Do not cache truncated response messages as per RFC

See https://tools.ietf.org/html/rfc1035#section-7.4

Author: clue

src/Query/CachingExecutor.php

@@ -43,8 +43,10 @@ function ($message) use ($nameserver, $query, $id, $cache, $executor, &$pending)
                     // perform DNS lookup if not already cached
                     return $pending = $executor->query($nameserver, $query)->then(
                         function (Message $message) use ($cache, $id) {
-                            // DNS response message received => store in cache and return
-                            $cache->set($id, $message, CachingExecutor::TTL);
+                            // DNS response message received => store in cache when not truncated and return
+                            if (!$message->header->isTruncated()) {
+                                $cache->set($id, $message, CachingExecutor::TTL);
+                            }
 
                             return $message;
                         }

tests/Query/CachingExecutorTest.php

@@ -82,6 +82,26 @@ public function testQueryWillReturnResolvedPromiseWhenCacheReturnsMissAndFallbac
         $promise->then($this->expectCallableOnceWith($message), $this->expectCallableNever());
     }
 
+    public function testQueryWillReturnResolvedPromiseWhenCacheReturnsMissAndFallbackExecutorResolvesWithTruncatedResponseButShouldNotSaveTruncatedMessageToCache()
+    {
+        $message = new Message();
+        $message->header->set('tc', 1);
+        $fallback = $this->getMockBuilder('React\Dns\Query\ExecutorInterface')->getMock();
+        $fallback->expects($this->once())->method('query')->willReturn(\React\Promise\resolve($message));
+
+        $cache = $this->getMockBuilder('React\Cache\CacheInterface')->getMock();
+        $cache->expects($this->once())->method('get')->with('reactphp.org:1:1')->willReturn(\React\Promise\resolve(null));
+        $cache->expects($this->never())->method('set');
+
+        $executor = new CachingExecutor($fallback, $cache);
+
+        $query = new Query('reactphp.org', Message::TYPE_A, Message::CLASS_IN);
+
+        $promise = $executor->query('8.8.8.8', $query);
+
+        $promise->then($this->expectCallableOnceWith($message), $this->expectCallableNever());
+    }
+
     public function testQueryWillReturnRejectedPromiseWhenCacheReturnsMissAndFallbackExecutorRejects()
     {
         $query = new Query('reactphp.org', Message::TYPE_A, Message::CLASS_IN);