Fix memory leak

Author: ForbesLindesay

lib/core.js

@@ -2,6 +2,16 @@
 
 var asap = require('asap')
 
+// Use Symbol if it's available, but otherwise just
+// generate a random string as this is probably going
+// to be unique
+var handleSymbol = typeof Symbol === 'function' ?
+    Symbol() :
+    (
+      '_promise_internal_key_handle_' +
+      Math.random().toString(35).substr(2, 10) + '_'
+    )
+
 module.exports = Promise;
 function Promise(fn) {
   if (typeof this !== 'object') throw new TypeError('Promises must be constructed via new')
@@ -16,6 +26,7 @@ function Promise(fn) {
       handle(new Handler(onFulfilled, onRejected, resolve, reject))
     })
   }
+  this.then[handleSymbol] = handle;
 
   function handle(deferred) {
     if (state === null) {
@@ -46,7 +57,18 @@ function Promise(fn) {
       if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {
         var then = newValue.then
         if (typeof then === 'function') {
-          doResolve(then.bind(newValue), resolve, reject)
+          if (typeof then[handleSymbol] === 'function') {
+            // to prevent a memory leak, we adopt the value of the other promise
+            // allowing this promise to be garbage collected as soon as nobody
+            // has a reference to it
+            handle = (self[handleSymbol] = then[handleSymbol]);
+            self.then = then;
+            deferreds.forEach(function (deferred) {
+              handle(deferred);
+            });
+          } else {
+            doResolve(then.bind(newValue), resolve, reject)
+          }
           return
         }
       }

package.json

@@ -4,9 +4,9 @@
   "description": "Bare bones Promises/A+ implementation",
   "main": "index.js",
   "scripts": {
-    "test": "mocha --timeout 200 --slow 99999 && npm run test-memory-leak",
-    "test-resolve": "mocha test/resolver-tests.js -R spec --timeout 200 --slow 999999",
-    "test-extensions": "mocha test/extensions-tests.js -R spec --timeout 200 --slow 999999",
+    "test": "mocha --timeout 200 --slow 99999 -R dot && npm run test-memory-leak",
+    "test-resolve": "mocha test/resolver-tests.js --timeout 200 --slow 999999",
+    "test-extensions": "mocha test/extensions-tests.js --timeout 200 --slow 999999",
     "test-memory-leak": "node --expose-gc test/memory-leak.js"
   },
   "repository": {

test/memory-leak.js

@@ -23,7 +23,9 @@ function next() {
       if (typeof global.gc === 'function') {
         global.gc()
         sampleB = process.memoryUsage()
+        console.log('Memory usage at start:');
         console.dir(sampleA)
+        console.log('Memory usage at end:');
         console.dir(sampleB)
         assert(sampleA.rss * 1.2 > sampleB.rss, 'RSS should not grow by more than 20%')
         assert(sampleA.heapTotal * 1.2 > sampleB.heapTotal, 'heapTotal should not grow by more than 20%')