MDL-65858 backup: Improve behaviour of ajax calls

When a backup (or restore) is in progress and the user
has another tab open that is taking time to generate and
holding the session lock, a condition arises where
multiple ajax calls are "queued" because the long page
is holding the session lock. When this lock is released,
the queued ajax requests complete.  However, as the ajax
requests also lock this can block other requests until they
complete. This patch decreases the frequency of ajax calls
and stops them from holding the session lock.

Author: Matt Porritt

backup/externallib.php

@@ -71,6 +71,9 @@ public static function get_async_backup_progress($backupids, $contextid) {
         require_once($CFG->dirroot . '/backup/util/includes/backup_includes.php');
         require_once($CFG->dirroot . '/backup/util/includes/restore_includes.php');
 
+        // Release session lock.
+        \core\session\manager::write_close();
+
         // Parameter validation.
         self::validate_parameters(
                 self::get_async_backup_progress_parameters(),
@@ -142,6 +145,9 @@ public static function get_async_backup_links_backup_parameters() {
      * @since Moodle 3.7
      */
     public static function get_async_backup_links_backup($filename, $contextid) {
+        // Release session lock.
+        \core\session\manager::write_close();
+
         // Parameter validation.
         self::validate_parameters(
                 self::get_async_backup_links_backup_parameters(),
@@ -206,6 +212,9 @@ public static function get_async_backup_links_restore_parameters() {
      * @since Moodle 3.7
      */
     public static function get_async_backup_links_restore($backupid, $contextid) {
+        // Release session lock.
+        \core\session\manager::write_close();
+
         // Parameter validation.
         self::validate_parameters(
                 self::get_async_backup_links_restore_parameters(),

backup/util/ui/amd/build/async_backup.min.js

@@ -39,7 +39,7 @@ define(['jquery', 'core/ajax', 'core/str', 'core/notification', 'core/templates'
      * Module level variables.
      */
     var Asyncbackup = {};
-    var checkdelay = 5000; //  How often we check for progress updates.
+    var checkdelay = 15000; //  How often we check for progress updates.
     var backupid; //  The backup id to get the progress for.
     var contextid; //  The course this backup progress is for.
     var restoreurl; //  The URL to view course restores.