add allowSubstitutes = false example

README.md

@@ -82,11 +82,20 @@ $ nix-build-uncached ./ci.nix
 
 If your package set you are building has packages at the top level scope that
 have the attribute `allowSubstitutes = false;` set, than `nix-build-uncached`
-will build/download them everytime. A workaround is to use the following
-[gist](https://gist.github.com/7bc9210b1c4d0afe390f0d425b50e02f):
+will build/download them everytime. This attribute is set for some builders such
+as `writeText` or `writeScriptBin`. A workaround is to use the following
+[nix library](./scripts/force_cached.nix) and save it as
+`force_cached.nix`. Than wrap your attribute set like this:
 
 ```nix
-# TODO example
+let
+  pkgs = import <nixpkgs> {};
+in (pkgs.callPackage ./force_cached.nix {}) {
+  hello = pkgs.writeScriptBin "hello" ''
+    #!/bin/sh
+    exec ${pkgs.hello}/bin/hello
+  '';
+}
 ```
 
 

scripts/force_cached.nix

@@ -0,0 +1,39 @@
+{ coreutils }:
+
+attrs:
+with builtins;
+let
+  # Copied from <nixpkgs/lib>
+  isDerivation = x: isAttrs x && x ? type && x.type == "derivation";
+
+  # Return true if `nix-build` would traverse that attribute set to look for
+  # more derivations to build.
+  hasRecurseIntoAttrs = x: isAttrs x && (x.recurseForDerivations or false);
+
+  # Wraps derivations that disallow substitutes so that they can be cached.
+  toCachedDrv = drv:
+    if !(drv.allowSubstitutes or true) then
+      derivation
+        {
+          name = "${drv.name}-to-cached";
+          system = drv.system;
+          builder = "/bin/sh";
+          args = [ "-c" "${coreutils}/bin/ln -s ${drv} $out" ];
+        }
+    else
+      drv;
+
+  op = _: val:
+    if isDerivation val then
+      toCachedDrv val
+    else if hasRecurseIntoAttrs val then
+      forceCached val
+    else
+      val
+  ;
+
+  # Traverses a tree of derivation and wrap all of those that disallow
+  # substitutes.
+  forceCached = attrs: mapAttrs op attrs;
+in
+forceCached attrs

scripts/force_cached_example.nix

@@ -0,0 +1,8 @@
+let
+  pkgs = import <nixpkgs> {};
+in (pkgs.callPackage ./force_cached.nix {}) {
+  hello = pkgs.writeScriptBin "hello" ''
+    #!/bin/sh
+    exec ${pkgs.hello}/bin/hello
+  '';
+}