Throw an exception when loading a plugin that is already loading.

If a plugin used an API in its constructor that also loading the same plugin, the caching mechanism of
`PluginManager#getCachedPlugin` were bypassed and the protection against instantiating a plugin class more than once was
disabled.

This fix throws a better exception to explain the situation and provides a better way to use for example the
`Navigation` API inside of a constructor.

Fixes #4317

lib/exceptions/PluginAlreadyLoadingException.php

+<?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($pluginClassName)
+    {
+        return parent::__construct(
+            sprintf(
+                _("Plugin '%s' wird bereits geladen."),
+                $pluginClassName
+            )
+        );
+    }
+}

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());
     }
 
     /**

lib/plugins/engine/PluginManager.php

@@ -586,9 +586,14 @@ class PluginManager
         $plugin_class = '';
         $plugin = null;
 
-        if (isset($this->plugin_cache[$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);
@@ -596,6 +601,7 @@ class PluginManager
 
         if ($plugin_class) {
             $plugin = app()->get($class);
+            NotificationCenter::postNotification('PluginDidLoad', $this, compact('class', 'plugin'));
         }
 
         return $this->plugin_cache[$class] = $plugin;