Deep-clone model settings in lib/builtin-models

Fix the code loading builtin models to always clone the JSON object
used as model settings/definition. This is needed to allow applications
to modify model settings while not affecting settings of new models
created in the local registry of another app.

lib/builtin-models.js

@@ -4,6 +4,9 @@
 // License text available at https://opensource.org/licenses/MIT
 
 'use strict';
+
+const assert = require('assert');
+
 module.exports = function(registry) {
   // NOTE(bajtos) we must use static require() due to browserify limitations
 
@@ -52,8 +55,33 @@ module.exports = function(registry) {
     require('../common/models/checkpoint.js'));
 
   function createModel(definitionJson, customizeFn) {
+    // Clone the JSON definition to allow applications
+    // to modify model settings while not affecting
+    // settings of new models created in the local registry
+    // of another app.
+    // This is needed because require() always returns the same
+    // object instance it loaded during the first call.
+    definitionJson = cloneDeepJson(definitionJson);
+
     var Model = registry.createModel(definitionJson);
     customizeFn(Model);
     return Model;
   }
 };
+
+// Because we are cloning objects created by JSON.parse,
+// the cloning algorithm can stay much simpler than a general-purpose
+// "cloneDeep" e.g. from lodash.
+function cloneDeepJson(obj) {
+  const result = Array.isArray(obj) ? [] : {};
+  assert.equal(Object.getPrototypeOf(result), Object.getPrototypeOf(obj));
+  for (const key in obj) {
+    const value = obj[key];
+    if (typeof value === 'object') {
+      result[key] = cloneDeepJson(value);
+    } else {
+      result[key] = value;
+    }
+  }
+  return result;
+}