Handle on_headers option in MockHandler (#1580)

* Handle on_headers option

* Tests for correct handling of on_headers option

src/Handler/MockHandler.php

@@ -1,6 +1,7 @@
 <?php
 namespace GuzzleHttp\Handler;
 
+use GuzzleHttp\Exception\RequestException;
 use GuzzleHttp\HandlerStack;
 use GuzzleHttp\Promise\PromiseInterface;
 use GuzzleHttp\Promise\RejectedPromise;
@@ -73,6 +74,18 @@ class MockHandler implements \Countable
         $this->lastOptions = $options;
         $response = array_shift($this->queue);
 
+        if (isset($options['on_headers'])) {
+            if (!is_callable($options['on_headers'])) {
+                throw new \InvalidArgumentException('on_headers must be callable');
+            }
+            try {
+                $options['on_headers']($response);
+            } catch (\Exception $e) {
+                $msg = 'An error was encountered during the on_headers event';
+                $response = new RequestException($msg, $request, $response, $e);
+            }
+        }
+
         if (is_callable($response)) {
             $response = call_user_func($response, $request, $options);
         }

tests/Handler/MockHandlerTest.php

@@ -114,6 +114,35 @@ class MockHandlerTest extends \PHPUnit_Framework_TestCase
         $this->assertSame($r, $p->wait());
     }
 
+    /**
+     * @expectedException \InvalidArgumentException
+     */
+    public function testEnsuresOnHeadersIsCallable()
+    {
+        $res = new Response();
+        $mock = new MockHandler([$res]);
+        $request = new Request('GET', 'http://example.com');
+        $mock($request, ['on_headers' => 'error!']);
+    }
+
+    /**
+     * @expectedException \GuzzleHttp\Exception\RequestException
+     * @expectedExceptionMessage An error was encountered during the on_headers event
+     * @expectedExceptionMessage test
+     */
+    public function testRejectsPromiseWhenOnHeadersFails()
+    {
+        $res = new Response();
+        $mock = new MockHandler([$res]);
+        $request = new Request('GET', 'http://example.com');
+        $promise = $mock($request, [
+            'on_headers' => function () {
+                throw new \Exception('test');
+            }
+        ]);
+
+        $promise->wait();
+    }
     public function testInvokesOnFulfilled()
     {
         $res = new Response();