# This file imports the pinned nixpkgs sets and applies relevant
# modifications, such as our overlays.
#
# The actual source pinning happens via niv in //third_party/sources
#
# Note that the attribute exposed by this (third_party.nixpkgs) is
# "special" in that the fixpoint used as readTree's config parameter
# in //default.nix passes this attribute as the `pkgs` argument to all
# readTree derivations.

{ depot ? { }, externalArgs ? { }, depotOverlays ? true, ... }:

let
  # import the nixos-unstable package set, or optionally use the
  # source (e.g. a path) specified by the `nixpkgsBisectPath`
  # argument. This is intended for use-cases where the depot is
  # bisected against nixpkgs to find the root cause of an issue in a
  # channel bump.
  nixpkgsSrc = externalArgs.nixpkgsBisectPath or depot.third_party.sources.nixpkgs;

  # Stable package set is imported, but not exposed, to overlay
  # required packages into the unstable set.
  stableNixpkgs = import depot.third_party.sources.nixpkgs-stable { };

  # Overlay for packages that should come from the stable channel
  # instead (e.g. because something is broken in unstable).
  # Use `stableNixpkgs` from above.
  stableOverlay = _unstableSelf: _unstableSuper: {

    # emacs27 is gone from unstable, but people should upgrade at
    # their own pace.
    emacs27 = builtins.trace "emacs27 is deprecated, please migrate to emacs28"
      stableNixpkgs.emacs27;
  };

  # Overlay to expose the nixpkgs commits we are using to other Nix code.
  commitsOverlay = _: _: {
    nixpkgsCommits = {
      unstable = depot.third_party.sources.nixpkgs.rev;
      stable = depot.third_party.sources.nixpkgs-stable.rev;
    };
  };
in
import nixpkgsSrc {
  # allow users to inject their config into builds (e.g. to test CA derivations)
  config =
    (if externalArgs ? nixpkgsConfig then externalArgs.nixpkgsConfig else { })
    // {
      allowUnfree = true;
      allowBroken = true;
    };

  overlays = [
    commitsOverlay
    stableOverlay
  ] ++ (if depotOverlays then [
    depot.third_party.overlays.haskell
    depot.third_party.overlays.emacs
    depot.third_party.overlays.tvl
    depot.third_party.overlays.ecl-static
    depot.third_party.overlays.dhall
  ] else [ ]);
}