                nix'.perl-bindings
                git
                boehmgc
              ];
          };

        in stdenv.mkDerivation {

          name = "hydra-${version}";

          src = self;

          buildInputs =
            [ makeWrapper autoconf automake libtool unzip nukeReferences pkgconfig sqlite libpqxx
              gitAndTools.topGit mercurial darcs subversion bazaar openssl bzip2 libxslt
              guile # optional, for Guile + Guix support
              perlDeps perl nix'
              postgresql95 # for running the tests
              boost
              nlohmann_json
            ];

          hydraPath = lib.makeBinPath (
            [ sqlite subversion openssh nix' coreutils findutils pixz
              gzip bzip2 lzma gnutar unzip git gitAndTools.topGit mercurial darcs gnused bazaar
            ] ++ lib.optionals stdenv.isLinux [ rpm dpkg cdrkit ] );

          configureFlags = [ "--with-docbook-xsl=${docbook_xsl}/xml/xsl/docbook" ];

          shellHook = ''
            PATH=$(pwd)/src/hydra-evaluator:$(pwd)/src/script:$(pwd)/src/hydra-eval-jobs:$(pwd)/src/hydra-queue-runner:$PATH
            PERL5LIB=$(pwd)/src/lib:$PERL5LIB
          '';

          preConfigure = "autoreconf -vfi";

          NIX_LDFLAGS = [ "-lpthread" ];

          enableParallelBuilding = true;

          preCheck = ''
            patchShebangs .
            export LOGNAME=''${LOGNAME:-foo}
          '';

          postInstall = ''
            mkdir -p $out/nix-support

            for i in $out/bin/*; do
                read -n 4 chars < $i
                if [[ $chars =~ ELF ]]; then continue; fi
                wrapProgram $i \
                    --prefix PERL5LIB ':' $out/libexec/hydra/lib:$PERL5LIB \
                    --prefix PATH ':' $out/bin:$hydraPath \
                    --set HYDRA_RELEASE ${version} \
                    --set HYDRA_HOME $out/libexec/hydra \
                    --set NIX_RELEASE ${nix'.name or "unknown"}
            done
          '';

          dontStrip = true;

          meta.description = "Build of Hydra on ${system}";
          passthru.perlDeps = perlDeps;
        };
      };

      hydraJobs = {

        build.x86_64-linux = packages.hydra;

        manual =
          pkgs.runCommand "hydra-manual-${version}" {}
          ''
            mkdir -p $out/share
            cp -prvd ${pkgs.hydra}/share/doc $out/share/

            mkdir $out/nix-support
            echo "doc manual $out/share/doc/hydra" >> $out/nix-support/hydra-build-products
          '';

        tests.install.x86_64-linux =
          with import (nixpkgs + "/nixos/lib/testing.nix") { system = "x86_64-linux"; };
          simpleTest {
            machine = hydraServer;
            testScript =
              ''
                $machine->waitForJob("hydra-init");
                $machine->waitForJob("hydra-server");
                $machine->waitForJob("hydra-evaluator");
                $machine->waitForJob("hydra-queue-runner");
                $machine->waitForOpenPort("3000");
                $machine->succeed("curl --fail http://localhost:3000/");
              '';
          };

        tests.api.x86_64-linux =
          with import (nixpkgs + "/nixos/lib/testing.nix") { system = "x86_64-linux"; };
          simpleTest {
            machine = hydraServer;
            testScript =
              let dbi = "dbi:Pg:dbname=hydra;user=root;"; in
              ''
                $machine->waitForJob("hydra-init");

                # Create an admin account and some other state.
                $machine->succeed
                    ( "su - hydra -c \"hydra-create-user root --email-address 'alice\@example.org' --password foobar --role admin\""
                    , "mkdir /run/jobset /tmp/nix"
                    , "chmod 755 /run/jobset /tmp/nix"
                    , "cp ${./tests/api-test.nix} /run/jobset/default.nix"
                    , "chmod 644 /run/jobset/default.nix"
                    , "chown -R hydra /run/jobset /tmp/nix"
                    );

                $machine->succeed("systemctl stop hydra-evaluator hydra-queue-runner");
                $machine->waitForJob("hydra-server");
                $machine->waitForOpenPort("3000");

                # Run the API tests.
                $machine->mustSucceed("su - hydra -c 'perl -I ${pkgs.hydra.perlDeps}/lib/perl5/site_perl ${./tests/api-test.pl}' >&2");
              '';
        };

        tests.notifications.x86_64-linux =
          with import (nixpkgs + "/nixos/lib/testing.nix") { system = "x86_64-linux"; };
          simpleTest {
            machine = { pkgs, ... }: {
              imports = [ hydraServer ];
              services.hydra-dev.extraConfig = ''
                <influxdb>
                  url = http://127.0.0.1:8086
                  db = hydra
                </influxdb>
              '';
              services.influxdb.enable = true;
            };
            testScript = ''
              $machine->waitForJob("hydra-init");

              # Create an admin account and some other state.
              $machine->succeed
                  ( "su - hydra -c \"hydra-create-user root --email-address 'alice\@example.org' --password foobar --role admin\""
                  , "mkdir /run/jobset"
                  , "chmod 755 /run/jobset"
                  , "cp ${./tests/api-test.nix} /run/jobset/default.nix"
                  , "chmod 644 /run/jobset/default.nix"
                  , "chown -R hydra /run/jobset"
                  );

              # Wait until InfluxDB can receive web requests
              $machine->waitForJob("influxdb");
              $machine->waitForOpenPort("8086");

              # Create an InfluxDB database where hydra will write to
              $machine->succeed(
                "curl -XPOST 'http://127.0.0.1:8086/query' \\
                --data-urlencode 'q=CREATE DATABASE hydra'");

              # Wait until hydra-server can receive HTTP requests
              $machine->waitForJob("hydra-server");
              $machine->waitForOpenPort("3000");

              # Setup the project and jobset
              $machine->mustSucceed(
                "su - hydra -c 'perl -I ${pkgs.hydra.perlDeps}/lib/perl5/site_perl ${./tests/setup-notifications-jobset.pl}' >&2");

              # Wait until hydra has build the job and
              # the InfluxDBNotification plugin uploaded its notification to InfluxDB
              $machine->waitUntilSucceeds(
                "curl -s -H 'Accept: application/csv' \\
                -G 'http://127.0.0.1:8086/query?db=hydra' \\
                --data-urlencode 'q=SELECT * FROM hydra_build_status' | grep success");
            '';
        };

      };

      checks.build = hydraJobs.build.x86_64-linux;
      checks.install = hydraJobs.tests.install.x86_64-linux;

      packages.hydra = pkgs.hydra;
      defaultPackage = pkgs.hydra;

      nixosModules.hydra = {
        imports = [ ./hydra-module.nix ];
        nixpkgs.overlays = [ self.overlay ];
      };

    };
}