From c63e5984979f55ae25059a1bb0ccc83fc64ea52a Mon Sep 17 00:00:00 2001
From: Eric Frias <efrias@syncad.com>
Date: Mon, 14 Apr 2014 16:27:08 -0400
Subject: [PATCH] Fix a race that occurred when notifying a thread that was
 blocked on a promise to wake up, but that thread simultaneously awoke for
 another reason (probably a timeout)

---
 src/thread/future.cpp | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/thread/future.cpp b/src/thread/future.cpp
index ebf68a6..7783834 100644
--- a/src/thread/future.cpp
+++ b/src/thread/future.cpp
@@ -79,8 +79,15 @@ namespace fc {
 #endif
   }
   void promise_base::_notify(){
-    if( _blocked_thread != nullptr ) 
-      _blocked_thread->notify(ptr(this,true));
+    // copy _blocked_thread into a local so that if the thread unblocks (e.g., 
+    // because of a timeout) before we get a chance to notify it, we won't be
+    // calling notify on a null pointer
+    thread* blocked_thread;
+    { synchronized(_spin_yield)
+      blocked_thread = _blocked_thread;
+    }
+    if( blocked_thread ) 
+      blocked_thread->notify(ptr(this,true));
   }
   promise_base::~promise_base() { }
   void promise_base::_set_timeout(){