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   1  <?php
   2  
   3  /**
   4   * @file
   5   * A database-mediated implementation of a locking mechanism.
   6   */
   7  
   8  /**
   9   * @defgroup lock Functions to coordinate long-running operations across requests.
  10   * @{
  11   * In most environments, multiple Drupal page requests (a.k.a. threads or
  12   * processes) will execute in parallel. This leads to potential conflicts or
  13   * race conditions when two requests execute the same code at the same time. A
  14   * common example of this is a rebuild like menu_rebuild() where we invoke many
  15   * hook implementations to get and process data from all active modules, and
  16   * then delete the current data in the database to insert the new afterwards.
  17   *
  18   * This is a cooperative, advisory lock system. Any long-running operation
  19   * that could potentially be attempted in parallel by multiple requests should
  20   * try to acquire a lock before proceeding. By obtaining a lock, one request
  21   * notifies any other requests that a specific opertation is in progress which
  22   * must not be executed in parallel.
  23   *
  24   * To use this API, pick a unique name for the lock. A sensible choice is the
  25   * name of the function performing the operation. A very simple example use of
  26   * this API:
  27   * @code
  28   * function mymodule_long_operation() {
  29   *   if (lock_acquire('mymodule_long_operation')) {
  30   *     // Do the long operation here.
  31   *     // ...
  32   *     lock_release('mymodule_long_operation');
  33   *   }
  34   * }
  35   * @endcode
  36   *
  37   * If a function acquires a lock it should always release it when the
  38   * operation is complete by calling lock_release(), as in the example.
  39   *
  40   * A function that has acquired a lock may attempt to renew a lock (extend the
  41   * duration of the lock) by calling lock_acquire() again during the operation.
  42   * Failure to renew a lock is indicative that another request has acquired
  43   * the lock, and that the current operation may need to be aborted.
  44   *
  45   * If a function fails to acquire a lock it may either immediately return, or
  46   * it may call lock_wait() if the rest of the current page request requires
  47   * that the operation in question be complete.  After lock_wait() returns,
  48   * the function may again attempt to acquire the lock, or may simply allow the
  49   * page request to proceed on the  assumption that a parallel request completed
  50   * the operation.
  51   *
  52   * lock_acquire() and lock_wait() will automatically break (delete) a lock
  53   * whose duration has exceeded the timeout specified when it was acquired.
  54   *
  55   * Alternative implementations of this API (such as APC) may be substituted
  56   * by setting the 'lock_inc' variable to an alternate include filepath.  Since
  57   * this is an API intended to support alternative implementations, code using
  58   * this API should never rely upon specific implementation details (for example
  59   * no code should look for or directly modify a lock in the {semaphore} table).
  60   */
  61  
  62  /**
  63   * Initialize the locking system.
  64   */
  65  function lock_init() {
  66    global $locks;
  67  
  68    $locks = array();
  69  }
  70  
  71  /**
  72   * Helper function to get this request's unique id.
  73   */
  74  function _lock_id() {
  75    static $lock_id;
  76  
  77    if (!isset($lock_id)) {
  78      // Assign a unique id.
  79      $lock_id = uniqid(mt_rand(), TRUE);
  80      // We only register a shutdown function if a lock is used.
  81      register_shutdown_function('lock_release_all', $lock_id);
  82    }
  83    return $lock_id;
  84  }
  85  
  86  /**
  87   * Acquire (or renew) a lock, but do not block if it fails.
  88   *
  89   * @param $name
  90   *   The name of the lock.
  91   * @param $timeout
  92   *   A number of seconds (float) before the lock expires.
  93   * @return
  94   *   TRUE if the lock was acquired, FALSE if it failed.
  95   */
  96  function lock_acquire($name, $timeout = 30.0) {
  97    global $locks;
  98  
  99    // Insure that the timeout is at least 1 ms.
 100    $timeout = max($timeout, 0.001);
 101    list($usec, $sec) = explode(' ', microtime());
 102    $expire = (float)$usec + (float)$sec + $timeout;
 103    if (isset($locks[$name])) {
 104      // Try to extend the expiration of a lock we already acquired.
 105      db_query("UPDATE {semaphore} SET expire = %f WHERE name = '%s' AND value = '%s'", $expire, $name, _lock_id());
 106      if (!db_affected_rows()) {
 107        // The lock was broken.
