events: fix memory leak, don't leak event names

Before this commit, events were set to undefined rather than deleted
from the EventEmitter's backing dictionary for performance reasons:
`delete obj.key` causes a transition of the dictionary's hidden class
and that can be costly.

Unfortunately, that introduces a memory leak when many events are added
and then removed again. The strings containing the event names are never
reclaimed by the garbage collector because they remain part of the
dictionary.

That's why this commit makes EventEmitter delete events again. This
effectively reverts commit 0397223.

Fixes #5970.

lib/events.js

@@ -197,7 +197,7 @@ EventEmitter.prototype.removeListener = function(type, listener) {
 
   if (list === listener ||
       (typeof list.listener === 'function' && list.listener === listener)) {
-    this._events[type] = undefined;
+    delete this._events[type];
     if (this._events.removeListener)
       this.emit('removeListener', type, listener);
 
@@ -215,7 +215,7 @@ EventEmitter.prototype.removeListener = function(type, listener) {
 
     if (list.length === 1) {
       list.length = 0;
-      this._events[type] = undefined;
+      delete this._events[type];
     } else {
       list.splice(position, 1);
     }
@@ -238,7 +238,7 @@ EventEmitter.prototype.removeAllListeners = function(type) {
     if (arguments.length === 0)
       this._events = {};
     else if (this._events[type])
-      this._events[type] = undefined;
+      delete this._events[type];
     return this;
   }
 
@@ -262,7 +262,7 @@ EventEmitter.prototype.removeAllListeners = function(type) {
     while (listeners.length)
       this.removeListener(type, listeners[listeners.length - 1]);
   }
-  this._events[type] = undefined;
+  delete this._events[type];
 
   return this;
 };

test/simple/test-event-emitter-memory-leak.js

@@ -0,0 +1,42 @@
+// 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.
+
+// Flags: --expose-gc
+
+// Add and remove a lot of different events to an EventEmitter, then check
+// that we didn't leak the event names.
+var common = require('../common');
+var assert = require('assert');
+var events = require('events');
+
+assert.equal(typeof gc, 'function', 'Run this test with --expose-gc');
+var before = process.memoryUsage().heapUsed;
+var e = new events.EventEmitter();
+
+for (var i = 0; i < 2.5e5; ++i) {
+  var name = 'a-pretty-long-event-name-' + i;
+  e.on(name, assert.fail);
+  e.removeListener(name, assert.fail);
+}
+while (gc());
+
+var after = process.memoryUsage().heapUsed;
+assert(after - before < 1024*1024, 'EventEmitter leaks event names.');