containers/helpers: add nix-json-trans

It’s a way to use the nix language capabilities to transform json. Essentially
like `jq`, but without introducing yet another DSL (the nixpkgs library is
reused).
One disadvantage it that it reads the json documents into memory, so no
streaming is performed. That shouldn’t matter for small-ish files, though.

containers/default.nix

@@ -25,6 +25,7 @@ let
             prependPrefix = ./helpers/prepend-prefix.nix;
             debugDockerImage = ./helpers/debug-docker-image.nix;
             createStandaloneDockerImage = ./helpers/create-standalone-docker-image.nix;
+          nixJsonTrans = ./helpers/nix-json-trans.nix;
           };
         })];
       };

containers/helpers/nix-json-trans.nix

+{ lib, pkgs, stdenv }:
+{ libPath ? <nixpkgs/lib> }:
+
+# TODO: Do we need all of lib?
+let
+  trans = pkgs.writeScript "nix-json-trans" ''
+    #!${stdenv.shell}
+    DIR=$(mktemp -d)
+    cat >$DIR/script-input.nix <<EOF
+    with import ${libPath};
+    $(cat "$1")
+    EOF
+
+    JSON="$(realpath "$2")"
+
+    cat >$DIR/script.nix <<EOF
+    let
+      f = import $DIR/script-input.nix;
+      json = builtins.fromJSON (builtins.readFile $JSON);
+    in
+      assert builtins.typeOf $JSON == "path";
+      assert builtins.typeOf f == "lambda";
+      f json
+    EOF
+
+    source ${pkgs.setupLocalNixStore}
+    ${pkgs.nix}/bin/nix-instantiate \
+      --eval --json --strict \
+      --show-trace \
+      $DIR/script.nix
+  '';
+
+  tests =
+    let
+      call = nixFile: jsonFile: "${trans} ${nixFile} ${jsonFile}";
+      eq = nixFile: jsonFile: resultFile: ''
+        ${pkgs.diffutils}/bin/diff <(${call nixFile jsonFile}) ${resultFile}
+      '';
+      mkjson = json: pkgs.writeText "test.json" (builtins.toJSON json);
+    in {
+
+      idJson =
+        let
+          json = mkjson {
+            foo = [ 1 2 3 null "bar" ];
+          };
+          idScr = pkgs.writeText "id-converter" "id";
+        in eq idScr json json;
+
+      replaceAttrsJson =
+        eq (pkgs.writeText "maybe-its-neuer"
+              ''mapAttrs (_: _: "manuel neuer")'')
+          (mkjson { foo = "richard stallman"; bar = "linus torvalds"; })
+          (mkjson { foo = "manuel neuer"; bar = "manuel neuer"; });
+    };
+
+in pkgs.withTests tests trans