 108        unset($locks[$name]);
 109      }
 110    }
 111    else {
 112      // Optimistically try to acquire the lock, then retry once if it fails.
 113      // The first time through the loop cannot be a retry.
 114      $retry = FALSE;
 115      // We always want to do this code at least once.
 116      do {
 117        if (@db_query("INSERT INTO {semaphore} (name, value, expire) VALUES ('%s', '%s', %f)", $name, _lock_id(), $expire)) {
 118          // We track all acquired locks in the global variable.
 119          $locks[$name] = TRUE;
 120          // We never need to try again.
 121          $retry = FALSE;
 122        }
 123        else {
 124          // Suppress the error. If this is our first pass through the loop,
 125          // then $retry is FALSE. In this case, the insert must have failed
 126          // meaning some other request acquired the lock but did not release it.
 127          // We decide whether to retry by checking lock_may_be_available()
 128          // Since this will break the lock in case it is expired.
 129          $retry = $retry ? FALSE : lock_may_be_available($name);
 130        }
 131        // We only retry in case the first attempt failed, but we then broke
 132        // an expired lock.
 133      } while ($retry);
 134    }
 135    return isset($locks[$name]);
 136  }
 137  
 138  /**
 139   * Check if lock acquired by a different process may be available.
 140   *
 141   * If an existing lock has expired, it is removed.
 142   *
 143   * @param $name
 144   *   The name of the lock.
 145   * @return
 146   *   TRUE if there is no lock or it was removed, FALSE otherwise.
 147   */
 148  function lock_may_be_available($name) {
 149    $lock = db_fetch_array(db_query("SELECT expire, value FROM {semaphore} WHERE name = '%s'", $name));
 150    if (!$lock) {
 151      return TRUE;
 152    }
 153    $expire = (float) $lock['expire'];
 154    list($usec, $sec) = explode(' ', microtime());
 155    $now = (float)$usec + (float)$sec;
 156    if ($now > $lock['expire']) {
 157      // We check two conditions to prevent a race condition where another
 158      // request acquired the lock and set a new expire time.  We add a small
 159      // number to $expire to avoid errors with float to string conversion.
 160      db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s' AND expire <= %f", $name, $lock['value'], 0.0001 + $expire);
 161      return (bool)db_affected_rows();
 162    }
 163    return FALSE;
 164  }
 165  
 166  /**
 167   * Wait for a lock to be available.
 168   *
 169   * This function may be called in a request that fails to acquire a desired
 170   * lock. This will block further execution until the lock is available or the
 171   * specified delay in seconds is reached.  This should not be used with locks
 172   * that are acquired very frequently, since the lock is likely to be acquired
 173   * again by a different request during the sleep().
 174   *
 175   * @param $name
 176   *   The name of the lock.
 177   * @param $delay
 178   *   The maximum number of seconds to wait, as an integer.
 179   * @return
 180   *   TRUE if the lock holds, FALSE if it is available.
 181   */
 182  function lock_wait($name, $delay = 30) {
 183  
 184    while ($delay--) {
 185      // This function should only be called by a request that failed to get a
 186      // lock, so we sleep first to give the parallel request a chance to finish
 187      // and release the lock.
 188      sleep(1);
 189      if (lock_may_be_available($name)) {
 190        // No longer need to wait.
 191        return FALSE;
 192      }
 193    }
 194    // The caller must still wait longer to get the lock.
 195    return TRUE;
 196  }
 197  
 198  /**
 199   * Release a lock previously acquired by lock_acquire().
 200   *
 201   * This will release the named lock if it is still held by the current request.
 202   *
 203   * @param $name
 204   *   The name of the lock.
 205   */
 206  function lock_release($name) {
 207    global $locks;
 208  
 209    unset($locks[$name]);
 210    db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s'", $name, _lock_id());
 211  }
 212  
 213  /**
 214   * Release all previously acquired locks.
 215   */
 216  function lock_release_all($lock_id = NULL) {
 217    global $locks;
 218  
 219    $locks = array();
 220    if (empty($lock_id)) {
 221      $lock_id = _lock_id();
 222    }
 223  
 224    db_query("DELETE FROM {semaphore} WHERE value = '%s'", _lock_id());
 225  }
 226  
 227  /**
 228   * @} End of "defgroup locks".
 229   */