<?php
// This file is part of Moodle - http://moodle.org/
//
// Moodle is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**
 * Utility helper for automated backups run through cron.
 *
 * @package    core
 * @subpackage backup
 * @copyright  2010 Sam Hemelryk
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

defined('MOODLE_INTERNAL') || die();

/**
 * This class is an abstract class with methods that can be called to aid the
 * running of automated backups over cron.
 */
abstract class backup_cron_automated_helper {

    /** Automated backups are active and ready to run */
    const STATE_OK = 0;
    /** Automated backups are disabled and will not be run */
    const STATE_DISABLED = 1;
    /** Automated backups are all ready running! */
    const STATE_RUNNING = 2;

    /** Course automated backup completed successfully */
    const BACKUP_STATUS_OK = 1;
    /** Course automated backup errored */
    const BACKUP_STATUS_ERROR = 0;
    /** Course automated backup never finished */
    const BACKUP_STATUS_UNFINISHED = 2;
    /** Course automated backup was skipped */
    const BACKUP_STATUS_SKIPPED = 3;
    /** Course automated backup had warnings */
    const BACKUP_STATUS_WARNING = 4;
    /** Course automated backup has yet to be run */
    const BACKUP_STATUS_NOTYETRUN = 5;
    /** Course automated backup has been added to adhoc task queue */
    const BACKUP_STATUS_QUEUED = 6;

    /** Run if required by the schedule set in config. Default. **/
    const RUN_ON_SCHEDULE = 0;
    /** Run immediately. **/
    const RUN_IMMEDIATELY = 1;

    const AUTO_BACKUP_DISABLED = 0;
    const AUTO_BACKUP_ENABLED = 1;
    const AUTO_BACKUP_MANUAL = 2;

    /** Automated backup storage in course backup filearea */
    const STORAGE_COURSE = 0;
    /** Automated backup storage in specified directory */
    const STORAGE_DIRECTORY = 1;
    /** Automated backup storage in course backup filearea and specified directory */
    const STORAGE_COURSE_AND_DIRECTORY = 2;

    /**
     * Get the courses to backup.
     *
     * When there are multiple courses to backup enforce some order to the record set.
     * The following is the preference order.
     * First backup courses that do not have an entry in backup_courses first,
     * as they are likely new and never been backed up. Do the oldest modified courses first.
     * Then backup courses that have previously been backed up starting with the oldest next start time.
     * Finally, all else being equal, defer to the sortorder of the courses.
     *
     * @param null|int $now timestamp to use in course selection.
     * @return moodle_recordset The recordset of matching courses.
     */
    protected static function get_courses($now = null) {
        global $DB;
        if ($now == null) {
            $now = time();
        }

        $sql = 'SELECT c.*,
                       COALESCE(bc.nextstarttime, 1) nextstarttime
                  FROM {course} c
             LEFT JOIN {backup_courses} bc ON bc.courseid = c.id
                 WHERE bc.nextstarttime IS NULL OR bc.nextstarttime < ?
              ORDER BY nextstarttime ASC,
                       c.timemodified DESC,
                       c.sortorder';

        $params = array(
            $now,  // Only get courses where the backup start time is in the past.
        );
        $rs = $DB->get_recordset_sql($sql, $params);

        return $rs;
    }

    /**
     * Runs the automated backups if required
     *
     * @param bool $rundirective
     */
    public static function run_automated_backup($rundirective = self::RUN_ON_SCHEDULE) {
        $now = time();

        $lock = self::get_automated_backup_lock($rundirective);
        if (!$lock) {
            return;
        }

        try {
            mtrace("Checking courses");
            mtrace("Skipping deleted courses", '...');
            mtrace(sprintf("%d courses", self::remove_deleted_courses_from_schedule()));
            mtrace('Running required automated backups...');
            cron_trace_time_and_memory();

            mtrace("Getting admin info");
            $admin = get_admin();
            if (!$admin) {
                mtrace("Error: No admin account was found");
                return;
            }

            $rs = self::get_courses($now); // Get courses to backup.
            $emailpending = self::check_and_push_automated_backups($rs, $admin);
            $rs->close();

