Make res.render callback is always async, even for sync view engines

closes #2668

History.md

@@ -7,6 +7,7 @@ This incorporates all changes after 4.10.1 up to 4.13.1.
     - `app.param(fn)`
     - `req.param()` -- use `req.params`, `req.body`, or `req.query` instead
   * change:
+    - `res.render` callback is always async, even for sync view engines
     - The leading `:` character in `name` for `app.param(name, fn)` is no longer removed
     - Use `router` module for routing
     - Use `path-is-absolute` module for absolute path detection

lib/view.js

@@ -122,8 +122,31 @@ View.prototype.lookup = function lookup(name) {
  */
 
 View.prototype.render = function render(options, callback) {
+  var sync = true;
+
   debug('render "%s"', this.path);
-  this.engine(this.path, options, callback);
+
+  // render, normalizing sync callbacks
+  this.engine(this.path, options, function onRender() {
+    if (!sync) {
+      return callback.apply(this, arguments);
+    }
+
+    // copy arguments
+    var args = new Array(arguments.length);
+    var cntx = this;
+
+    for (var i = 0; i < arguments.length; i++) {
+      args[i] = arguments[i];
+    }
+
+    // force callback to be async
+    return process.nextTick(function renderTick() {
+      return callback.apply(cntx, args);
+    });
+  });
+
+  sync = false;
 };
 
 /**