darwin: add impure-cmds

On darwin, there are some commands neither opensource nor able to build in nixpkgs.
We have no choice but to use those system-shipped impure ones.

pkgs/os-specific/darwin/impure-cmds/default.nix

···
+{ lib, runCommandLocal }:
+
+# On darwin, there are some commands neither opensource nor able to build in nixpkgs.
+# We have no choice but to use those system-shipped impure ones.
+
+let
+  commands = {
+    ditto = "/usr/bin/ditto"; # ditto is not opensource
+    sudo  = "/usr/bin/sudo";  # sudo must be owned by uid 0 and have the setuid bit set
+  };
+
+  mkImpureDrv = name: path:
+    runCommandLocal "${name}-impure-darwin" {
+      __impureHostDeps = [ path ];
+
+      meta = {
+        platforms = lib.platforms.darwin;
+      };
+    } ''
+      if ! [ -x ${path} ]; then
+        echo Cannot find command ${path}
+        exit 1
+      fi
+
+      mkdir -p $out/bin
+      ln -s ${path} $out/bin
+
+      manpage="/usr/share/man/man1/${name}.1"
+      if [ -f $manpage ]; then
+        mkdir -p $out/share/man/man1
+        ln -s $manpage $out/share/man/man1
+      fi
+    '';
+in lib.mapAttrs mkImpureDrv commands

pkgs/top-level/all-packages.nix

···
 
   subsurface = libsForQt514.callPackage ../applications/misc/subsurface { };
 
-  sudo = callPackage ../tools/security/sudo { };
+  sudo = if stdenv.isDarwin then darwin.sudo else callPackage ../tools/security/sudo { };
 
   suidChroot = callPackage ../tools/system/suid-chroot { };
 

pkgs/top-level/darwin-packages.nix

···
 
 let
   apple-source-releases = callPackage ../os-specific/darwin/apple-source-releases { };
+
+  impure-cmds = callPackage ../os-specific/darwin/impure-cmds { };
 in
 
-(apple-source-releases // {
+(impure-cmds // apple-source-releases // {
 
   callPackage = newScope (darwin.apple_sdk.frameworks // darwin);