Add requirement that exception listener implement interface

Per suggestions from others in the PR, and with their help, I've added
in a check that will validate that the exception listener implements
the required interface. In addition, I've updated the tests and added a
new test to verify that this works as expected.

Author: swquinn

src/Sentry/SentryBundle/DependencyInjection/Configuration.php

@@ -44,6 +44,10 @@ public function getConfigTreeBuilder()
                 ->end()
                 ->scalarNode('exception_listener')
                     ->defaultValue('Sentry\SentryBundle\EventListener\ExceptionListener')
+                    ->validate()
+                    ->ifTrue($this->getExceptionListenerInvalidationClosure())
+                        ->thenInvalid('The "sentry.exception_listener" parameter should be a FQCN of a class implementing the SentryExceptionListenerInterface interface')
+                    ->end()
                 ->end()
                 ->arrayNode('skip_capture')
                     ->treatNullLike(array())
@@ -72,4 +76,18 @@ public function getConfigTreeBuilder()
 
         return $treeBuilder;
     }
+
+    /**
+     * @return \Closure
+     */
+    private function getExceptionListenerInvalidationClosure()
+    {
+        return function ($value) {
+            $implements = class_implements($value);
+            if ($implements === false) {
+                return true;
+            }
+            return !in_array('Sentry\SentryBundle\EventListener\SentryExceptionListenerInterface', $implements, true);
+        };
+    }
 }

test/Sentry/SentryBundle/Test/DependencyInjection/ExtensionTest.php

@@ -133,6 +133,19 @@ public function test_that_it_uses_options_value()
         );
     }
 
+    /**
+     * @expectedException Symfony\Component\Config\Definition\Exception\InvalidConfigurationException
+     */
+    public function test_that_it_is_invalid_if_exception_listener_fails_to_implement_required_interface()
+    {
+        $class = 'Sentry\SentryBundle\Test\Fixtures\InvalidExceptionListener';
+        $container = $this->getContainer(array(
+            static::CONFIG_ROOT => array(
+                'exception_listener' => $class,
+            ),
+        ));
+    }
+
     public function test_that_it_uses_defined_class_as_exception_listener_class_by_default()
     {
         $container = $this->getContainer();
@@ -145,14 +158,15 @@ public function test_that_it_uses_defined_class_as_exception_listener_class_by_d
 
     public function test_that_it_uses_exception_listener_value()
     {
+        $class = 'Sentry\SentryBundle\Test\Fixtures\CustomExceptionListener';
         $container = $this->getContainer(array(
             static::CONFIG_ROOT => array(
-                'exception_listener' => 'exceptionListenerClass',
+                'exception_listener' => $class,
             ),
         ));
 
         $this->assertSame(
-            'exceptionListenerClass',
+            $class,
             $container->getParameter('sentry.exception_listener')
         );
     }

test/Sentry/SentryBundle/Test/Fixtures/CustomExceptionListener.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Sentry\SentryBundle\Test\Fixtures;
+
+use Sentry\SentryBundle\EventListener\ExceptionListener;
+
+/**
+ * @package Sentry\SentryBundle\Tests\Fixtures
+ */
+class CustomExceptionListener extends ExceptionListener
+{
+}

test/Sentry/SentryBundle/Test/Fixtures/InvalidExceptionListener.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Sentry\SentryBundle\Test\Fixtures;
+
+/**
+ * An invalid <code>ExceptionListener</code> for testing.
+ *
+ * This exception listener does not implement, or extend a class that
+ * implements, the appropriate interface that the dependency injection
+ * container requires.
+ *
+ * @package Sentry\SentryBundle\Tests\Fixtures
+ */
+class InvalidExceptionListener
+{
+}