events: pass the original listener added by once

When removing a `once` listener, the listener being passed to
the `removeListener` callback is the wrapper. This unwraps the
listener so that `removeListener` is passed the actual listener.

PR-URL: https://github.com/nodejs/node/pull/6394
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/events.js

@@ -308,7 +308,7 @@ EventEmitter.prototype.prependOnceListener =
 // emits a 'removeListener' event iff the listener was removed
 EventEmitter.prototype.removeListener =
     function removeListener(type, listener) {
-      var list, events, position, i;
+      var list, events, position, i, originalListener;
 
       if (typeof listener !== 'function')
         throw new TypeError('"listener" argument must be a function');
@@ -327,7 +327,7 @@ EventEmitter.prototype.removeListener =
         else {
           delete events[type];
           if (events.removeListener)
-            this.emit('removeListener', type, listener);
+            this.emit('removeListener', type, list.listener || listener);
         }
       } else if (typeof list !== 'function') {
         position = -1;
@@ -335,6 +335,7 @@ EventEmitter.prototype.removeListener =
         for (i = list.length; i-- > 0;) {
           if (list[i] === listener ||
               (list[i].listener && list[i].listener === listener)) {
+            originalListener = list[i].listener;
             position = i;
             break;
           }
@@ -356,7 +357,7 @@ EventEmitter.prototype.removeListener =
         }
 
         if (events.removeListener)
-          this.emit('removeListener', type, listener);
+          this.emit('removeListener', type, originalListener || listener);
       }
 
       return this;

test/parallel/test-event-emitter-remove-listeners.js

@@ -102,3 +102,14 @@ e6.emit('hello');
 
 // Interal listener array [listener3]
 e6.emit('hello');
+
+const e7 = new events.EventEmitter();
+
+const listener5 = () => {};
+
+e7.once('hello', listener5);
+e7.on('removeListener', common.mustCall((eventName, listener) => {
+  assert.strictEqual(eventName, 'hello');
+  assert.strictEqual(listener, listener5);
+}));
+e7.emit('hello');