{ ccpkgs                       # Checked out ccpkgs git repo
    ? { outPath = ./.;
        revCount = 1234;
        shortRev = "gabcdef";
      }
, nixpkgs                      # Checked out nixpkgs git repe
    ? { outPath = ./upstream;
        revCount = 56789;
        shortRev = "gfedcba";
      }
, stableBranch ? false
, supportedSystems ? [ "x86_64-linx" ] }:


# Free up the nixpkgs and ccpkgs name for binding to final product.
#
let
  # *Git - git checkout
  #
  nixpkgsGit = nixpkgs;
  ccpkgsGit = ccpkgs;

# Tarball hydra release product for the ccpkgs channel.
#
# 1. Generate an unpacked channel for the associated nixpkgs in the store.
# 2. Symlink this into the top-level directory as upstream (nixpkgs is used).
# 3. Stick in the standard version files.
# 4. Let releaseTools.sourceTarball handle the details of exposing it as a hydra product.
#
in let

  # pkgs - Packages sucked in from the given ccpkgs git version.
  #
  pkgs = import ccpkgsGit { upstream = nixpkgsGit; };

  # version        - Version number (e.g., "17.09")
  # version*Suffix - Version suffix (e.g., "5678.gfedcba")
  #
  # Code lifted from nixos/release.nix.
  #
  version =
    let
      versionNixpkgs = pkgs.lib.fileContents (nixpkgsGit + /.version);
      versionCcpkgs  = pkgs.lib.fileContents ./.version;
    in
      assert (versionNixpkgs == versionCcpkgs); versionCcpkgs;

  versionSeparator = if stableBranch then "." else "beta";

  mkVersion = base: git: rec {
    count = toString (git.revCount - base);
    commit = git.shortRev;
    suffix = "${versionSeparator}${count}.${commit}";
  };

  nixpkgsVersion = mkVersion 114283 nixpkgsGit;
  ccpkgsVersion  = mkVersion      0 ccpkgsGit;

  versionSuffix = "${versionSeparator}${ccpkgsVersion.count}.${nixpkgsVersion.count}.${ccpkgsVersion.commit}.${nixpkgsVersion.commit}";

  # nixpkgs - The store path containing the unpacked nixpkgs channel.
  #
  # 1. Generate a channel tarball from the git repo via the nixos/release.nix expression for hydra.
  # 2. Unpack this into the store using the nix/unpack-channel.nix expression used by nix-channel.
  #
  nixpkgs =
    let
      # channel - Store path containing the channel release tarballs.
      #
      # The nixos.channel attribute from nixos/release.nix builds the nixpkgs channel tarballs.
      #
      channel =
        ( import (nixpkgsGit + /nixos/release.nix) {
            inherit stableBranch supportedSystems;
            nixpkgs = nixpkgsGit;
          } ).channel;

      # nixpkgs - The store path containing the unpacked nixpkgs channel.
      #
      # The nix provided nix/unpack-channel.nix function extracts the nixpkgs from the channel,
      #
      nixpkgs =
        ( import <nix/unpack-channel.nix> rec {
           name = "nixos";
           channelName = "${name}-${version}";
           src = channel + /tarballs + "/${name}-${version}${nixpkgsVersion.suffix}.tar.xz";
           binaryCacheURL = https://cache.nixos.org;
          } );
    in
      nixpkgs;

  # jobs - The jobs hydra is to run.
  #
  jobs = {
    # Tweak the nixos make-channel code to include the upstream channel.
    #
    # 1. Replace the git repo nixpkgs with a copy of the unpacked nixpkgs channel.
    #
    channel =
      ( import (nixpkgsGit + /nixos/lib/make-channel.nix) {
          inherit pkgs version versionSuffix;
          nixpkgs = ccpkgsGit;
      } ).overrideAttrs ( original: {
          name = "ccpkgs-channel";
          distPhase =
            ''
              rm -fr upstream
              cp -rd ${nixpkgs}/nixos-${version} upstream
            '' + original.distPhase;
      } );

    # tested - Aggregate package set required to built for automatic channel release
    #
    tested = pkgs.lib.hydraJob (pkgs.releaseTools.aggregate {
      name = "ccpkgs-${version}";

      constituents =
           builtins.attrValues jobs.pkgs
        ++ builtins.attrValues jobs.temporary;

      meta = {
        description = "Release-critical builds for the ccpkgs channel";
        maintainers = with pkgs.lib.maintainers; [ ];
      };
    });

    # pkgs - Attribute set of overlayed pkgs.
    #
    pkgs =
      builtins.listToAttrs
        ( builtins.map ( name: { inherit name; value = pkgs.${name}; } )
            ( builtins.attrNames ( import ./pkgs/all-packages.nix { } { } ) ) );

    # temporary - Attribute set of overlayed pkgs.
    #
    temporary =
      builtins.listToAttrs
      ( map ( name: { inherit name; value = pkgs.${name}; } )
          ( builtins.attrNames ( import ./temporary/all-temporary.nix { } { } ) ) );
  };

in jobs