Add documentation for maintenance page and UPDATE_LOCK.php

Author: marc1706

development/index.rst

@@ -21,6 +21,7 @@ Contents:
    migrations/index
    request/request
    testing/index
+   update_system/maintenance_page
    development/phpstorm
 
 Indices and tables

development/update_system/maintenance_page.rst

@@ -0,0 +1,90 @@
+================
+Maintenance Page
+================
+
+Overview
+========
+
+The maintenance page in phpBB 4.0 is designed to be a "zero-dependency" interceptor page. It resides in ``includes/startup.php`` and triggers before the autoloader, database, session, or language systems are initialized.
+
+This architecture ensures that even if the database is offline or core files are being overwritten during an update, the user is served a professional, localized maintenance page rather than a PHP error.
+
+Functional Flow
+---------------
+
+1. **Detection**: The system checks for the existence of ``store/UPDATE_LOCK.php``.
+2. **Execution**: If the file exists, ``send_maintenance_screen()`` is called.
+3. **Data Loading**: The function ``include``'s the lock file to retrieve metadata.
+4. **Rendering**: A static HTML/CSS page is output with the necessary data added as a JSON payload for client-side logic.
+5. **Polling**: The page includes a ``<meta http-equiv="refresh" content="30">`` tag to refresh the page every 30 seconds and automatically reconnect the user once the lock file is removed.
+
+The Update Lock File
+====================
+
+The ``store/UPDATE_LOCK.php`` is an auto-generated PHP array-export. It acts as the "source of truth" for the maintenance screen.
+
+Data Schema
+-----------
+
+The file must return an associative array with the following structure:
+
+initiated
+    *(int)* Unix timestamp of the start of maintenance. Used by the frontend to calculate "Started on" strings.
+
+config
+    *(array)* Global metadata.
+
+    * ``default_lang``: Fallback ISO language code, should be the default board language and fitting language strings **MUST** be provided for this language in the ``lang`` array.
+    * ``sitename``: The name of the board to be used in titles and messaging, same as the site name stored in the board config.
+
+lang
+    *(array)* A collection of language-specific blocks. Each block is keyed by the language ISO code and contains the following keys:
+
+    * ``BOARD_MAINTENANCE``: The main body text for the board maintenance message.
+    * ``BOARD_MAINTENANCE_START``: The localized "Started on" label (supports ``%1$s`` for the date).
+    * ``BOARD_MAINTENANCE_TITLE``: The HTML title tag template (supports ``%1$s`` for the sitename).
+
+links
+    *(array)* A list of external community links. Each entry is an array with ``title`` and ``url``.
+
+Example Implementation
+----------------------
+
+.. code-block:: php
+
+    <?php
+    /**
+     * phpBB Update Lock File
+     * This file is auto-generated. Do not edit manually.
+     */
+
+    if (!defined('IN_PHPBB'))
+    {
+        exit;
+    }
+
+    return [
+        'initiated' => 1738200000,
+        'config' => [
+           'default_lang' => 'en',
+           'sitename' => 'yourdomain.com',
+        ],
+        'lang' => [
+           'en' => [
+              'BOARD_MAINTENANCE'        => 'The board is currently down for maintenance.',
+              'BOARD_MAINTENANCE_START'  => 'Started on %1$s',
+              'BOARD_MAINTENANCE_TITLE'  => 'Board maintenance - %1$s',
+           ]
+        ],
+        'links' => [
+           ['title' => 'Discord', 'url' => 'https://discord.gg/example'],
+           ['title' => 'Status Page', 'url' => 'https://status.example.com'],
+        ],
+    ];
+
+Best Practices for Developers
+=============================
+
+1. **Atomic Writes**: When creating the lock file, always write to a temporary file (e.g., ``UPDATE_LOCK.php.tmp``) and use ``rename()`` to move it to its final destination. This prevents the interceptor from including a half-written file.
+2. **Minimalism**: Do not include heavy logic in the lock file. It is intended only for data storage.
+3. **Localization**: Always provide at least the key corresponding to your default language in the ``lang`` array to satisfy the ``default_lang`` fallback.