events: avoid retaining removed event names

Restore shape mode for preallocated EventEmitters while deleting dynamic
names from ordinary EventEmitters. Keeping every removed name as an
undefined property causes unbounded memory growth when event names are
unstructured.

Refs: #64373
Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

lib/events.js

@@ -86,6 +86,7 @@ const { addAbortListener } = require('internal/events/abort_listener');
 
 const kCapture = Symbol('kCapture');
 const kErrorMonitor = Symbol('events.errorMonitor');
+const kShapeMode = Symbol('shapeMode');
 const kEmitting = Symbol('events.emitting');
 const kMaxEventTargetListeners = Symbol('events.maxEventTargetListeners');
 const kMaxEventTargetListenersWarned =
@@ -334,6 +335,9 @@ EventEmitter.init = function(opts) {
       this._events === ObjectGetPrototypeOf(this)._events) {
     this._events = { __proto__: null };
     this._eventsCount = 0;
+    this[kShapeMode] = false;
+  } else {
+    this[kShapeMode] = true;
   }
 
   this._maxListeners ||= undefined;
@@ -687,12 +691,13 @@ EventEmitter.prototype.removeListener =
       if (list === listener || list.listener === listener) {
         this._eventsCount -= 1;
 
-        // Leave the key in place with an `undefined` value: repeatedly
-        // adding and removing a listener for the same event this way keeps
-        // the `events` object in the same shape and avoids both a `delete`
-        // (which would put the object into dictionary mode) and allocating
-        // a fresh object when the last listener is removed.
-        events[type] = undefined;
+        if (this[kShapeMode]) {
+          events[type] = undefined;
+        } else if (this._eventsCount === 0) {
+          this._events = { __proto__: null };
+        } else {
+          delete events[type];
+        }
 
         if (events.removeListener !== undefined)
           this.emit('removeListener', type, list.listener || listener);
@@ -752,6 +757,7 @@ EventEmitter.prototype.removeAllListeners =
           else
             delete events[type];
         }
+        this[kShapeMode] = false;
         return this;
       }
 
@@ -764,6 +770,7 @@ EventEmitter.prototype.removeAllListeners =
         this.removeAllListeners('removeListener');
         this._events = { __proto__: null };
         this._eventsCount = 0;
+        this[kShapeMode] = false;
         return this;
       }
 

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

@@ -144,6 +144,19 @@ function listener2() {}
   assert.deepStrictEqual(ee, ee.removeListener('foo', () => {}));
 }
 
+{
+  const ee = new EventEmitter();
+  ee.on('persistent', listener1);
+
+  for (let i = 0; i < 100; i++) {
+    const eventName = `event-${i}`;
+    ee.on(eventName, listener2);
+    ee.removeListener(eventName, listener2);
+  }
+
+  assert.deepStrictEqual(Reflect.ownKeys(ee._events), ['persistent']);
+}
+
 {
   const ee = new EventEmitter();
   const listener = () => {};