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<?php

/**

 * @file

 * A database-mediated implementation of a locking mechanism.

 */

/**

 * @defgroup lock Locking mechanisms

 * @{

 * Functions to coordinate long-running operations across requests.

 *

 * In most environments, multiple Drupal page requests (a.k.a. threads or

 * processes) will execute in parallel. This leads to potential conflicts or

 * race conditions when two requests execute the same code at the same time. A

 * common example of this is a rebuild like menu_rebuild() where we invoke many

 * hook implementations to get and process data from all active modules, and

 * then delete the current data in the database to insert the new afterwards.

 *

 * This is a cooperative, advisory lock system. Any long-running operation

 * that could potentially be attempted in parallel by multiple requests should

 * try to acquire a lock before proceeding. By obtaining a lock, one request

 * notifies any other requests that a specific operation is in progress which

 * must not be executed in parallel.

 *

 * To use this API, pick a unique name for the lock. A sensible choice is the

 * name of the function performing the operation. A very simple example use of

 * this API:

 * @code

 * function mymodule_long_operation() {

 *   if (lock_acquire('mymodule_long_operation')) {

 *     // Do the long operation here.

 *     // ...

 *     lock_release('mymodule_long_operation');

 *   }

 * }

 * @endcode

 *

 * If a function acquires a lock it should always release it when the

 * operation is complete by calling lock_release(), as in the example.

 *

 * A function that has acquired a lock may attempt to renew a lock (extend the

 * duration of the lock) by calling lock_acquire() again during the operation.

 * Failure to renew a lock is indicative that another request has acquired

 * the lock, and that the current operation may need to be aborted.

 *

 * If a function fails to acquire a lock it may either immediately return, or

 * it may call lock_wait() if the rest of the current page request requires

 * that the operation in question be complete. After lock_wait() returns,

 * the function may again attempt to acquire the lock, or may simply allow the

 * page request to proceed on the assumption that a parallel request completed

 * the operation.

 *

 * lock_acquire() and lock_wait() will automatically break (delete) a lock

 * whose duration has exceeded the timeout specified when it was acquired.

 *

 * Alternative implementations of this API (such as APC) may be substituted

 * by setting the 'lock_inc' variable to an alternate include filepath. Since

 * this is an API intended to support alternative implementations, code using

 * this API should never rely upon specific implementation details (for example

 * no code should look for or directly modify a lock in the {semaphore} table).

 */

/**

 * Initialize the locking system.

 */

function lock_initialize() {

  global $locks;

  $locks = array();

}

/**

 * Helper function to get this request's unique id.

 */

function _lock_id() {

  // Do not use drupal_static(). This identifier refers to the current

  // client request, and must not be changed under any circumstances

  // else the shutdown handler may fail to release our locks.

  static $lock_id;

  if (!isset($lock_id)) {

    // Assign a unique id.

    $lock_id = uniqid(mt_rand(), TRUE);

    // We only register a shutdown function if a lock is used.

    drupal_register_shutdown_function('lock_release_all', $lock_id);

  }

  return $lock_id;

}

/**

 * Acquire (or renew) a lock, but do not block if it fails.

 *

 * @param $name

 *   The name of the lock.

 * @param $timeout

 *   A number of seconds (float) before the lock expires (minimum of 0.001).

 *

 * @return

 *   TRUE if the lock was acquired, FALSE if it failed.

 */

function lock_acquire($name, $timeout = 30.0) {

  global $locks;

  // Insure that the timeout is at least 1 ms.

  $timeout = max($timeout, 0.001);

  $expire = microtime(TRUE) + $timeout;

  if (isset($locks[$name])) {

    // Try to extend the expiration of a lock we already acquired.

    $success = (bool) db_update('semaphore')

      ->fields(array('expire' => $expire))

      ->condition('name', $name)

      ->condition('value', _lock_id())

      ->execute();

    if (!$success) {

      // The lock was broken.

      unset($locks[$name]);

    }

    return $success;

  }

  else {

    // Optimistically try to acquire the lock, then retry once if it fails.

    // The first time through the loop cannot be a retry.

    $retry = FALSE;

    // We always want to do this code at least once.

    do {

      try {

        db_insert('semaphore')

          ->fields(array(

            'name' => $name,

            'value' => _lock_id(),

            'expire' => $expire,

          ))

          ->execute();

        // We track all acquired locks in the global variable.

        $locks[$name] = TRUE;

        // We never need to try again.

        $retry = FALSE;

      }

      catch (PDOException $e) {

        // Suppress the error. If this is our first pass through the loop,

        // then $retry is FALSE. In this case, the insert must have failed

        // meaning some other request acquired the lock but did not release it.

        // We decide whether to retry by checking lock_may_be_available()

        // Since this will break the lock in case it is expired.

        $retry = $retry ? FALSE : lock_may_be_available($name);

      }

      // We only retry in case the first attempt failed, but we then broke

      // an expired lock.

    } while ($retry);

  }

  return isset($locks[$name]);

}

/**

 * Check if lock acquired by a different process may be available.

 *

 * If an existing lock has expired, it is removed.

 *

 * @param $name

 *   The name of the lock.

 *

 * @return

 *   TRUE if there is no lock or it was removed, FALSE otherwise.

 */

function lock_may_be_available($name) {

  $lock = db_query('SELECT expire, value FROM {semaphore} WHERE name = :name', array(':name' => $name))->fetchAssoc();

  if (!$lock) {

    return TRUE;

  }

  $expire = (float) $lock['expire'];

  $now = microtime(TRUE);

  if ($now > $expire) {

    // We check two conditions to prevent a race condition where another

    // request acquired the lock and set a new expire time. We add a small

    // number to $expire to avoid errors with float to string conversion.

    return (bool) db_delete('semaphore')

      ->condition('name', $name)

      ->condition('value', $lock['value'])

      ->condition('expire', 0.0001 + $expire, '<=')

      ->execute();

  }

  return FALSE;

}

/**

 * Wait for a lock to be available.

 *

 * This function may be called in a request that fails to acquire a desired

 * lock. This will block further execution until the lock is available or the

 * specified delay in seconds is reached. This should not be used with locks

 * that are acquired very frequently, since the lock is likely to be acquired

 * again by a different request while waiting.

 *

 * @param $name

 *   The name of the lock.

 * @param $delay

 *   The maximum number of seconds to wait, as an integer.

 *

 * @return

 *   TRUE if the lock holds, FALSE if it is available.

 */

function lock_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 (lock_may_be_available($name)) {

      // No longer need to wait.

      return FALSE;

    }

  }

  // The caller must still wait longer to get the lock.

  return TRUE;

}

/**

 * Release a lock previously acquired by lock_acquire().

 *

 * This will release the named lock if it is still held by the current request.

 *

 * @param $name

 *   The name of the lock.

 */

function lock_release($name) {

  global $locks;

  unset($locks[$name]);

  db_delete('semaphore')

    ->condition('name', $name)

    ->condition('value', _lock_id())

    ->execute();

}

/**

 * Release all previously acquired locks.

 */

function lock_release_all($lock_id = NULL) {

  global $locks;

  $locks = array();

  if (empty($lock_id)) {

    $lock_id = _lock_id();

  }

  db_delete('semaphore')

    ->condition('value', $lock_id)

    ->execute();

}

/**

 * @} End of "defgroup lock".

 */