/**
  * Do the job.
  * Throw exceptions on errors (the job will be retried).
  */
 public function execute()
 {
     global $CFG;
     if (!empty($CFG->usetags)) {
         require_once $CFG->dirroot . '/tag/lib.php';
         tag_cron();
     }
 }

/**
 * Execute cron tasks
 */
function cron_run()
{
    global $DB, $CFG, $OUTPUT;
    if (CLI_MAINTENANCE) {
        echo "CLI maintenance mode active, cron execution suspended.\n";
        exit(1);
    }
    if (moodle_needs_upgrading()) {
        echo "Moodle upgrade pending, cron execution suspended.\n";
        exit(1);
    }
    require_once $CFG->libdir . '/adminlib.php';
    require_once $CFG->libdir . '/gradelib.php';
    if (!empty($CFG->showcronsql)) {
        $DB->set_debug(true);
    }
    if (!empty($CFG->showcrondebugging)) {
        set_debugging(DEBUG_DEVELOPER, true);
    }
    set_time_limit(0);
    $starttime = microtime();
    // Increase memory limit
    raise_memory_limit(MEMORY_EXTRA);
    // Emulate normal session - we use admin accoutn by default
    cron_setup_user();
    // Start output log
    $timenow = time();
    mtrace("Server Time: " . date('r', $timenow) . "\n\n");
    // Run cleanup core cron jobs, but not every time since they aren't too important.
    // These don't have a timer to reduce load, so we'll use a random number
    // to randomly choose the percentage of times we should run these jobs.
    $random100 = rand(0, 100);
    if ($random100 < 20) {
        // Approximately 20% of the time.
        mtrace("Running clean-up tasks...");
        cron_trace_time_and_memory();
        // Delete users who haven't confirmed within required period
        if (!empty($CFG->deleteunconfirmed)) {
            $cuttime = $timenow - $CFG->deleteunconfirmed * 3600;
            $rs = $DB->get_recordset_sql("SELECT *\n                                             FROM {user}\n                                            WHERE confirmed = 0 AND firstaccess > 0\n                                                  AND firstaccess < ?", array($cuttime));
            foreach ($rs as $user) {
                delete_user($user);
                // we MUST delete user properly first
                $DB->delete_records('user', array('id' => $user->id));
                // this is a bloody hack, but it might work
                mtrace(" Deleted unconfirmed user for " . fullname($user, true) . " ({$user->id})");
            }
            $rs->close();
        }
        // Delete users who haven't completed profile within required period
        if (!empty($CFG->deleteincompleteusers)) {
            $cuttime = $timenow - $CFG->deleteincompleteusers * 3600;
            $rs = $DB->get_recordset_sql("SELECT *\n                                             FROM {user}\n                                            WHERE confirmed = 1 AND lastaccess > 0\n                                                  AND lastaccess < ? AND deleted = 0\n                                                  AND (lastname = '' OR firstname = '' OR email = '')", array($cuttime));
            foreach ($rs as $user) {
                if (isguestuser($user) or is_siteadmin($user)) {
                    continue;
                }
                delete_user($user);
                mtrace(" Deleted not fully setup user {$user->username} ({$user->id})");
            }
            $rs->close();
        }
        // Delete old logs to save space (this might need a timer to slow it down...)
        if (!empty($CFG->loglifetime)) {
            // value in days
            $loglifetime = $timenow - $CFG->loglifetime * 3600 * 24;
            $DB->delete_records_select("log", "time < ?", array($loglifetime));
            mtrace(" Deleted old log records");
        }
        // 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);
//.........这里部分代码省略.........

//.........这里部分代码省略.........
        if (!empty($CFG->cachetext)) {
            // Defined in config.php
            $cachelifetime = time() - $CFG->cachetext - 60;
            // Add an extra minute to allow for really heavy sites
            if ($DB->delete_records_select('cache_text', "timemodified < ?", array($cachelifetime))) {
                mtrace("Deleted old cache_text records");
            }
        }
        flush();
        if (!empty($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();