events: emitter.listeners() should not have side effects

Fixes #3803

lib/events.js

@@ -237,10 +237,9 @@ EventEmitter.prototype.removeAllListeners = function(type) {
 };
 
 EventEmitter.prototype.listeners = function(type) {
-  if (!this._events) this._events = {};
+  if (!this._events || !this._events[type]) return [];
-  if (!this._events[type]) this._events[type] = [];
   if (!isArray(this._events[type])) {
-    this._events[type] = [this._events[type]];
+    return [this._events[type]];
   }
   return this._events[type].slice(0);
 };

test/simple/test-event-emitter-listeners-side-effects.js

@@ -0,0 +1,61 @@
+
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var events = require('events');
+
+var EventEmitter = require('events').EventEmitter;
+var assert = require('assert');
+
+var e = new EventEmitter;
+var fl;  // foo listeners
+
+fl = e.listeners('foo');
+assert(Array.isArray(fl));
+assert(fl.length === 0);
+assert(typeof e._events == 'undefined');
+
+e.on('foo', assert.fail);
+fl = e.listeners('foo');
+assert(e._events.foo === assert.fail);
+assert(Array.isArray(fl));
+assert(fl.length === 1);
+assert(fl[0] === assert.fail);
+
+e.listeners('bar');
+assert(!e._events.hasOwnProperty('bar'));
+
+e.on('foo', assert.ok);
+fl = e.listeners('foo');
+
+assert(Array.isArray(e._events.foo));
+assert(e._events.foo.length === 2);
+assert(e._events.foo[0] === assert.fail);
+assert(e._events.foo[1] === assert.ok);
+
+assert(Array.isArray(fl));
+assert(fl.length === 2);
+assert(fl[0] === assert.fail);
+assert(fl[1] === assert.ok);
+
+console.log('ok');