            // Send email to admin if necessary.
            if ($emailpending) {
                self::send_backup_status_to_admin($admin);
            }
        } finally {
            // Everything is finished release lock.
            $lock->release();
            mtrace('Automated backups complete.');
        }
    }

    /**
     * Gets the results from the last automated backup that was run based upon
     * the statuses of the courses that were looked at.
     *
     * @return array
     */
    public static function get_backup_status_array() {
        global $DB;

        $result = array(
            self::BACKUP_STATUS_ERROR => 0,
            self::BACKUP_STATUS_OK => 0,
            self::BACKUP_STATUS_UNFINISHED => 0,
            self::BACKUP_STATUS_SKIPPED => 0,
            self::BACKUP_STATUS_WARNING => 0,
            self::BACKUP_STATUS_NOTYETRUN => 0,
            self::BACKUP_STATUS_QUEUED => 0,
        );

        $statuses = $DB->get_records_sql('SELECT DISTINCT bc.laststatus,
                                            COUNT(bc.courseid) AS statuscount
                                            FROM {backup_courses} bc
                                            GROUP BY bc.laststatus');

        foreach ($statuses as $status) {
            if (empty($status->statuscount)) {
                $status->statuscount = 0;
            }
            $result[(int)$status->laststatus] += $status->statuscount;
        }

        return $result;
    }

    /**
     * Collect details for all statuses of the courses
     * and send report to admin.
     *
     * @param stdClass $admin
     * @return array
     */
    private static function send_backup_status_to_admin($admin) {
        global $DB, $CFG;

        mtrace("Sending email to admin");
        $message = "";

        $count = self::get_backup_status_array();
        $haserrors = ($count[self::BACKUP_STATUS_ERROR] != 0 || $count[self::BACKUP_STATUS_UNFINISHED] != 0);

        // Build the message text.
        // Summary.
        $message .= get_string('summary') . "\n";
        $message .= "==================================================\n";
        $message .= '  ' . get_string('courses') . ': ' . array_sum($count) . "\n";
        $message .= '  ' . get_string('ok') . ': ' . $count[self::BACKUP_STATUS_OK] . "\n";
        $message .= '  ' . get_string('skipped') . ': ' . $count[self::BACKUP_STATUS_SKIPPED] . "\n";
        $message .= '  ' . get_string('error') . ': ' . $count[self::BACKUP_STATUS_ERROR] . "\n";
        $message .= '  ' . get_string('unfinished') . ': ' . $count[self::BACKUP_STATUS_UNFINISHED] . "\n";
        $message .= '  ' . get_string('backupadhocpending') . ': ' . $count[self::BACKUP_STATUS_QUEUED] . "\n";
        $message .= '  ' . get_string('warning') . ': ' . $count[self::BACKUP_STATUS_WARNING] . "\n";
        $message .= '  ' . get_string('backupnotyetrun') . ': ' . $count[self::BACKUP_STATUS_NOTYETRUN]."\n\n";

        // Reference.
        if ($haserrors) {
            $message .= "  ".get_string('backupfailed')."\n\n";
            $desturl = "$CFG->wwwroot/report/backups/index.php";
            $message .= "  ".get_string('backuptakealook', '', $desturl)."\n\n";
            // Set message priority.
            $admin->priority = 1;
            // Reset error and unfinished statuses to ok if longer than 24 hours.
            $sql = "laststatus IN (:statuserror,:statusunfinished) AND laststarttime < :yesterday";
            $params = [
                'statuserror' => self::BACKUP_STATUS_ERROR,
                'statusunfinished' => self::BACKUP_STATUS_UNFINISHED,
                'yesterday' => time() - 86400,
            ];
            $DB->set_field_select('backup_courses', 'laststatus', self::BACKUP_STATUS_OK, $sql, $params);
        } else {
            $message .= "  ".get_string('backupfinished')."\n";
        }

        // Build the message subject.
        $site = get_site();
        $prefix = format_string($site->shortname, true, array('context' => context_course::instance(SITEID))).": ";
        if ($haserrors) {
            $prefix .= "[".strtoupper(get_string('error'))."] ";
        }
        $subject = $prefix.get_string('automatedbackupstatus', 'backup');

