Avoid garbage references when resolving rejects on legacy PHP <= 5.6

These garbage references only show up when a DNS lookup rejects with an
Exception on legacy PHP <= 5.6. These issues have been fixed upstream in
the Promise component for current PHP versions with
https://github.com/reactphp/promise/releases/tag/v2.6.0 but this PR
explicitly unsets known garbage references to avoid unexpected memory
growth on legacy PHP versions.

The downstream Socket component implicitly depends on this because its
test suite currently fails. This changeset can be considered a new
feature in this component which fixes a bug in the test suite of a
downstream component.

Author: clue

src/Query/CachingExecutor.php

@@ -52,10 +52,14 @@ function (Message $message) use ($cache, $id, $that) {
                         }
                     );
                 }
-            )->then($resolve, $reject);
+            )->then($resolve, function ($e) use ($reject, &$pending) {
+                $reject($e);
+                $pending = null;
+            });
         }, function ($_, $reject) use (&$pending, $query) {
             $reject(new \RuntimeException('DNS query for ' . $query->name . ' has been cancelled'));
             $pending->cancel();
+            $pending = null;
         });
     }
 

src/Query/CoopExecutor.php

@@ -75,10 +75,11 @@ public function query($nameserver, Query $query)
         $counts =& $this->counts;
         return new Promise(function ($resolve, $reject) use ($promise) {
             $promise->then($resolve, $reject);
-        }, function () use ($promise, $key, $query, &$pending, &$counts) {
+        }, function () use (&$promise, $key, $query, &$pending, &$counts) {
             if (--$counts[$key] < 1) {
                 unset($pending[$key], $counts[$key]);
                 $promise->cancel();
+                $promise = null;
             }
             throw new \RuntimeException('DNS query for ' . $query->name . ' has been cancelled');
         });

tests/FunctionalResolverTest.php

@@ -98,4 +98,74 @@ public function testInvalidResolverDoesNotResolveGoogle()
         $promise = $this->resolver->resolve('google.com');
         $promise->then($this->expectCallableNever(), $this->expectCallableOnce());
     }
+
+    public function testResolveShouldNotCauseGarbageReferencesWhenUsingInvalidNameserver()
+    {
+        if (class_exists('React\Promise\When')) {
+            $this->markTestSkipped('Not supported on legacy Promise v1 API');
+        }
+
+        $factory = new Factory();
+        $this->resolver = $factory->create('255.255.255.255', $this->loop);
+
+        gc_collect_cycles();
+
+        $promise = $this->resolver->resolve('google.com');
+        unset($promise);
+
+        $this->assertEquals(0, gc_collect_cycles());
+    }
+
+    public function testResolveCachedShouldNotCauseGarbageReferencesWhenUsingInvalidNameserver()
+    {
+        if (class_exists('React\Promise\When')) {
+            $this->markTestSkipped('Not supported on legacy Promise v1 API');
+        }
+
+        $factory = new Factory();
+        $this->resolver = $factory->createCached('255.255.255.255', $this->loop);
+
+        gc_collect_cycles();
+
+        $promise = $this->resolver->resolve('google.com');
+        unset($promise);
+
+        $this->assertEquals(0, gc_collect_cycles());
+    }
+
+    public function testCancelResolveShouldNotCauseGarbageReferences()
+    {
+        if (class_exists('React\Promise\When')) {
+            $this->markTestSkipped('Not supported on legacy Promise v1 API');
+        }
+
+        $factory = new Factory();
+        $this->resolver = $factory->create('127.0.0.1', $this->loop);
+
+        gc_collect_cycles();
+
+        $promise = $this->resolver->resolve('google.com');
+        $promise->cancel();
+        $promise = null;
+
+        $this->assertEquals(0, gc_collect_cycles());
+    }
+
+    public function testCancelResolveCachedShouldNotCauseGarbageReferences()
+    {
+        if (class_exists('React\Promise\When')) {
+            $this->markTestSkipped('Not supported on legacy Promise v1 API');
+        }
+
+        $factory = new Factory();
+        $this->resolver = $factory->createCached('127.0.0.1', $this->loop);
+
+        gc_collect_cycles();
+
+        $promise = $this->resolver->resolve('google.com');
+        $promise->cancel();
+        $promise = null;
+
+        $this->assertEquals(0, gc_collect_cycles());
+    }
 }

tests/Query/CoopExecutorTest.php

@@ -205,4 +205,29 @@ public function testQueryTwiceWillQueryBaseExecutorTwiceIfFirstQueryHasAlreadyBe
 
         $promise2->then(null, $this->expectCallableNever());
     }
+
+    public function testCancelQueryShouldNotCauseGarbageReferences()
+    {
+        if (class_exists('React\Promise\When')) {
+            $this->markTestSkipped('Not supported on legacy Promise v1 API');
+        }
+
+        $deferred = new Deferred(function () {
+            throw new \RuntimeException();
+        });
+
+        $base = $this->getMockBuilder('React\Dns\Query\ExecutorInterface')->getMock();
+        $base->expects($this->once())->method('query')->willReturn($deferred->promise());
+        $connector = new CoopExecutor($base);
+
+        gc_collect_cycles();
+
+        $query = new Query('reactphp.org', Message::TYPE_A, Message::CLASS_IN);
+
+        $promise = $connector->query('8.8.8.8', $query);
+        $promise->cancel();
+        $promise = null;
+
+        $this->assertEquals(0, gc_collect_cycles());
+    }
 }