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includes/lock.inc

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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. Limit of name's length is 255 characters.
+ * @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".
+ */