        // Send the message.
        $eventdata = new \core\message\message();
        $eventdata->courseid          = SITEID;
        $eventdata->modulename        = 'moodle';
        $eventdata->userfrom          = $admin;
        $eventdata->userto            = $admin;
        $eventdata->subject           = $subject;
        $eventdata->fullmessage       = $message;
        $eventdata->fullmessageformat = FORMAT_PLAIN;
        $eventdata->fullmessagehtml   = '';
        $eventdata->smallmessage      = '';

        $eventdata->component         = 'moodle';
        $eventdata->name         = 'backup';

        return message_send($eventdata);
    }

    /**
     * Loop through courses and push to course ad-hoc task if required
     *
     * @param \record_set $courses
     * @param stdClass $admin
     * @return boolean
     */
    private static function check_and_push_automated_backups($courses, $admin) {
        global $DB;

        $now = time();
        $emailpending = false;

        $nextstarttime = self::calculate_next_automated_backup(null, $now);
        $showtime = "undefined";
        if ($nextstarttime > 0) {
            $showtime = date('r', $nextstarttime);
        }

        foreach ($courses as $course) {
            $backupcourse = $DB->get_record('backup_courses', array('courseid' => $course->id));
            if (!$backupcourse) {
                $backupcourse = new stdClass;
                $backupcourse->courseid = $course->id;
                $backupcourse->laststatus = self::BACKUP_STATUS_NOTYETRUN;
                $DB->insert_record('backup_courses', $backupcourse);
                $backupcourse = $DB->get_record('backup_courses', array('courseid' => $course->id));
            }

            // Check if we are going to be running the backup now.
            $shouldrunnow = ($backupcourse->nextstarttime > 0 && $backupcourse->nextstarttime < $now);

            // Check if the course is not scheduled to run right now, or it has been put in queue.
            if (!$shouldrunnow || $backupcourse->laststatus == self::BACKUP_STATUS_QUEUED) {
                $backupcourse->nextstarttime = $nextstarttime;
                $DB->update_record('backup_courses', $backupcourse);
                mtrace('Skipping course id ' . $course->id . ': Not scheduled for backup until ' . $showtime);
            } else {
                $skipped = self::should_skip_course_backup($backupcourse, $course, $nextstarttime);
                if (!$skipped) { // If it should not be skipped.

                    // Only make the backup if laststatus isn't 2-UNFINISHED (uncontrolled error or being backed up).
                    if ($backupcourse->laststatus != self::BACKUP_STATUS_UNFINISHED) {
                        // Add every non-skipped courses to backup adhoc task queue.
                        mtrace('Putting backup of course id ' . $course->id . ' in adhoc task queue');

                        // We have to send an email because we have included at least one backup.
                        $emailpending = true;
                        // Create adhoc task for backup.
                        self::push_course_backup_adhoc_task($backupcourse, $admin);
                    }
                }
            }
        }

        return $emailpending;
    }

    /**
     * Check if we can skip this course backup.
     *
     * @param stdClass $backupcourse
     * @param stdClass $course
     * @param int $nextstarttime
     * @return boolean
     */
    private static function should_skip_course_backup($backupcourse, $course, $nextstarttime) {
        global $DB;

        $config = get_config('backup');
        $now = time();
         // Assume that we are not skipping anything.
         $skipped = false;
         $skippedmessage = '';

        // The last backup is considered as successful when OK or SKIPPED.
        $lastbackupwassuccessful = ($backupcourse->laststatus == self::BACKUP_STATUS_SKIPPED ||
        $backupcourse->laststatus == self::BACKUP_STATUS_OK) && (
        $backupcourse->laststarttime > 0 && $backupcourse->lastendtime > 0);

        // If config backup_auto_skip_hidden is set to true, skip courses that are not visible.
        if ($config->backup_auto_skip_hidden) {
            $skipped = ($config->backup_auto_skip_hidden && !$course->visible);
            $skippedmessage = 'Not visible';
        }

