MAGETWO-94172: Elasticsearch 5.0+ exception is shown if customer searches product before reindexing.

Add try catch block to catch exceptions thrown in Elastic search and return empty search results response.

Author: admanesachin

app/code/Magento/Elasticsearch/Elasticsearch5/SearchAdapter/Adapter.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Elasticsearch\Elasticsearch5\SearchAdapter;
 
 use Magento\Framework\App\ObjectManager;
@@ -12,6 +13,7 @@
 use Magento\Elasticsearch\SearchAdapter\Aggregation\Builder as AggregationBuilder;
 use Magento\Elasticsearch\SearchAdapter\ConnectionManager;
 use \Magento\Elasticsearch\SearchAdapter\ResponseFactory;
+use Psr\Log\LoggerInterface;
 
 /**
  * Elasticsearch Search Adapter
@@ -47,25 +49,56 @@ class Adapter implements AdapterInterface
      */
     private $queryContainerFactory;
 
+    /**
+     * Empty response from Elasticsearch.
+     *
+     * @var array
+     */
+    private static $emptyRawResponse = [
+        "hits" =>
+            [
+                "hits" => []
+            ],
+        "aggregations" =>
+            [
+                "price_bucket" => [],
+                "category_bucket" =>
+                    [
+                        "buckets" => []
+
+                    ]
+            ]
+    ];
+
+    /**
+     * @var LoggerInterface
+     */
+    private $logger;
+
     /**
      * @param ConnectionManager $connectionManager
      * @param Mapper $mapper
      * @param ResponseFactory $responseFactory
      * @param AggregationBuilder $aggregationBuilder
      * @param \Magento\Elasticsearch\SearchAdapter\QueryContainerFactory $queryContainerFactory
+     * @param LoggerInterface $logger
      */
     public function __construct(
         ConnectionManager $connectionManager,
         Mapper $mapper,
         ResponseFactory $responseFactory,
         AggregationBuilder $aggregationBuilder,
-        \Magento\Elasticsearch\SearchAdapter\QueryContainerFactory $queryContainerFactory
-    ) {
+        \Magento\Elasticsearch\SearchAdapter\QueryContainerFactory $queryContainerFactory,
+        LoggerInterface $logger = null
+    )
+    {
         $this->connectionManager = $connectionManager;
         $this->mapper = $mapper;
         $this->responseFactory = $responseFactory;
         $this->aggregationBuilder = $aggregationBuilder;
         $this->queryContainerFactory = $queryContainerFactory;
+        $this->logger = $logger ?: ObjectManager::getInstance()
+            ->get(LoggerInterface::class);
     }
 
     /**
@@ -78,23 +111,12 @@ public function query(RequestInterface $request)
         $aggregationBuilder = $this->aggregationBuilder;
         $query = $this->mapper->buildQuery($request);
         $aggregationBuilder->setQuery($this->queryContainerFactory->create(['query' => $query]));
-        if ($client->indexExists($query['index'])) {
+        try {
             $rawResponse = $client->query($query);
-        } else {
-            $rawResponse = [
-                "hits" =>
-                    [
-                        "hits" => []
-                    ],
-                "aggregations" =>
-                    [
-                        "price_bucket" => [],
-                        "category_bucket" =>
-                            [
-                                "buckets" => []
-                            ]
-                    ]
-            ];
+        } catch (\Exception $e) {
+            $this->logger->critical($e);
+            // return empty search result in case an exception is thrown from Elasticsearch
+            $rawResponse = self::$emptyRawResponse;
         }
         $rawDocuments = isset($rawResponse['hits']['hits']) ? $rawResponse['hits']['hits'] : [];
         $queryResponse = $this->responseFactory->create(