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root / drupal7 / includes / lock.inc @ 6d8023f2

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<?php
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/**
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 * @file
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 * A database-mediated implementation of a locking mechanism.
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 */
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/**
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 * @defgroup lock Locking mechanisms
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 * @{
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 * Functions to coordinate long-running operations across requests.
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 *
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 * In most environments, multiple Drupal page requests (a.k.a. threads or
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 * processes) will execute in parallel. This leads to potential conflicts or
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 * race conditions when two requests execute the same code at the same time. A
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 * common example of this is a rebuild like menu_rebuild() where we invoke many
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 * hook implementations to get and process data from all active modules, and
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 * then delete the current data in the database to insert the new afterwards.
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 *
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 * This is a cooperative, advisory lock system. Any long-running operation
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 * that could potentially be attempted in parallel by multiple requests should
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 * try to acquire a lock before proceeding. By obtaining a lock, one request
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 * notifies any other requests that a specific operation is in progress which
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 * must not be executed in parallel.
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 *
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 * To use this API, pick a unique name for the lock. A sensible choice is the
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 * name of the function performing the operation. A very simple example use of
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 * this API:
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 * @code
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 * function mymodule_long_operation() {
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 *   if (lock_acquire('mymodule_long_operation')) {
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 *     // Do the long operation here.
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 *     // ...
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 *     lock_release('mymodule_long_operation');
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 *   }
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 * }
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 * @endcode
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 *
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 * If a function acquires a lock it should always release it when the
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 * operation is complete by calling lock_release(), as in the example.
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 *
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 * A function that has acquired a lock may attempt to renew a lock (extend the
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 * duration of the lock) by calling lock_acquire() again during the operation.
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 * Failure to renew a lock is indicative that another request has acquired
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 * the lock, and that the current operation may need to be aborted.
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 *
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 * If a function fails to acquire a lock it may either immediately return, or
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 * it may call lock_wait() if the rest of the current page request requires
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 * that the operation in question be complete. After lock_wait() returns,
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 * the function may again attempt to acquire the lock, or may simply allow the
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 * page request to proceed on the assumption that a parallel request completed
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 * the operation.
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 *
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 * lock_acquire() and lock_wait() will automatically break (delete) a lock
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 * whose duration has exceeded the timeout specified when it was acquired.
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 *
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 * Alternative implementations of this API (such as APC) may be substituted
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 * by setting the 'lock_inc' variable to an alternate include filepath. Since
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 * this is an API intended to support alternative implementations, code using
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 * this API should never rely upon specific implementation details (for example
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 * no code should look for or directly modify a lock in the {semaphore} table).
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 */
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/**
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 * Initialize the locking system.
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 */
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function lock_initialize() {
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  global $locks;
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  $locks = array();
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}
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/**
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 * Helper function to get this request's unique id.
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 */
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function _lock_id() {
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  // Do not use drupal_static(). This identifier refers to the current
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  // client request, and must not be changed under any circumstances
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  // else the shutdown handler may fail to release our locks.
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  static $lock_id;
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  if (!isset($lock_id)) {
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    // Assign a unique id.
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    $lock_id = uniqid(mt_rand(), TRUE);
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    // We only register a shutdown function if a lock is used.
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    drupal_register_shutdown_function('lock_release_all', $lock_id);
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  }
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  return $lock_id;
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}
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/**
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 * Acquire (or renew) a lock, but do not block if it fails.
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 *
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 * @param $name
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 *   The name of the lock. Limit of name's length is 255 characters.
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 * @param $timeout
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 *   A number of seconds (float) before the lock expires (minimum of 0.001).
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 *
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 * @return
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 *   TRUE if the lock was acquired, FALSE if it failed.
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 */
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function lock_acquire($name, $timeout = 30.0) {
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  global $locks;
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  // Insure that the timeout is at least 1 ms.
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  $timeout = max($timeout, 0.001);
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  $expire = microtime(TRUE) + $timeout;
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  if (isset($locks[$name])) {
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    // Try to extend the expiration of a lock we already acquired.
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    $success = (bool) db_update('semaphore')
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      ->fields(array('expire' => $expire))
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      ->condition('name', $name)
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      ->condition('value', _lock_id())
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      ->execute();
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    if (!$success) {
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      // The lock was broken.
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      unset($locks[$name]);
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    }
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    return $success;
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  }
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  else {
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    // Optimistically try to acquire the lock, then retry once if it fails.
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    // The first time through the loop cannot be a retry.
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    $retry = FALSE;
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    // We always want to do this code at least once.
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    do {
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      try {
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        db_insert('semaphore')
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          ->fields(array(
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            'name' => $name,
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            'value' => _lock_id(),
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            'expire' => $expire,
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          ))
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          ->execute();
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        // We track all acquired locks in the global variable.