        // If config backup_auto_skip_modif_days is set to true, skip courses
        // that have not been modified since the number of days defined.
        if (!$skipped && $lastbackupwassuccessful && $config->backup_auto_skip_modif_days) {
            $timenotmodifsincedays = $now - ($config->backup_auto_skip_modif_days * DAYSECS);
            // Check log if there were any modifications to the course content.
            $logexists = self::is_course_modified($course->id, $timenotmodifsincedays);
            $skipped = ($course->timemodified <= $timenotmodifsincedays && !$logexists);
            $skippedmessage = 'Not modified in the past '.$config->backup_auto_skip_modif_days.' days';
        }

        // If config backup_auto_skip_modif_prev is set to true, skip courses
        // that have not been modified since previous backup.
        if (!$skipped && $lastbackupwassuccessful && $config->backup_auto_skip_modif_prev) {
            // Check log if there were any modifications to the course content.
            $logexists = self::is_course_modified($course->id, $backupcourse->laststarttime);
            $skipped = ($course->timemodified <= $backupcourse->laststarttime && !$logexists);
            $skippedmessage = 'Not modified since previous backup';
        }

        if ($skipped) { // Must have been skipped for a reason.
            $backupcourse->laststatus = self::BACKUP_STATUS_SKIPPED;
            $backupcourse->nextstarttime = $nextstarttime;
            $DB->update_record('backup_courses', $backupcourse);
            mtrace('Skipping course id ' . $course->id . ': ' . $skippedmessage);
        }

        return $skipped;
    }

    /**
     * Create course backup adhoc task
     *
     * @param stdClass $backupcourse
     * @param stdClass $admin
     * @return void
     */
    private static function push_course_backup_adhoc_task($backupcourse, $admin) {
        global $DB;

        $asynctask = new \core\task\course_backup_task();
        $asynctask->set_blocking(false);
        $asynctask->set_custom_data(array(
            'courseid' => $backupcourse->courseid,
            'adminid' => $admin->id
        ));
        \core\task\manager::queue_adhoc_task($asynctask);

        $backupcourse->laststatus = self::BACKUP_STATUS_QUEUED;
        $DB->update_record('backup_courses', $backupcourse);
    }

    /**
     * Works out the next time the automated backup should be run.
     *
     * @param mixed $ignoredtimezone all settings are in server timezone!
     * @param int $now timestamp, should not be in the past, most likely time()
     * @return int timestamp of the next execution at server time
     */
    public static function calculate_next_automated_backup($ignoredtimezone, $now) {

        $config = get_config('backup');

        $backuptime = new DateTime('@' . $now);
        $backuptime->setTimezone(core_date::get_server_timezone_object());
        $backuptime->setTime($config->backup_auto_hour, $config->backup_auto_minute);

        while ($backuptime->getTimestamp() < $now) {
            $backuptime->add(new DateInterval('P1D'));
        }

        // Get number of days from backup date to execute backups.
        $automateddays = substr($config->backup_auto_weekdays, $backuptime->format('w')) . $config->backup_auto_weekdays;
        $daysfromnow = strpos($automateddays, "1");

        // Error, there are no days to schedule the backup for.
        if ($daysfromnow === false) {
            return 0;
        }

        if ($daysfromnow > 0) {
            $backuptime->add(new DateInterval('P' . $daysfromnow . 'D'));
        }

        return $backuptime->getTimestamp();
    }

    /**
     * Launches a automated backup routine for the given course
     *
     * @param stdClass $course
     * @param int $starttime
     * @param int $userid
     * @return bool
     */
    public static function launch_automated_backup($course, $starttime, $userid) {

        $outcome = self::BACKUP_STATUS_OK;
        $config = get_config('backup');
        $dir = $config->backup_auto_destination;
        $storage = (int)$config->backup_auto_storage;

        $bc = new backup_controller(backup::TYPE_1COURSE, $course->id, backup::FORMAT_MOODLE, backup::INTERACTIVE_NO,
                backup::MODE_AUTOMATED, $userid);

        try {

            // Set the default filename.
            $format = $bc->get_format();
            $type = $bc->get_type();
            $id = $bc->get_id();
            $users = $bc->get_plan()->get_setting('users')->get_value();
            $anonymised = $bc->get_plan()->get_setting('anonymize')->get_value();
            $incfiles = (bool)$config->backup_auto_files;
            $bc->get_plan()->get_setting('filename')->set_value(backup_plan_dbops::get_default_backup_filename($format, $type,
                    $id, $users, $anonymised, false, $incfiles));

            $bc->set_status(backup::STATUS_AWAITING);

            $bc->execute_plan();
            $results = $bc->get_results();
            $outcome = self::outcome_from_results($results);
            $file = $results['backup_destination']; // May be empty if file already moved to target location.

