stdenv-darwin: allow easier testing of bootstrap tools

This un-hardcodes the bootstrap tools passed into the Darwin stdenv and
thus allows us to quickly iterate on improving the design of the full
bootstrap process. We can easily change the contents of the bootstrap
tools and evaluate an entire bootstrap all the way up to real packages.

pkgs/stdenv/darwin/default.nix

···
-{ system      ? builtins.currentSystem
-, allPackages ? import ../../top-level/all-packages.nix
-, platform    ? null
-, config      ? {}
+{ system         ? builtins.currentSystem
+, allPackages    ? import ../../top-level/all-packages.nix
+, platform       ? null
+, config         ? {}
+
+# Allow passing in bootstrap files directly so we can test the stdenv bootstrap process when changing the bootstrap tools
+, bootstrapFiles ? let
+  fetch = { file, sha256, executable ? true }: import <nix/fetchurl.nix> {
+    url = "http://tarballs.nixos.org/stdenv-darwin/x86_64/4f07c88d467216d9692fefc951deb5cd3c4cc722/${file}";
+    inherit sha256 system executable;
+  }; in {
+    sh      = fetch { file = "sh";    sha256 = "1siix3wakzil31r2cydmh3v8a1nyq4605dwiabqc5lx73j4xzrzi"; };
+    bzip2   = fetch { file = "bzip2"; sha256 = "0zvqm977k11b5cl4ixxb5h0ds24g6z0f8m28z4pqxzpa353lqbla"; };
+    mkdir   = fetch { file = "mkdir"; sha256 = "13frk8lsfgzlb65p9l26cvxf06aag43yjk7vg9msn7ix3v8cmrg1"; };
+    cpio    = fetch { file = "cpio";  sha256 = "0ms5i9m1vdksj575sf1djwgm7zhnvfrrb44dxnfh9avr793rc2w4"; };
+    tarball = fetch { file = "bootstrap-tools.cpio.bz2"; sha256 = "1lz1b0grl4642h6n635xvi6imf0yyy1zyzdr9ing5aphzz0z5iic"; executable = false; };
+  }
 }:
 
 let
   libSystemProfile = ''
     (import "${./standard-sandbox.sb}")
   '';
-
-  fetch = { file, sha256, executable ? true }: import <nix/fetchurl.nix> {
-    url = "http://tarballs.nixos.org/stdenv-darwin/x86_64/4f07c88d467216d9692fefc951deb5cd3c4cc722/${file}";
-    inherit sha256 system executable;
-  };
-
-  bootstrapFiles = {
-    sh    = fetch { file = "sh";    sha256 = "1siix3wakzil31r2cydmh3v8a1nyq4605dwiabqc5lx73j4xzrzi"; };
-    bzip2 = fetch { file = "bzip2"; sha256 = "0zvqm977k11b5cl4ixxb5h0ds24g6z0f8m28z4pqxzpa353lqbla"; };
-    mkdir = fetch { file = "mkdir"; sha256 = "13frk8lsfgzlb65p9l26cvxf06aag43yjk7vg9msn7ix3v8cmrg1"; };
-    cpio  = fetch { file = "cpio";  sha256 = "0ms5i9m1vdksj575sf1djwgm7zhnvfrrb44dxnfh9avr793rc2w4"; };
-  };
-
-  tarball = fetch { file = "bootstrap-tools.cpio.bz2"; sha256 = "1lz1b0grl4642h6n635xvi6imf0yyy1zyzdr9ing5aphzz0z5iic"; executable = false; };
 in rec {
   allPackages = import ../../top-level/all-packages.nix;
 
···
   '';
 
   bootstrapTools = derivation rec {
-    inherit system tarball;
+    inherit system;
 
     name    = "bootstrap-tools";
     builder = bootstrapFiles.sh; # Not a filename! Attribute 'sh' on bootstrapFiles
     args    = [ ./unpack-bootstrap-tools.sh ];
 
-    inherit (bootstrapFiles) mkdir bzip2 cpio;
+    inherit (bootstrapFiles) mkdir bzip2 cpio tarball;
 
     __sandboxProfile = binShClosure + libSystemProfile;
   };
···
       inherit cc;
     };
   };
+
+  stdenvDarwin = stage5;
 }

pkgs/stdenv/darwin/make-bootstrap-tools.nix

···
-with import ../../top-level/all-packages.nix { system = "x86_64-darwin"; };
+{ system ? builtins.currentSystem }:
+
+with import ../../top-level/all-packages.nix { inherit system; };
 
 rec {
   # We want coreutils without ACL support.
···
     '';
   };
 
-  unpack = stdenv.mkDerivation {
+  bootstrapFiles = {
+    sh      = "${build}/on-server/sh";
+    bzip2   = "${build}/on-server/bzip2";
+    mkdir   = "${build}/on-server/mkdir";
+    cpio    = "${build}/on-server/cpio";
+    tarball = "${build}/on-server/bootstrap-tools.cpio.bz2";
+  };
+
+  unpack = stdenv.mkDerivation (bootstrapFiles // {
     name = "unpack";
 
     # This is by necessity a near-duplicate of unpack-bootstrap-tools.sh. If we refer to it directly,
···
       EOF
     '';
 
-    tarball = "${build}/on-server/bootstrap-tools.cpio.bz2";
-
-    mkdir = "${build}/on-server/mkdir";
-    bzip2 = "${build}/on-server/bzip2";
-    cpio  = "${build}/on-server/cpio";
-
     allowedReferences = [ "out" ];
-  };
+  });
 
   test = stdenv.mkDerivation {
     name = "test";
···
 
       $out/bin/hello
     '';
+  };
+
+  # The ultimate test: bootstrap a whole stdenv from the tools specified above and get a package set out of it
+  test-pkgs = let
+    stdenv = import ./. { inherit system bootstrapFiles; };
+  in import ../../top-level/all-packages.nix {
+    inherit system;
+    bootStdenv = stdenv.stdenvDarwin;
   };
 }

pkgs/stdenv/default.nix

···
   # Linux standard environment.
   stdenvLinux = (import ./linux { inherit system allPackages platform config lib; }).stdenvLinux;
 
-  stdenvDarwin = (import ./darwin { inherit system allPackages platform config;}).stage5;
+  stdenvDarwin = (import ./darwin { inherit system allPackages platform config;}).stdenvDarwin;
 
   # Select the appropriate stdenv for the platform `system'.
   stdenv =