MDL-67052 locks: Added a native MySQL / MariaDB lock implementation

config-dist.php

@@ -521,9 +521,9 @@
 //      $CFG->supportuserid = -20;
 //
 // Moodle 2.7 introduces a locking api for critical tasks (e.g. cron).
-// The default locking system to use is DB locking for Postgres, and file locking for
-// MySQL, Oracle and SQLServer. If $CFG->preventfilelocking is set, then the default
-// will always be DB locking. It can be manually set to one of the lock
+// The default locking system to use is DB locking for Postgres, MySQL, MariaDB and
+// file locking for Oracle and SQLServer. If $CFG->preventfilelocking is set, then the
+// default will always be DB locking. It can be manually set to one of the lock
 // factory classes listed below, or one of your own custom classes implementing the
 // \core\lock\lock_factory interface.
 //
@@ -537,6 +537,8 @@
 //
 // "\\core\\lock\\db_record_lock_factory" - DB locking based on table rows.
 //
+// "\\core\\lock\\mysql_lock_factory" - DB locking based on MySQL / MariaDB locks.
+//
 // "\\core\\lock\\postgres_lock_factory" - DB locking based on postgres advisory locks.
 //
 // Settings used by the lock factories

lib/classes/lock/mysql_lock_factory.php

@@ -0,0 +1,190 @@
+<?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/>.
+
+/**
+ * MySQL / MariaDB locking factory.
+ *
+ * @package    core
+ * @category   lock
+ * @copyright  Brendan Heywood <[email protected]>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace core\lock;
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * MySQL / MariaDB locking factory.
+ *
+ * @package   core
+ * @category  lock
+ * @copyright Brendan Heywood <[email protected]>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class mysql_lock_factory implements lock_factory {
+
+    /** @var string $dbprefix - used as a namespace for these types of locks */
+    protected $dbprefix = '';
+
+    /** @var \moodle_database $db Hold a reference to the global $DB */
+    protected $db;
+
+    /** @var array $openlocks - List of held locks - used by auto-release */
+    protected $openlocks = [];
+
+    /**
+     * Return a unique prefix based on the database name and prefix.
+     * @param string $type - Used to prefix lock keys.
+     * @return string.
+     */
+    protected function get_unique_db_prefix($type) {
+        global $CFG;
+        $prefix = $CFG->dbname . ':';
+        if (isset($CFG->prefix)) {
+            $prefix .= $CFG->prefix;
+        }
+        $prefix .= '_' . $type . '_';
+        return $prefix;
+    }
+
+    /**
+     * Lock constructor.
+     * @param string $type - Used to prefix lock keys.
+     */
+    public function __construct($type) {
+        global $DB;
+
+        $this->dbprefix = $this->get_unique_db_prefix($type);
+        // Save a reference to the global $DB so it will not be released while we still have open locks.
+        $this->db = $DB;
+
+        \core_shutdown_manager::register_function([$this, 'auto_release']);
+    }
+
+    /**
+     * Is available.
+     * @return boolean - True if this lock type is available in this environment.
+     */
+    public function is_available() {
+        return $this->db->get_dbfamily() === 'mysql';
+    }
+
+    /**
+     * Return information about the blocking behaviour of the lock type on this platform.
+     * @return boolean - Defer to the DB driver.
+     */
+    public function supports_timeout() {
+        return true;
+    }
+
+    /**
+     * Will this lock type will be automatically released when a process ends.
+     *
+     * @return boolean - Via shutdown handler.
+     */
+    public function supports_auto_release() {
+        return true;
+    }
+
+    /**
+     * Multiple locks for the same resource can NOT be held by a single process.
+     *
+     * Hard coded to false and workaround inconsistent support in different
+     * versions of MySQL / MariaDB.
+     *
+     * @return boolean - false
+     */
+    public function supports_recursion() {
+        return false;
+    }
+
+    /**
+     * Create and get a lock
+     * @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
+     * @param int $timeout - The number of seconds to wait for a lock before giving up.
+     * @param int $maxlifetime - Unused by this lock type.
+     * @return boolean - true if a lock was obtained.
+     */
+    public function get_lock($resource, $timeout, $maxlifetime = 86400) {
+
+        // We sha1 to avoid long key names hitting the mysql str limit.
+        $resourcekey = sha1($this->dbprefix . $resource);
+
+        // Even though some versions of MySQL and MariaDB can support stacked locks
+        // just never stack them and always fail fast.
+        if (isset($this->openlocks[$resourcekey])) {
+            return false;
+        }
+
+        $params = [
+            'resourcekey' => $resourcekey,
+            'timeout' => $timeout
+        ];
+
+        $result = $this->db->get_record_sql('SELECT GET_LOCK(:resourcekey, :timeout) AS locked', $params);
+        $locked = $result->locked == 1;
+
+        if ($locked) {
+            $this->openlocks[$resourcekey] = 1;
+            return new lock($resourcekey, $this);
+        }
+        return false;
+    }
+
+    /**
+     * Release a lock that was previously obtained with @lock.
+     * @param lock $lock - a lock obtained from this factory.
+     * @return boolean - true if the lock is no longer held (including if it was never held).
+     */
+    public function release_lock(lock $lock) {
+
+        $params = [
+            'resourcekey' => $lock->get_key()
+        ];
+        $result = $this->db->get_record_sql('SELECT RELEASE_LOCK(:resourcekey) AS unlocked', $params);
+        $result = $result->unlocked == 1;
+        if ($result) {
+            unset($this->openlocks[$lock->get_key()]);
+        }
+        return $result;
+    }
+
+    /**
+     * Extend a lock that was previously obtained with @lock.
+     * @param lock $lock - a lock obtained from this factory.
+     * @param int $maxlifetime - the new lifetime for the lock (in seconds).
+     * @return boolean - true if the lock was extended.
+     */
+    public function extend_lock(lock $lock, $maxlifetime = 86400) {
+        // Not supported by this factory.
+        return false;
+    }
+
+    /**
+     * Auto release any open locks on shutdown.
+     * This is required, because we may be using persistent DB connections.
+     */
+    public function auto_release() {
+        // Called from the shutdown handler. Must release all open locks.
+        foreach ($this->openlocks as $key => $unused) {
+            $lock = new lock($key, $this);
+            $lock->release();
+        }
+    }
+
+}
+

lib/upgrade.txt

@@ -19,6 +19,7 @@ information provided here is intended especially for developers.
   - $plugin->incompatible = 36;
     incompatible takes a single int corresponding to the first incompatible branch. Any Moodle versions including and
     above this will be prevented from installing the plugin, and a message will be given when attempting installation.
+* Added a native MySQL / MariaDB lock implementation
 
 === 3.8 ===
 * Add CLI option to notify all cron tasks to stop: admin/cli/cron.php --stop