            // If we need to copy the backup file to an external dir and it is not writable, change status to error.
            // This is a feature to prevent moodledata to be filled up and break a site when the admin misconfigured
            // the automated backups storage type and destination directory.
            if ($storage !== 0 && (empty($dir) || !file_exists($dir) || !is_dir($dir) || !is_writable($dir))) {
                $bc->log('Specified backup directory is not writable - ', backup::LOG_ERROR, $dir);
                $dir = null;
                $outcome = self::BACKUP_STATUS_ERROR;
            }

            // Copy file only if there was no error.
            if ($file && !empty($dir) && $storage !== 0 && $outcome != self::BACKUP_STATUS_ERROR) {
                $filename = backup_plan_dbops::get_default_backup_filename($format, $type, $course->id, $users, $anonymised,
                        !$config->backup_shortname);
                if (!$file->copy_content_to($dir.'/'.$filename)) {
                    $bc->log('Attempt to copy backup file to the specified directory failed - ',
                            backup::LOG_ERROR, $dir);
                    $outcome = self::BACKUP_STATUS_ERROR;
                }
                if ($outcome != self::BACKUP_STATUS_ERROR && $storage === 1) {
                    if (!$file->delete()) {
                        $outcome = self::BACKUP_STATUS_WARNING;
                        $bc->log('Attempt to delete the backup file from course automated backup area failed - ',
                                backup::LOG_WARNING, $file->get_filename());
                    }
                }
            }

        } catch (moodle_exception $e) {
            $bc->log('backup_auto_failed_on_course', backup::LOG_ERROR, $course->shortname); // Log error header.
            $bc->log('Exception: ' . $e->errorcode, backup::LOG_ERROR, $e->a, 1); // Log original exception problem.
            $bc->log('Debug: ' . $e->debuginfo, backup::LOG_DEBUG, null, 1); // Log original debug information.
            $outcome = self::BACKUP_STATUS_ERROR;
        }

        // Delete the backup file immediately if something went wrong.
        if ($outcome === self::BACKUP_STATUS_ERROR) {

            // Delete the file from file area if exists.
            if (!empty($file)) {
                $file->delete();
            }

            // Delete file from external storage if exists.
            if ($storage !== 0 && !empty($filename) && file_exists($dir.'/'.$filename)) {
                @unlink($dir.'/'.$filename);
            }
        }

        $bc->destroy();
        unset($bc);

        return $outcome;
    }

    /**
     * Returns the backup outcome by analysing its results.
     *
     * @param array $results returned by a backup
     * @return int {@link self::BACKUP_STATUS_OK} and other constants
     */
    public static function outcome_from_results($results) {
        $outcome = self::BACKUP_STATUS_OK;
        foreach ($results as $code => $value) {
            // Each possible error and warning code has to be specified in this switch
            // which basically analyses the results to return the correct backup status.
            switch ($code) {
                case 'missing_files_in_pool':
                    $outcome = self::BACKUP_STATUS_WARNING;
                    break;
            }
            // If we found the highest error level, we exit the loop.
            if ($outcome == self::BACKUP_STATUS_ERROR) {
                break;
            }
        }
        return $outcome;
    }

    /**
     * Removes deleted courses fromn the backup_courses table so that we don't
     * waste time backing them up.
     *
     * @return int
     */
    public static function remove_deleted_courses_from_schedule() {
        global $DB;
        $skipped = 0;
        $sql = "SELECT bc.courseid FROM {backup_courses} bc WHERE bc.courseid NOT IN (SELECT c.id FROM {course} c)";
        $rs = $DB->get_recordset_sql($sql);
        foreach ($rs as $deletedcourse) {
            // Doesn't exist, so delete from backup tables.
            $DB->delete_records('backup_courses', array('courseid' => $deletedcourse->courseid));
            $skipped++;
        }
        $rs->close();
        return $skipped;
    }

