From facbb77e6aa62ab30fe72f7536dfd456f51b01ff Mon Sep 17 00:00:00 2001
From: Kris Kowal <kris.kowal@cixar.com>
Date: Thu, 23 Aug 2012 16:51:00 -0700
Subject: [PATCH] Break coercion cycles with cooperating promises

Cooperating promise libraries must implement `valueOf` to extract the
fulfillment value from a fulfilled promise (or return the selfsame
promise for an unresolved or rejected promise).  This breaks cycles in
coercion, where one library uses `resolve` to coerce a foreign promise
to a local promise, which would in turn call `then` on the foreign
promise, which would in turn attempt to coerce the promise with its own
`resolve`, which would then in turn call `then` on our coerced promise,
ad nauseum.

Fixes #106.
---
 q.js | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/q.js b/q.js
index d509990d..8268a703 100644
--- a/q.js
+++ b/q.js
@@ -775,6 +775,15 @@ function resolve(object) {
     if (isPromise(object)) {
         return object;
     }
+    // In order to break infinite recursion or loops between `then` and
+    // `resolve`, it is necessary to attempt to extract fulfilled values
+    // out of foreign promise implementations before attempting to wrap
+    // them as unresolved promises.  It is my hope that other
+    // implementations will implement `valueOf` to synchronously extract
+    // the fulfillment value from their fulfilled promises.  If the
+    // other promise library does not implement `valueOf`, the
+    // implementations on primordial prototypes are harmless.
+    object = valueOf(object);
     // assimilate thenables, CommonJS/Promises/A
     if (object && typeof object.then === "function") {
         var deferred = defer();