/core/lib/Drupal/Core/Lock/LockBackendAbstract.php

<?php

/**
 * @file
 * Contains \Drupal\Core\Lock\LockBackendAbstract.
 */

namespace Drupal\Core\Lock;

/**
 * Non backend related common methods implementation for lock backends.
 *
 * @ingroup lock
 */
abstract class LockBackendAbstract implements LockBackendInterface {

  /**
   * Current page lock token identifier.
   *
   * @var string
   */
  protected $lockId;

  /**
   * Existing locks for this page.
   *
   * @var array
   */
  protected $locks = array();

  /**
   * {@inheritdoc}
   */
  public function wait($name, $delay = 30) {
    // Pause the process for short periods between calling
    // lock_may_be_available(). This prevents hitting the database with constant
    // database queries while waiting, which could lead to performance issues.
    // However, if the wait period is too long, there is the potential for a
    // large number of processes to be blocked waiting for a lock, especially
    // if the item being rebuilt is commonly requested. To address both of these
    // concerns, begin waiting for 25ms, then add 25ms to the wait period each
    // time until it reaches 500ms. After this point polling will continue every
    // 500ms until $delay is reached.

    // $delay is passed in seconds, but we will be using usleep(), which takes
    // microseconds as a parameter. Multiply it by 1 million so that all
    // further numbers are equivalent.
    $delay = (int) $delay * 1000000;

    // Begin sleeping at 25ms.
    $sleep = 25000;
    while ($delay > 0) {
      // This function should only be called by a request that failed to get a
      // lock, so we sleep first to give the parallel request a chance to finish
      // and release the lock.
      usleep($sleep);
      // After each sleep, increase the value of $sleep until it reaches
      // 500ms, to reduce the potential for a lock stampede.
      $delay = $delay - $sleep;
      $sleep = min(500000, $sleep + 25000, $delay);
      if ($this->lockMayBeAvailable($name)) {
        // No longer need to wait.
        return FALSE;
      }
    }
    // The caller must still wait longer to get the lock.
    return TRUE;
  }

  /**
   * {@inheritdoc}
   */
  public function getLockId() {
    if (!isset($this->lockId)) {
      $this->lockId = uniqid(mt_rand(), TRUE);
    }
    return $this->lockId;
  }
}