    /**
     * Try to get lock for automated backup.
     * @param int $rundirective
     *
     * @return \core\lock\lock|boolean - An instance of \core\lock\lock if the lock was obtained, or false.
     */
    public static function get_automated_backup_lock($rundirective = self::RUN_ON_SCHEDULE) {
        $config = get_config('backup');
        $active = (int)$config->backup_auto_active;
        $weekdays = (string)$config->backup_auto_weekdays;

        mtrace("Checking automated backup status", '...');
        $locktype = 'automated_backup';
        $resource = 'queue_backup_jobs_running';
        $lockfactory = \core\lock\lock_config::get_lock_factory($locktype);

        // In case of automated backup also check that it is scheduled for at least one weekday.
        if ($active === self::AUTO_BACKUP_DISABLED ||
                ($rundirective == self::RUN_ON_SCHEDULE && $active === self::AUTO_BACKUP_MANUAL) ||
                ($rundirective == self::RUN_ON_SCHEDULE && strpos($weekdays, '1') === false)) {
            mtrace('INACTIVE');
            return false;
        }

        if (!$lock = $lockfactory->get_lock($resource, 10)) {
            return false;
        }

        mtrace('OK');
        return $lock;
    }

    /**
     * Removes excess backups from a specified course.
     *
     * @param stdClass $course Course object
     * @param int $now Starting time of the process
     * @return bool Whether or not backups is being removed
     */
    public static function remove_excess_backups($course, $now = null) {
        $config = get_config('backup');
        $maxkept = (int)$config->backup_auto_max_kept;
        $storage = $config->backup_auto_storage;
        $deletedays = (int)$config->backup_auto_delete_days;

        if ($maxkept == 0 && $deletedays == 0) {
            // Means keep all backup files and never delete backup after x days.
            return true;
        }

        if (!isset($now)) {
            $now = time();
        }

        // Clean up excess backups in the course backup filearea.
        $deletedcoursebackups = false;
        if ($storage == self::STORAGE_COURSE || $storage == self::STORAGE_COURSE_AND_DIRECTORY) {
            $deletedcoursebackups = self::remove_excess_backups_from_course($course, $now);
        }

        // Clean up excess backups in the specified external directory.
        $deleteddirectorybackups = false;
        if ($storage == self::STORAGE_DIRECTORY || $storage == self::STORAGE_COURSE_AND_DIRECTORY) {
            $deleteddirectorybackups = self::remove_excess_backups_from_directory($course, $now);
        }

        if ($deletedcoursebackups || $deleteddirectorybackups) {
            return true;
        } else {
            return false;
        }
    }

    /**
     * Removes excess backups in the course backup filearea from a specified course.
     *
     * @param stdClass $course Course object
     * @param int $now Starting time of the process
     * @return bool Whether or not backups are being removed
     */
    protected static function remove_excess_backups_from_course($course, $now) {
        $fs = get_file_storage();
        $context = context_course::instance($course->id);
        $component = 'backup';
        $filearea = 'automated';
        $itemid = 0;
        $backupfiles = array();
        $backupfilesarea = $fs->get_area_files($context->id, $component, $filearea, $itemid, 'timemodified DESC', false);
        // Store all the matching files into timemodified => stored_file array.
        foreach ($backupfilesarea as $backupfile) {
            $backupfiles[$backupfile->get_timemodified()] = $backupfile;
        }

        $backupstodelete = self::get_backups_to_delete($backupfiles, $now);
        if ($backupstodelete) {
            foreach ($backupstodelete as $backuptodelete) {
                $backuptodelete->delete();
            }
            mtrace('Deleted ' . count($backupstodelete) . ' old backup file(s) from the automated filearea');
            return true;
        } else {
            return false;
        }
    }

    /**
     * Removes excess backups in the specified external directory from a specified course.
     *
     * @param stdClass $course Course object
     * @param int $now Starting time of the process
     * @return bool Whether or not backups are being removed
     */
    protected static function remove_excess_backups_from_directory($course, $now) {
        $config = get_config('backup');
        $dir = $config->backup_auto_destination;

        $isnotvaliddir = !file_exists($dir) || !is_dir($dir) || !is_writable($dir);
        if ($isnotvaliddir) {
            mtrace('Error: ' . $dir . ' does not appear to be a valid directory');
            return false;
        }

