Cancellation handler must only be called once

Author: clue

src/React/Promise/Deferred.php

@@ -128,11 +128,14 @@ public function cancel()
             return;
         }
 
+        $canceller = $this->canceller;
+        $this->canceller = null;
+
         try {
             $that = $this;
 
             call_user_func(
-                $this->canceller,
+                $canceller,
                 function ($value = null) use ($that) {
                     $that->resolve($value);
                 },

tests/React/Promise/DeferredTest.php

@@ -114,6 +114,14 @@ public function shouldInvokeCancellationHandlerAndStayPendingWhenCallingCancel()
         $d->then($this->expectCallableNever(), $this->expectCallableNever());
     }
 
+    /** @test */
+    public function shouldInvokeCancellationHandlerOnlyOnceWhenCallingCancelMultipleTimes()
+    {
+        $d = new Deferred($this->expectCallableOnce());
+        $d->cancel();
+        $d->cancel();
+    }
+
     /** @test */
     public function shouldResolveWhenCancellationHandlerResolves()
     {