fix(runtime.js): partials compile not caching (#1600)

Reintroduce "merge" function, no called "mergeIfNeeded", that only creates a new partials
object if both "env.partials" and "options.partials" are set.

closes #1598

lib/handlebars/runtime.js

@@ -124,6 +124,15 @@ export function template(templateSpec, env) {
       }
       return value;
     },
+    mergeIfNeeded: function(param, common) {
+      let obj = param || common;
+
+      if (param && common && (param !== common)) {
+        obj = Utils.extend({}, common, param);
+      }
+
+      return obj;
+    },
     // An empty object to use as replacement for null-contexts
     nullContext: Object.seal({}),
 
@@ -161,7 +170,8 @@ export function template(templateSpec, env) {
       container.helpers = Utils.extend({}, env.helpers, options.helpers);
 
       if (templateSpec.usePartial) {
-        container.partials = Utils.extend({}, env.partials, options.partials);
+        // Use mergeIfNeeded here to prevent compiling global partials multiple times
+        container.partials = container.mergeIfNeeded(options.partials, env.partials);
       }
       if (templateSpec.usePartial || templateSpec.useDecorators) {
         container.decorators = Utils.extend({}, env.decorators, options.decorators);

spec/regressions.js

@@ -334,4 +334,31 @@ describe('Regressions', function() {
 
     shouldCompileTo('{{helpa length="foo"}}', [obj, helpers], 'foo');
   });
+
+  describe('GH-1598: Performance degradation for partials since v4.3.0', function() {
+    // Do not run test for runs without compiler
+    if (!Handlebars.compile) {
+      return;
+    }
+
+    var newHandlebarsInstance;
+    beforeEach(function() {
+      newHandlebarsInstance = Handlebars.create();
+    });
+    afterEach(function() {
+      sinon.restore();
+    });
+
+    it('should only compile global partials once', function() {
+      var templateSpy = sinon.spy(newHandlebarsInstance, 'template');
+      newHandlebarsInstance.registerPartial({
+        'dude': 'I am a partial'
+      });
+      var string = 'Dudes: {{> dude}} {{> dude}}';
+      newHandlebarsInstance.compile(string)(); // This should compile template + partial once
+      newHandlebarsInstance.compile(string)(); // This should only compile template
+      equal(templateSpy.callCount, 3);
+      sinon.restore();
+    });
+  });
 });