From cd6966f1b447e8c45d499bd8b385a57fbfe70e60 Mon Sep 17 00:00:00 2001
From: Michele Guerini Rocco <m.guerinirocco2@campus.unimib.it>
Date: Wed, 4 May 2022 10:17:17 +0200
Subject: [PATCH] default.nix: prevent nixpkgs tarball GC

When adding a GC root with `nix-build -A inputDerivation` the Nixpkgs
tarball wasn't being saved because it's technically not a build input.
---
 default.nix | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/default.nix b/default.nix
index 9a17a88..cd39407 100644
--- a/default.nix
+++ b/default.nix
@@ -2,11 +2,11 @@
 
 let
   # Pinned Nixpkgs for reproducibility
-  nixpkgs = import (builtins.fetchTarball
+  nixpkgs = builtins.fetchTarball
     { name   = "nixpkgs-21.05-ee90403e147";
       url    = "https://github.com/nixos/nixpkgs/archive/ee90403e147.tar.gz";
       sha256 = "1mk3s4ncfa8z8mr6vrgjh74s8dci12yam7plpc1bqgz12wld73ax";
-    }) {};
+    };
 
   # Needed for HTML manual
   katex = builtins.fetchTarball {
@@ -20,7 +20,7 @@ let
       !builtins.elem path [ "configure.mk" "default.nix" "result" ]
       && baseNameOf path != "build") ./.;
 
-  inherit (nixpkgs) lib pkgs;
+  inherit (import nixpkgs {}) lib pkgs;
 
 in
   pkgs.stdenv.mkDerivation rec {
@@ -41,6 +41,9 @@ in
       (texlive.combine {
         inherit (texlive) scheme-small xetex fontspec;
       })
+
+      # save nixpkgs tarball from gc
+      nixpkgs
     ];
 
     buildInputs = lib.optional static pkgs.glibc.static;