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   1  <?php
   2  // This file is part of Moodle - http://moodle.org/
   3  //
   4  // Moodle is free software: you can redistribute it and/or modify
   5  // it under the terms of the GNU General Public License as published by
   6  // the Free Software Foundation, either version 3 of the License, or
   7  // (at your option) any later version.
   8  //
   9  // Moodle is distributed in the hope that it will be useful,
  10  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  // GNU General Public License for more details.
  13  //
  14  // You should have received a copy of the GNU General Public License
  15  // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
  16  
  17  /**
  18   * This is a db record locking factory.
  19   *
  20   * @package    core
  21   * @category   lock
  22   * @copyright  Damyon Wiese 2013
  23   * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  24   */
  25  
  26  namespace core\lock;
  27  
  28  defined('MOODLE_INTERNAL') || die();
  29  
  30  /**
  31   * This is a db record locking factory.
  32   *
  33   * This lock factory uses record locks relying on sql of the form "SET XXX where YYY" and checking if the
  34   * value was set. It supports timeouts, autorelease and can work on any DB. The downside - is this
  35   * will always be slower than some shared memory type locking function.
  36   *
  37   * @package   core
  38   * @category  lock
  39   * @copyright Damyon Wiese 2013
  40   * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  41   */
  42  class db_record_lock_factory implements lock_factory {
  43  
  44      /** @var \moodle_database $db Hold a reference to the global $DB */
  45      protected $db;
  46  
  47      /** @var string $type Used to prefix lock keys */
  48      protected $type;
  49  
  50      /** @var array $openlocks - List of held locks - used by auto-release */
  51      protected $openlocks = array();
  52  
  53      /**
  54       * Is available.
  55       * @return boolean - True if this lock type is available in this environment.
  56       */
  57      public function is_available() {
  58          return true;
  59      }
  60  
  61      /**
  62       * Almighty constructor.
  63       * @param string $type - Used to prefix lock keys.
  64       */
  65      public function __construct($type) {
  66          global $DB;
  67  
  68          $this->type = $type;
  69          // Save a reference to the global $DB so it will not be released while we still have open locks.
  70          $this->db = $DB;
  71  
  72          \core_shutdown_manager::register_function(array($this, 'auto_release'));
  73      }
  74  
  75      /**
  76       * Return information about the blocking behaviour of the lock type on this platform.
  77       * @return boolean - True
  78       */
  79      public function supports_timeout() {
  80          return true;
  81      }
  82  
  83      /**
  84       * Will this lock type will be automatically released when a process ends.
  85       *
  86       * @return boolean - True (shutdown handler)
  87       */
  88      public function supports_auto_release() {
  89          return true;
  90      }
  91  
  92      /**
  93       * Multiple locks for the same resource can be held by a single process.
  94       * @return boolean - False - not process specific.
  95       */
  96      public function supports_recursion() {
  97          return false;
  98      }
  99  
 100      /**
 101       * This function generates a unique token for the lock to use.
 102       * It is important that this token is not solely based on time as this could lead
 103       * to duplicates in a clustered environment (especially on VMs due to poor time precision).
 104       */
 105      protected function generate_unique_token() {
 106          return generate_uuid();
 107      }
 108  
 109      /**
 110       * Create and get a lock
 111       * @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
 112       * @param int $timeout - The number of seconds to wait for a lock before giving up.
 113       * @param int $maxlifetime - Unused by this lock type.
 114       * @return boolean - true if a lock was obtained.
 115       */
 116      public function get_lock($resource, $timeout, $maxlifetime = 86400) {
 117  
 118          $token = $this->generate_unique_token();
 119          $now = time();
 120          $giveuptime = $now + $timeout;
 121          $expires = $now + $maxlifetime;
 122  
 123          if (!$this->db->record_exists('lock_db', array('resourcekey' => $resource))) {
 124              $record = new \stdClass();
 125              $record->resourcekey = $resource;
 126              $result = $this->db->insert_record('lock_db', $record);
 127          }
 128  
 129          $params = array('expires' => $expires,
 130                          'token' => $token,
 131                          'resourcekey' => $resource,
 132                          'now' => $now);
 133          $sql = 'UPDATE {lock_db}
 134                     SET
 135                         expires = :expires,
 136                         owner = :token
 137                   WHERE
 138                         resourcekey = :resourcekey AND
 139                         (owner IS NULL OR expires < :now)';
 140  
 141          do {
 142              $now = time();
 143              $params['now'] = $now;
 144              $this->db->execute($sql, $params);
 145  
 146              $countparams = array('owner' => $token, 'resourcekey' => $resource);
 147              $result = $this->db->count_records('lock_db', $countparams);
 148              $locked = $result === 1;
 149              if (!$locked) {
 150                  usleep(rand(10000, 250000)); // Sleep between 10 and 250 milliseconds.
 151              }
 152              // Try until the giveup time.
 153          } while (!$locked && $now < $giveuptime);
 154  
 155          if ($locked) {
 156              $this->openlocks[$token] = 1;
 157              return new lock($token, $this);
 158          }
 159  
 160          return false;
 161      }
 162  
 163      /**
 164       * Release a lock that was previously obtained with @lock.
 165       * @param lock $lock - a lock obtained from this factory.
 166       * @return boolean - true if the lock is no longer held (including if it was never held).
 167       */
 168      public function release_lock(lock $lock) {
 169          $params = array('noexpires' => null,
 170                          'token' => $lock->get_key(),
 171                          'noowner' => null);
 172  
 173          $sql = 'UPDATE {lock_db}
 174                      SET
 175                          expires = :noexpires,
 176                          owner = :noowner
 177                      WHERE
 178                          owner = :token';
 179          $result = $this->db->execute($sql, $params);
 180          if ($result) {
 181              unset($this->openlocks[$lock->get_key()]);
 182          }
 183          return $result;
 184      }
 185  
 186      /**
 187       * Extend a lock that was previously obtained with @lock.
 188       * @param lock $lock - a lock obtained from this factory.
 189       * @param int $maxlifetime - the new lifetime for the lock (in seconds).
 190       * @return boolean - true if the lock was extended.
 191       */
 192      public function extend_lock(lock $lock, $maxlifetime = 86400) {
 193          $now = time();
 194          $expires = $now + $maxlifetime;
 195          $params = array('expires' => $expires,
 196                          'token' => $lock->get_key());
 197  
 198          $sql = 'UPDATE {lock_db}
 199                      SET
 200                          expires = :expires,
 201                      WHERE
 202                          owner = :token';
 203  
 204          $this->db->execute($sql, $params);
 205          $countparams = array('owner' => $lock->get_key());
 206          $result = $this->count_records('lock_db', $countparams);
 207  
 208          return $result === 0;
 209      }
 210  
 211      /**
 212       * Auto release any open locks on shutdown.
 213       * This is required, because we may be using persistent DB connections.
 214       */
 215      public function auto_release() {
 216          // Called from the shutdown handler. Must release all open locks.
 217          foreach ($this->openlocks as $key => $unused) {
 218              $lock = new lock($key, $this);
 219              $lock->release();
 220          }
 221      }
 222  }