//.........这里部分代码省略.........
            }
        }
        flush();
        if (!empty($CFG->notifyloginfailures)) {
            notify_login_failures();
            mtrace('Notified login failured');
        }
        flush();
        //
        // generate new password emails for users
        //
        mtrace('checking for create_password');
        if ($DB->count_records('user_preferences', array('name' => 'create_password', 'value' => '1'))) {
            mtrace('creating passwords for new users');
            $newusers = $DB->get_records_sql("SELECT u.id as id, u.email, u.firstname,\n                                                     u.lastname, u.username,\n                                                     p.id as prefid\n                                                FROM {user} u\n                                                JOIN {user_preferences} p ON u.id=p.userid\n                                               WHERE p.name='create_password' AND p.value='1' AND u.email !='' ");
            foreach ($newusers as $newuserid => $newuser) {
                // email user
                if (setnew_password_and_mail($newuser)) {
                    // remove user pref
                    $DB->delete_records('user_preferences', array('id' => $newuser->prefid));
                } else {
                    trigger_error("Could not create and mail new user password!");
                }
            }
        }
        if (!empty($CFG->usetags)) {
            require_once $CFG->dirroot . '/tag/lib.php';
            tag_cron();
            mtrace('Executed tag cron');
        }
        // Accesslib stuff
        cleanup_contexts();
        mtrace('Cleaned up contexts');
        gc_cache_flags();
        mtrace('Cleaned cache flags');
        // If you suspect that the context paths are somehow corrupt
        // replace the line below with: build_context_path(true);
        build_context_path();
        mtrace('Built context paths');
        if (!empty($CFG->messagingdeletereadnotificationsdelay)) {
            $notificationdeletetime = time() - $CFG->messagingdeletereadnotificationsdelay;
            $DB->delete_records_select('message_read', 'notification=1 AND timeread<:notificationdeletetime', array('notificationdeletetime' => $notificationdeletetime));
            mtrace('Cleaned up read notifications');
        }
        mtrace("Finished clean-up tasks...");
    }
    // End of occasional clean-up tasks
    // Run automated backups if required.
    require_once $CFG->dirroot . '/backup/util/includes/backup_includes.php';
    require_once $CFG->dirroot . '/backup/util/helper/backup_cron_helper.class.php';
    backup_cron_automated_helper::run_automated_backup();
    /// Run the auth cron, if any
    /// before enrolments because it might add users that will be needed in enrol plugins
    $auths = get_enabled_auth_plugins();
    mtrace("Running auth crons if required...");
    foreach ($auths as $auth) {
        $authplugin = get_auth_plugin($auth);
        if (method_exists($authplugin, 'cron')) {
            mtrace("Running cron for auth/{$auth}...");
            $authplugin->cron();
            if (!empty($authplugin->log)) {
                mtrace($authplugin->log);
            }
        }
        unset($authplugin);
    }

//.........这里部分代码省略.........
        }
        // Delete old backup_controllers and logs.
        $loglifetime = get_config('backup', 'loglifetime');
        if (!empty($loglifetime)) {
            // Value in days.
            $loglifetime = $timenow - $loglifetime * 3600 * 24;
            // Delete child records from backup_logs.
            $DB->execute("DELETE FROM {backup_logs}\n                           WHERE EXISTS (\n                               SELECT 'x'\n                                 FROM {backup_controllers} bc\n                                WHERE bc.backupid = {backup_logs}.backupid\n                                  AND bc.timecreated < ?)", array($loglifetime));
            // Delete records from backup_controllers.
            $DB->execute("DELETE FROM {backup_controllers}\n                          WHERE timecreated < ?", array($loglifetime));
            mtrace(" Deleted old backup records");
        }
        // Delete old cached texts
        if (!empty($CFG->cachetext)) {
            // Defined in config.php
            $cachelifetime = time() - $CFG->cachetext - 60;
            // Add an extra minute to allow for really heavy sites
            $DB->delete_records_select('cache_text', "timemodified < ?", array($cachelifetime));
            mtrace(" Deleted old cache_text records");
        }
        if (!empty($CFG->usetags)) {
            require_once $CFG->dirroot . '/tag/lib.php';
            tag_cron();
            mtrace(' Executed tag cron');
        }
        // Context maintenance stuff
        context_helper::cleanup_instances();
        mtrace(' Cleaned up context instances');
        context_helper::build_all_paths(false);
        // If you suspect that the context paths are somehow corrupt
        // replace the line below with: context_helper::build_all_paths(true);
        mtrace(' Built context paths');
        // Remove expired cache flags
        gc_cache_flags();
        mtrace(' Cleaned cache flags');
        // Cleanup messaging
        if (!empty($CFG->messagingdeletereadnotificationsdelay)) {
            $notificationdeletetime = time() - $CFG->messagingdeletereadnotificationsdelay;
            $DB->delete_records_select('message_read', 'notification=1 AND timeread<:notificationdeletetime', array('notificationdeletetime' => $notificationdeletetime));
            mtrace(' Cleaned up read notifications');
        }
        mtrace(' Deleting temporary files...');
        cron_delete_from_temp();
        // Cleanup user password reset records
        // Delete any reset request records which are expired by more than a day.
        // (We keep recently expired requests around so we can give a different error msg to users who
        // are trying to user a recently expired reset attempt).
        $pwresettime = isset($CFG->pwresettime) ? $CFG->pwresettime : 1800;
        $earliestvalid = time() - $pwresettime - DAYSECS;
        $DB->delete_records_select('user_password_resets', "timerequested < ?", array($earliestvalid));
        mtrace(' Cleaned up old password reset records');
        mtrace("...finished clean-up tasks");
    }
    // End of occasional clean-up tasks
    // Send login failures notification - brute force protection in moodle is weak,
    // we should at least send notices early in each cron execution
    if (notify_login_failures()) {
        mtrace(' Notified login failures');
    }
    // Make sure all context instances are properly created - they may be required in auth, enrol, etc.
    context_helper::create_instances();
    mtrace(' Created missing context instances');
    // Session gc.
    mtrace("Running session gc tasks...");
    \core\session\manager::gc();
    mtrace("...finished stale session cleanup");

 /**
  * Do the job.
  * Throw exceptions on errors (the job will be retried).
  */
 public function execute()
 {
     // Remove expired cache flags.
     gc_cache_flags();
 }