Make it much easier to add one-off event listeners

Summary:
Ref T4657. Right now, you have to muck with `events.listeners` to install listeners. Instead, automatically install all subclasses of AutoEventListener.

Primarily, this makes it easier to resolve requests with "drop this file in `src/extensions/`, no warranty", which seems to have worked well so far in resolving things like custom remarkup rules, etc.

Test Plan:
  - Added such a listener, had it autoregister.
  - Clicked around and saw the effects of normal listeners.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4657

Differential Revision: https://secure.phabricator.com/D9262

src/__phutil_library_map__.php

@@ -1288,6 +1288,7 @@ phutil_register_library_map(array(
     'PhabricatorAuthUnlinkController' => 'applications/auth/controller/PhabricatorAuthUnlinkController.php',
     'PhabricatorAuthValidateController' => 'applications/auth/controller/PhabricatorAuthValidateController.php',
     'PhabricatorAuthenticationConfigOptions' => 'applications/config/option/PhabricatorAuthenticationConfigOptions.php',
+    'PhabricatorAutoEventListener' => 'infrastructure/events/PhabricatorAutoEventListener.php',
     'PhabricatorBarePageExample' => 'applications/uiexample/examples/PhabricatorBarePageExample.php',
     'PhabricatorBarePageView' => 'view/page/PhabricatorBarePageView.php',
     'PhabricatorBaseEnglishTranslation' => 'infrastructure/internationalization/translation/PhabricatorBaseEnglishTranslation.php',
@@ -4062,6 +4063,7 @@ phutil_register_library_map(array(
     'PhabricatorAuthUnlinkController' => 'PhabricatorAuthController',
     'PhabricatorAuthValidateController' => 'PhabricatorAuthController',
     'PhabricatorAuthenticationConfigOptions' => 'PhabricatorApplicationConfigOptions',
+    'PhabricatorAutoEventListener' => 'PhabricatorEventListener',
     'PhabricatorBarePageExample' => 'PhabricatorUIExample',
     'PhabricatorBarePageView' => 'AphrontPageView',
     'PhabricatorBaseEnglishTranslation' => 'PhabricatorTranslation',

src/infrastructure/events/PhabricatorAutoEventListener.php

@@ -0,0 +1,17 @@
+<?php
+
+/**
+ * Event listener which is registered automatically, without requiring
+ * configuration.
+ *
+ * Normally, event listeners must be registered via applications. This is
+ * appropriate for structured listeners in libraries, but it adds a lot of
+ * overhead and is cumbersome for one-off listeners.
+ *
+ * All concrete subclasses of this class are automatically registered at
+ * startup. This allows it to be used with custom one-offs that can be dropped
+ * into `phabricator/src/extensions/`.
+ */
+abstract class PhabricatorAutoEventListener extends PhabricatorEventListener {
+
+}

src/infrastructure/events/PhabricatorEventEngine.php

@@ -1,36 +1,50 @@
 <?php
 
-/**
- * @group events
- */
 final class PhabricatorEventEngine {
 
   public static function initialize() {
-    $listeners = PhabricatorEnv::getEnvConfig('events.listeners');
-    foreach ($listeners as $listener) {
+    // NOTE: If any of this fails, we just log it and move on. It's important
+    // to try to make it through here because users may have difficulty fixing
+    // fix the errors if we don't: for example, if we fatal here a user may not
+    // be able to run `bin/config` in order to remove an invalid listener.
+
+    // Load automatic listeners.
+    $listeners = id(new PhutilSymbolLoader())
+      ->setAncestorClass('PhabricatorAutoEventListener')
+      ->loadObjects();
+
+    // Load configured listeners.
+    $config_listeners = PhabricatorEnv::getEnvConfig('events.listeners');
+    foreach ($config_listeners as $listener_class) {
       try {
-        id(new $listener())->register();
+        $listeners[] = newv($listener_class, array());
       } catch (Exception $ex) {
-        // If the listener does not exist, or throws when registering, just
-        // log it and continue. In particular, this is important to let you
-        // run `bin/config` in order to remove an invalid listener.
         phlog($ex);
       }
     }
 
-    // Register the DarkConosole event logger.
-    id(new DarkConsoleEventPluginAPI())->register();
-    id(new ManiphestEdgeEventListener())->register();
+    // Add builtin listeners.
+    $listeners[] = new DarkConsoleEventPluginAPI();
+    $listeners[] = new ManiphestEdgeEventListener();
 
+    // Add application listeners.
     $applications = PhabricatorApplication::getAllInstalledApplications();
     foreach ($applications as $application) {
-      $listeners = $application->getEventListeners();
-      foreach ($listeners as $listener) {
+      $app_listeners = $application->getEventListeners();
+      foreach ($app_listeners as $listener) {
         $listener->setApplication($application);
-        $listener->register();
+        $listeners[] = $listener;
       }
     }
 
+    // Now, register all of the listeners.
+    foreach ($listeners as $listener) {
+      try {
+        $listener->register();
+      } catch (Exception $ex) {
+        phlog($ex);
+      }
+    }
   }
 
 }