Allow specifying allowUnfreePredicate instead of allowUnfree. The pre…

…dicate will have access to the arguments of mkDerivation call. Should be an improvement for #2188

pkgs/stdenv/generic/default.nix

@@ -16,6 +16,15 @@ let
 
   allowUnfree = config.allowUnfree or false || builtins.getEnv "NIXPKGS_ALLOW_UNFREE" == "1";
 
+  # Alow granular checks to allow only some unfree packages
+  # Example:
+  # {pkgs, ...}:
+  # {
+  #   allowUnfree = false;
+  #   allowUnfreePredicate = (x: pkgs.lib.hasPrefix "flashplayero-" x.name);
+  # }
+  allowUnfreePredicate = config.allowUnfreePredicate or (x: false);
+
   allowBroken = config.allowBroken or false || builtins.getEnv "NIXPKGS_ALLOW_BROKEN" == "1";
 
   unsafeGetAttrPos = builtins.unsafeGetAttrPos or (n: as: null);
@@ -57,7 +66,7 @@ let
               unsafeGetAttrPos "name" attrs;
           pos' = if pos != null then "‘" + pos.file + ":" + toString pos.line + "’" else "«unknown-file»";
         in
-        if !allowUnfree && (let l = lib.lists.toList attrs.meta.license or []; in lib.lists.elem "unfree" l || lib.lists.elem "unfree-redistributable" l) then
+        if !allowUnfree && (let l = lib.lists.toList attrs.meta.license or []; in lib.lists.elem "unfree" l || lib.lists.elem "unfree-redistributable" l) && !(allowUnfreePredicate attrs) then
           throw ''
             Package ‘${attrs.name}’ in ${pos'} has an unfree license, refusing to evaluate. You can set
               { nixpkgs.config.allowUnfree = true; }