        // Calculate backup filename regex, ignoring the date/time/info parts that can be
        // variable, depending of languages, formats and automated backup settings.
        $filename = backup::FORMAT_MOODLE . '-' . backup::TYPE_1COURSE . '-' . $course->id . '-';
        $regex = '#' . preg_quote($filename, '#') . '.*\.mbz$#';

        // Store all the matching files into filename => timemodified array.
        $backupfiles = array();
        foreach (scandir($dir) as $backupfile) {
            // Skip files not matching the naming convention.
            if (!preg_match($regex, $backupfile)) {
                continue;
            }

            // Read the information contained in the backup itself.
            try {
                $bcinfo = backup_general_helper::get_backup_information_from_mbz($dir . '/' . $backupfile);
            } catch (backup_helper_exception $e) {
                mtrace('Error: ' . $backupfile . ' does not appear to be a valid backup (' . $e->errorcode . ')');
                continue;
            }

            // Make sure this backup concerns the course and site we are looking for.
            if ($bcinfo->format === backup::FORMAT_MOODLE &&
                    $bcinfo->type === backup::TYPE_1COURSE &&
                    $bcinfo->original_course_id == $course->id &&
                    backup_general_helper::backup_is_samesite($bcinfo)) {
                $backupfiles[$bcinfo->backup_date] = $backupfile;
            }
        }

        $backupstodelete = self::get_backups_to_delete($backupfiles, $now);
        if ($backupstodelete) {
            foreach ($backupstodelete as $backuptodelete) {
                unlink($dir . '/' . $backuptodelete);
            }
            mtrace('Deleted ' . count($backupstodelete) . ' old backup file(s) from external directory');
            return true;
        } else {
            return false;
        }
    }

    /**
     * Get the list of backup files to delete depending on the automated backup settings.
     *
     * @param array $backupfiles Existing backup files
     * @param int $now Starting time of the process
     * @return array Backup files to delete
     */
    protected static function get_backups_to_delete($backupfiles, $now) {
        $config = get_config('backup');
        $maxkept = (int)$config->backup_auto_max_kept;
        $deletedays = (int)$config->backup_auto_delete_days;
        $minkept = (int)$config->backup_auto_min_kept;

        // Sort by keys descending (newer to older filemodified).
        krsort($backupfiles);
        $tokeep = $maxkept;
        if ($deletedays > 0) {
            $deletedayssecs = $deletedays * DAYSECS;
            $tokeep = 0;
            $backupfileskeys = array_keys($backupfiles);
            foreach ($backupfileskeys as $timemodified) {
                $mustdeletebackup = $timemodified < ($now - $deletedayssecs);
                if ($mustdeletebackup || $tokeep >= $maxkept) {
                    break;
                }
                $tokeep++;
            }

            if ($tokeep < $minkept) {
                $tokeep = $minkept;
            }
        }

        if (count($backupfiles) <= $tokeep) {
            // There are less or equal matching files than the desired number to keep, there is nothing to clean up.
            return false;
        } else {
            $backupstodelete = array_splice($backupfiles, $tokeep);
            return $backupstodelete;
        }
    }

    /**
     * Check logs to find out if a course was modified since the given time.
     *
     * @param int $courseid course id to check
     * @param int $since timestamp, from which to check
     *
     * @return bool true if the course was modified, false otherwise. This also returns false if no readers are enabled. This is
     * intentional, since we cannot reliably determine if any modification was made or not.
     */
    protected static function is_course_modified($courseid, $since) {
        $logmang = get_log_manager();
        $readers = $logmang->get_readers('core\log\sql_reader');
        $params = array('courseid' => $courseid, 'since' => $since);

        foreach ($readers as $readerpluginname => $reader) {
            $where = "courseid = :courseid and timecreated > :since and crud <> 'r'";

            // Prevent logs of prevous backups causing a false positive.
            if ($readerpluginname != 'logstore_legacy') {
                $where .= " and target <> 'course_backup'";
            }

            if ($reader->get_events_select_count($where, $params)) {
                return true;
            }
        }
        return false;
    }
}
