From 821cb43c51f67d60c3f8221327ab6d8ca5940b75 Mon Sep 17 00:00:00 2001
From: Marcus Eibrink-Lunzenauer <lunzenauer@elan-ev.de>
Date: Thu, 26 Jun 2025 13:04:56 +0000
Subject: Throw an exception when loading a plugin that is already loading.

Closes #4317

Merge request studip/studip!3126
---
 lib/exceptions/PluginAlreadyLoadingException.php | 19 +++++++++++++++++++
 lib/plugins/engine/PluginEngine.php              |  1 +
 lib/plugins/engine/PluginManager.php             | 10 ++++++++--
 3 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 lib/exceptions/PluginAlreadyLoadingException.php

diff --git a/lib/exceptions/PluginAlreadyLoadingException.php b/lib/exceptions/PluginAlreadyLoadingException.php
new file mode 100644
index 0000000..76fbce0
--- /dev/null
+++ b/lib/exceptions/PluginAlreadyLoadingException.php
@@ -0,0 +1,19 @@
+<?php
+
+/**
+ * This exception is thrown, if a plugin is already loading.
+ *
+ * There should always only be one instance of a plugin class.
+ */
+class PluginAlreadyLoadingException extends Exception
+{
+    public function __construct(string $pluginClassName)
+    {
+       parent::__construct(
+            sprintf(
+                _("Plugin '%s' wird bereits geladen."),
+                $pluginClassName
+            )
+        );
+    }
+}
diff --git a/lib/plugins/engine/PluginEngine.php b/lib/plugins/engine/PluginEngine.php
index 52f94af..a49f90b 100644
--- a/lib/plugins/engine/PluginEngine.php
+++ b/lib/plugins/engine/PluginEngine.php
@@ -60,6 +60,7 @@ class PluginEngine
         if (is_object($user) && $perm->have_perm('admin')) {
             self::getPlugins(AdministrationPlugin::class);
         }
+        NotificationCenter::postNotification('PluginsDidLoad', PluginManager::getInstance());
     }
 
     /**
diff --git a/lib/plugins/engine/PluginManager.php b/lib/plugins/engine/PluginManager.php
index 4bcb96b..3288382 100644
--- a/lib/plugins/engine/PluginManager.php
+++ b/lib/plugins/engine/PluginManager.php
@@ -594,16 +594,22 @@ class PluginManager
         $plugin_class = '';
         $plugin = null;
 
-        if (isset($this->plugin_cache[$class])) {
-            return $this->plugin_cache[$class];
+        if (array_key_exists($class, $this->plugin_cache)) {
+            if ($this->plugin_cache[$class] !== null) {
+                return $this->plugin_cache[$class];
+            }
+            throw new PluginAlreadyLoadingException($class);
         }
 
+        $this->plugin_cache[$class] = null;
+
         if ($plugin_info['core'] || !$this->isPluginsDisabled()) {
             $plugin_class = $this->loadPlugin($class, $path);
         }
 
         if ($plugin_class) {
             $plugin = app()->get($class);
+            NotificationCenter::postNotification('PluginDidLoad', $this, compact('class', 'plugin'));
         }
 
         return $this->plugin_cache[$class] = $plugin;
-- 
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