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        $locks[$name] = TRUE;
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        // We never need to try again.
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        $retry = FALSE;
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      }
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      catch (PDOException $e) {
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        // Suppress the error. If this is our first pass through the loop,
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        // then $retry is FALSE. In this case, the insert must have failed
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        // meaning some other request acquired the lock but did not release it.
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        // We decide whether to retry by checking lock_may_be_available()
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        // Since this will break the lock in case it is expired.
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        $retry = $retry ? FALSE : lock_may_be_available($name);
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      }
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      // We only retry in case the first attempt failed, but we then broke
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      // an expired lock.
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    } while ($retry);
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  }
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  return isset($locks[$name]);
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}
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/**
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 * Check if lock acquired by a different process may be available.
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 *
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 * If an existing lock has expired, it is removed.
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 *
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 * @param $name
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 *   The name of the lock.
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 *
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 * @return
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 *   TRUE if there is no lock or it was removed, FALSE otherwise.
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 */
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function lock_may_be_available($name) {
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  $lock = db_query('SELECT expire, value FROM {semaphore} WHERE name = :name', array(':name' => $name))->fetchAssoc();
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  if (!$lock) {
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    return TRUE;
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  }
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  $expire = (float) $lock['expire'];
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  $now = microtime(TRUE);
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  if ($now > $expire) {
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    // We check two conditions to prevent a race condition where another
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    // request acquired the lock and set a new expire time. We add a small
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    // number to $expire to avoid errors with float to string conversion.
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    return (bool) db_delete('semaphore')
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      ->condition('name', $name)
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      ->condition('value', $lock['value'])
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      ->condition('expire', 0.0001 + $expire, '<=')
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      ->execute();
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  }
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  return FALSE;
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}
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/**
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 * Wait for a lock to be available.
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 *
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 * This function may be called in a request that fails to acquire a desired
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 * lock. This will block further execution until the lock is available or the
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 * specified delay in seconds is reached. This should not be used with locks
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 * that are acquired very frequently, since the lock is likely to be acquired
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 * again by a different request while waiting.
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 *
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 * @param $name
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 *   The name of the lock.
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 * @param $delay
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 *   The maximum number of seconds to wait, as an integer.
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 *
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 * @return
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 *   TRUE if the lock holds, FALSE if it is available.
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 */
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function lock_wait($name, $delay = 30) {
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  // Pause the process for short periods between calling
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  // lock_may_be_available(). This prevents hitting the database with constant
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  // database queries while waiting, which could lead to performance issues.
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  // However, if the wait period is too long, there is the potential for a
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  // large number of processes to be blocked waiting for a lock, especially
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  // if the item being rebuilt is commonly requested. To address both of these
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  // concerns, begin waiting for 25ms, then add 25ms to the wait period each
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  // time until it reaches 500ms. After this point polling will continue every
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  // 500ms until $delay is reached.
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  // $delay is passed in seconds, but we will be using usleep(), which takes
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  // microseconds as a parameter. Multiply it by 1 million so that all
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  // further numbers are equivalent.
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  $delay = (int) $delay * 1000000;
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  // Begin sleeping at 25ms.
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  $sleep = 25000;
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  while ($delay > 0) {
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    // This function should only be called by a request that failed to get a
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    // lock, so we sleep first to give the parallel request a chance to finish
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    // and release the lock.
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    usleep($sleep);
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    // After each sleep, increase the value of $sleep until it reaches
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    // 500ms, to reduce the potential for a lock stampede.
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    $delay = $delay - $sleep;
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    $sleep = min(500000, $sleep + 25000, $delay);
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    if (lock_may_be_available($name)) {
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      // No longer need to wait.
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      return FALSE;
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    }
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  }
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  // The caller must still wait longer to get the lock.
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  return TRUE;
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}
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/**
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 * Release a lock previously acquired by lock_acquire().
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 *
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 * This will release the named lock if it is still held by the current request.
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 *
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 * @param $name
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 *   The name of the lock.
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 */
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function lock_release($name) {
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  global $locks;
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  unset($locks[$name]);
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  db_delete('semaphore')
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    ->condition('name', $name)
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    ->condition('value', _lock_id())
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    ->execute();
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}
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/**
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 * Release all previously acquired locks.
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 */
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function lock_release_all($lock_id = NULL) {
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  global $locks;
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  $locks = array();
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  if (empty($lock_id)) {
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    $lock_id = _lock_id();
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  }
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  db_delete('semaphore')
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    ->condition('value', $lock_id)
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    ->execute();
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}
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/**
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 * @} End of "defgroup lock".
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 */