Check that a plugin is loaded when accessing its pages

A plugin can be registered yet fail to load for example due to unmet
dependencies.

Fixes #17359

core/constant_inc.php

@@ -383,6 +383,7 @@
 define( 'ERROR_PLUGIN_UPGRADE_FAILED', 2503 );
 define( 'ERROR_PLUGIN_INSTALL_FAILED', 2504 );
 define( 'ERROR_PLUGIN_UPGRADE_NEEDED', 2505 );
+define( 'ERROR_PLUGIN_NOT_LOADED', 2506 );
 define( 'ERROR_PLUGIN_GENERIC', 2599 );
 
 # ERROR_COLUMNS_*

lang/strings_english.txt

@@ -1668,6 +1668,7 @@ $MANTIS_ERROR[ERROR_TAG_ALREADY_ATTACHED] = 'That tag already attached to that i
 $MANTIS_ERROR[ERROR_TOKEN_NOT_FOUND] = 'Token could not be found.';
 $MANTIS_ERROR[ERROR_EVENT_UNDECLARED] = 'Event "%1$s" has not yet been declared.';
 $MANTIS_ERROR[ERROR_PLUGIN_NOT_REGISTERED] = 'Plugin is not registered with MantisBT.';
+$MANTIS_ERROR[ERROR_PLUGIN_NOT_LOADED] = 'Plugin is not loaded, make sure its dependencies are met.';
 $MANTIS_ERROR[ERROR_PLUGIN_ALREADY_INSTALLED] = 'Plugin is already installed.';
 $MANTIS_ERROR[ERROR_PLUGIN_PAGE_NOT_FOUND] = 'Plugin page not found.';
 $MANTIS_ERROR[ERROR_PLUGIN_INSTALL_FAILED] = 'Plugin installation failed: %1$s.';

plugin.php

@@ -50,6 +50,11 @@
 	trigger_error( ERROR_PLUGIN_NOT_REGISTERED, ERROR );
 }
 
+# Plugin can be registered but fail to load e.g. due to unmet dependencies
+if( !plugin_is_loaded( $t_basename ) ) {
+	trigger_error( ERROR_PLUGIN_NOT_LOADED, ERROR );
+}
+
 $t_page = $t_plugin_path.$t_basename.DIRECTORY_SEPARATOR.
 		'pages'.DIRECTORY_SEPARATOR.$t_action.'.php';