Do not schedule connection pool checks on 4xx level errors

Closes #636

src/Elasticsearch/Transport.php

@@ -114,9 +114,11 @@ function ($response) {
             },
             //onFailure
             function ($response) {
-                //some kind of real faiure here, like a timeout
-                $this->connectionPool->scheduleCheck();
-                // log stuff
+                // Ignore 400 level errors, as that means the server responded just fine
+                if (!(isset($response['code']) && $response['code'] >=400 && $response['code'] < 500)) {
+                    // Otherwise schedule a check
+                    $this->connectionPool->scheduleCheck();
+                }
             });
 
         return $future;

tests/Elasticsearch/Tests/ConnectionPool/StaticConnectionPoolIntegrationTest.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types = 1);
+
+/**
+ * Class StaticConnectionPoolIntegrationTest
+ *
+ * @category   Tests
+ * @package    Elasticsearch
+ * @subpackage Tests/StaticConnectionPoolTest
+ * @author     Zachary Tong <zachary.tong@elasticsearch.com>
+ * @license    http://www.apache.org/licenses/LICENSE-2.0 Apache2
+ * @link       http://elasticsearch.org
+ */
+class StaticConnectionPoolIntegrationTest extends \PHPUnit_Framework_TestCase
+{
+    // Issue #636
+    public function test404Liveness() {
+        $client = \Elasticsearch\ClientBuilder::create()
+            ->setHosts([$_SERVER['ES_TEST_HOST']])
+            ->setConnectionPool(\Elasticsearch\ConnectionPool\StaticConnectionPool::class)
+            ->build();
+
+        $connection = $client->transport->getConnection();
+
+        // Ensure connection is dead
+        $connection->markDead();
+
+        // The index doesn't exist, but the server is up so this will return a 404
+        $this->assertFalse($client->indices()->exists(['index' => 'not_existing_index']));
+
+        // But the node should be marked as alive since the server responded
+        $this->assertTrue($connection->isAlive());
+    }
+}