refactor: Move nixpkgs attribute to `third_party.nixpkgs` r/2470

Please read b/108 to make sense of this.

This gets rid of the explicit list of exposed packages from nixpkgs,
and instead makes the entire package set available at
`third_party.nixpkgs`.

To accommodate this, a LOT of things have to be very slightly shuffled
around. Some of this was done in already submitted CLs, but this
change is unfortunately still quite noisy.

Pay extra attention to:

* overlay-like functionality that was partially moved to actual
  overlays (partially as in, the minimum required to get a green
  build)

* modified uses of the package set path, esp. in NixOS systems

Special notes:

* xanthous has been disabled in CI because of issues with the Haskell
  overlay
* //third_party/nix has been disabled because of other unclear
  dependency issues

Both of these will be tackled in a followup CL.

Change-Id: I2f9c60a4d275fdb5209264be0addfd7e06c53118
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2910
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

1 files changed, 56 insertions, 0 deletions

diff --git a/third_party/nixpkgs/default.nix b/third_party/nixpkgs/default.nix
new file mode 100644
index 0000000000..53adfe81e3
--- /dev/null
+++ b/third_party/nixpkgs/default.nix
@@ -0,0 +1,56 @@
+# This file imports the pinned nixpkgs sets and applies relevant
+# modifications, such as our overlays.
+#
+# 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, ... }:
+
+let
+  # This provides the sources of nixpkgs. We track both
+  # nixos-unstable, and the current stable channel of the latest NixOS
+  # release.
+
+  # Tracking nixos-unstable as of 2021-03-25.
+  unstableHashes = {
+    commit = "60dd94fb7e01a8288f6638eee71d7cb354c49327";
+    sha256 = "0skdwk9bdld295kzrymirs8xrzycqmhsclaz8s18jhcz75hb8sk3";
+  };
+
+  # Tracking nixos-20.09 as of 2021-03-25.
+  stableHashes = {
+    commit = "223d0d733a66b46504ea6b4c15f88b7cc4db58fb";
+    sha256 = "073327ris0frqa3kpid3nsjr9w8yx2z83xpsc24w898mrs9r7d5v";
+  };
+
+  nixpkgsSrc = fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${unstableHashes.commit}.tar.gz";
+    sha256 = unstableHashes.sha256;
+  };
+  stableNixpkgsSrc = fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${stableHashes.commit}.tar.gz";
+    sha256 = stableHashes.sha256;
+  };
+
+  # Stable package set is imported, but not exposed, to overlay
+  # required packages into the unstable set.
+  stableNixpkgs = import stableNixpkgsSrc {};
+
+  # Overlay for packages that should come from the stable channel
+  # instead (e.g. because something is broken in unstable).
+  stableOverlay = self: super: {
+    inherit (stableNixpkgs)
+      awscli # TODO(grfn): Move back to unstable once it is fixed
+      ;
+  };
+in import nixpkgsSrc {
+  config.allowUnfree = true;
+  config.allowBroken = true;
+  overlays = [
+    stableOverlay
+    depot.third_party.overlays.tvl
+    depot.third_party.overlays.haskell
+  ];
+}