Created middleware to catch rest errors and return JSON responses

config/autoload/middleware-pipeline.global.php

@@ -26,11 +26,5 @@ return [
             ],
             'priority' => 1,
         ],
-
-        'error' => [
-            'middleware' => [],
-            'error'    => true,
-            'priority' => -10000,
-        ],
     ],
 ];

module/Rest/config/middleware-pipeline.config.php

@@ -12,5 +12,14 @@ return [
             ],
             'priority' => 5,
         ],
+
+        'rest-error' => [
+            'path' => '/rest',
+            'middleware' => [
+                Middleware\Error\ResponseTypeMiddleware::class,
+            ],
+            'error'    => true,
+            'priority' => -10000,
+        ],
     ],
 ];

module/Rest/config/services.config.php

@@ -19,6 +19,8 @@ return [
 
             Middleware\CrossDomainMiddleware::class => InvokableFactory::class,
             Middleware\CheckAuthenticationMiddleware::class => AnnotatedFactory::class,
+
+            Middleware\Error\ResponseTypeMiddleware::class => AnnotatedFactory::class,
         ],
     ],
 

module/Rest/src/Middleware/Error/ResponseTypeMiddleware.php

@@ -0,0 +1,59 @@
+<?php
+namespace Shlinkio\Shlink\Rest\Middleware\Error;
+
+use Acelaya\ZsmAnnotatedServices\Annotation\Inject;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Shlinkio\Shlink\Rest\Util\RestUtils;
+use Zend\Diactoros\Response\JsonResponse;
+use Zend\I18n\Translator\TranslatorInterface;
+use Zend\Stratigility\ErrorMiddlewareInterface;
+
+class ResponseTypeMiddleware implements ErrorMiddlewareInterface
+{
+    /**
+     * @var TranslatorInterface
+     */
+    private $translator;
+
+    /**
+     * ResponseTypeMiddleware constructor.
+     * @param TranslatorInterface $translator
+     *
+     * @Inject({"translator"})
+     */
+    public function __construct(TranslatorInterface $translator)
+    {
+        $this->translator = $translator;
+    }
+
+    /**
+     * Process an incoming error, along with associated request and response.
+     *
+     * Accepts an error, a server-side request, and a response instance, and
+     * does something with them; if further processing can be done, it can
+     * delegate to `$out`.
+     *
+     * @see MiddlewareInterface
+     * @param mixed $error
+     * @param Request $request
+     * @param Response $response
+     * @param null|callable $out
+     * @return null|Response
+     */
+    public function __invoke($error, Request $request, Response $response, callable $out = null)
+    {
+        $accept = $request->getHeader('Accept');
+        if (! empty(array_intersect(['application/json', 'text/json', 'application/x-json'], $accept))) {
+            $status = $response->getStatusCode();
+            $status = $status >= 400 ? $status : 500;
+
+            return new JsonResponse([
+                'error' => RestUtils::UNKNOWN_ERROR,
+                'message' => $this->translator->translate('Unknown error'),
+            ], $status);
+        }
+
+        return $out($request, $response, $error);
+    }
+}