events: unwrap #once listeners in #listeners

Fixes: https://github.com/nodejs/node/issues/6873
PR-URL: https://github.com/nodejs/node/pull/6881
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/events.js

@@ -425,9 +425,9 @@ EventEmitter.prototype.listeners = function listeners(type) {
     if (!evlistener)
       ret = [];
     else if (typeof evlistener === 'function')
-      ret = [evlistener];
+      ret = [evlistener.listener || evlistener];
     else
-      ret = arrayClone(evlistener, evlistener.length);
+      ret = unwrapListeners(evlistener);
   }
 
   return ret;
@@ -475,3 +475,11 @@ function arrayClone(arr, i) {
     copy[i] = arr[i];
   return copy;
 }
+
+function unwrapListeners(arr) {
+  const ret = new Array(arr.length);
+  for (var i = 0; i < ret.length; ++i) {
+    ret[i] = arr[i].listener || arr[i];
+  }
+  return ret;
+}

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

@@ -36,3 +36,16 @@ function listener2() {}
   assert.deepStrictEqual(ee.listeners('foo'), [listener, listener2]);
   assert.deepStrictEqual(eeListenersCopy, [listener]);
 }
+
+{
+  const ee = new events.EventEmitter();
+  ee.once('foo', listener);
+  assert.deepStrictEqual(ee.listeners('foo'), [listener]);
+}
+
+{
+  const ee = new events.EventEmitter();
+  ee.on('foo', listener);
+  ee.once('foo', listener2);
+  assert.deepStrictEqual(ee.listeners('foo'), [listener, listener2]);
+}