basic flake.nix support with lib.packs

impure and sandbox options should be passed as:

$ nix build .#pkgs.zlib -L --impure --option sandbox false

flake.nix

@@ -0,0 +1,47 @@
+{
+  description = "Flake for NixPACK";
+
+  #inputs.spack = { url="github:spack/spack"; flake=false; };
+  inputs.spack = { url="github:flatironinstitute/spack/fi-nixpack"; flake=false; };
+  inputs.nixpkgs.url = "github:dguibert/nixpkgs/pu-nixpack";
+
+  outputs = inputs: let
+    nixpkgsFor = system: import inputs.nixpkgs {
+      inherit system;
+    };
+
+    nixosPacks = system: let
+      pkgs = nixpkgsFor system;
+      gccWithFortran = pkgs.wrapCC (pkgs.gcc.cc.override {
+        langFortran = true;
+      });
+    in inputs.self.lib.packs {
+      inherit system;
+      os = "nixos21";
+      global.verbose = "true";
+      spackConfig.config.source_cache="/tmp/spack_cache";
+      spackPython = "${pkgs.python3}/bin/python3";
+      spackEnv   = {
+        # pure environment PATH
+	#PATH="${pkgs.coreutils}/bin:${pkgs.gnumake}/bin:${pkgs.gnutar}/bin:${pkgs.gzip}/bin:${pkgs.gnused}/bin:${pkgs.glib}/bin"; # glib: locale
+        PATH="/run/current-system/sw/bin:${pkgs.gnumake}/bin:${pkgs.binutils.bintools}/bin";
+        LOCALE_ARCHIVE="/run/current-system/sw/lib/locale/locale-archive";
+      };
+
+      package = {
+        compiler = { name="gcc"; extern=gccWithFortran; version=gccWithFortran.version; };
+      };
+    };
+  in {
+    lib.packs = {
+      ...
+    }@args: import ./packs ({
+      inherit (inputs) spack nixpkgs;
+    } // args);
+
+    packages.x86_64-linux = nixosPacks "x86_64-linux";
+
+    defaultPackage.x86_64-linux = inputs.self.packages.x86_64-linux.hello;
+
+  };
+}

packs/default.nix

@@ -61,6 +61,7 @@ prefsUpdate = let
       spackConfig = lib.recursiveUpdate;
       spackPython = scalar;
       spackPath = scalar;
+      spackEnv = lib.recursiveUpdate;
       nixpkgsSrc = scalar;
       nixpkgs = scalar;
       verbose = scalar;
@@ -81,6 +82,7 @@ packsWithPrefs =
   , spackConfig ? {}
   , spackPython ? "/usr/bin/python3"
   , spackPath ? "/bin:/usr/bin"
+  , spackEnv ? { PATH=spackPath; }
   , nixpkgsSrc ? null
   , nixpkgs ? fetchGit ({
       url = "git://github.com/NixOS/nixpkgs";
@@ -102,33 +104,31 @@ lib.fix (packs: with packs; {
     ({ label = "withPrefs"; } // p));
 
 
-  makeSpackConfig = import ../spack/config.nix packs spackPath;
+  makeSpackConfig = import ../spack/config.nix packs spackEnv;
 
-  inherit spack spackPython spackPath;
+  inherit spack spackPython spackEnv;
   spackConfig = makeSpackConfig (lib.recursiveUpdate defaultSpackConfig packPrefs.spackConfig);
 
-  spackNixLib = derivation {
+  spackNixLib = derivation ({
     name = "nix-spack-py";
     inherit system;
     builder = ../spack/install.sh;
     src = ../spack/nixpack.py;
-    PATH = spackPath;
-  };
+  } // spackEnv);
 
   /* common attributes for running spack */
   spackBuilder = attrs: builtins.removeAttrs (derivation ({
     inherit (packs) system os spackConfig;
     builder = spackPython;
     PYTHONPATH = "${spackNixLib}:${spack}/lib/spack:${spack}/lib/spack/external";
-    PATH = spackPath;
     LC_ALL = "en_US.UTF-8"; # work around spack bugs processing log files
     repos = if attrs ? withRepos
       then if attrs.withRepos
         then repos
         else null
       else map (r: r + "/repo.yaml") repos;
     spackCache = if attrs.withRepos or false then spackCacheRepos else spackCache;
-  } // attrs)) ["PYTHONPATH" "PATH" "LC_ALL" "spackConfig" "spackCache" "passAsFile"];
+  } // spackEnv // attrs)) ["PYTHONPATH" "PATH" "LC_ALL" "spackConfig" "spackCache" "passAsFile"];
 
   /* pre-generated spack repo index cache (both with and without overlay repos) */
   makeSpackCache = withRepos: lib.when (builtins.isAttrs spackSrc)

spack/config.nix

@@ -1,7 +1,6 @@
-packs: spackPath: config: derivation ({
+packs: spackEnv: config: derivation ({
   inherit (packs.prefs) system;
   name = "spackConfig";
   builder = ./config.sh;
   sections = builtins.attrNames config;
-  PATH = spackPath;
-} // builtins.mapAttrs (n: v: builtins.toJSON { "${n}" = v; }) config)
+} // spackEnv // builtins.mapAttrs (n: v: builtins.toJSON { "${n}" = v; }) config)