lock.inc
A database-mediated implementation of a locking mechanism.
File
drupal-6.x/includes/lock.incView source
- <?php
-
- /**
- * @file
- * A database-mediated implementation of a locking mechanism.
- */
-
- /**
- * @defgroup lock 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 opertation 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_init() {
- global $locks;
-
- $locks = array();
- }
-
- /**
- * Helper function to get this request's unique id.
- */
- function _lock_id() {
- 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.
- 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 $timeout
- * A number of seconds (float) before the lock expires.
- * @return
- * TRUE if the lock was acquired, FALSE if it failed.
- */
- function lock_acquire($name, $timeout = 30.0) {
- global $locks;
-
- // Insure that the timeout is at least 1 ms.
- $timeout = max($timeout, 0.001);
- list($usec, $sec) = explode(' ', microtime());
- $expire = (float)$usec + (float)$sec + $timeout;
- if (isset($locks[$name])) {
- // Try to extend the expiration of a lock we already acquired.
- db_query("UPDATE {semaphore} SET expire = %f WHERE name = '%s' AND value = '%s'", $expire, $name, _lock_id());
- if (!db_affected_rows()) {
- // The lock was broken.
- unset($locks[$name]);
- }
- }
- 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 {
- if (@db_query("INSERT INTO {semaphore} (name, value, expire) VALUES ('%s', '%s', %f)", $name, _lock_id(), $expire)) {
- // We track all acquired locks in the global variable.
- $locks[$name] = TRUE;
- // We never need to try again.
- $retry = FALSE;
- }
- else {
- // 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_fetch_array(db_query("SELECT expire, value FROM {semaphore} WHERE name = '%s'", $name));
- if (!$lock) {
- return TRUE;
- }
- $expire = (float) $lock['expire'];
- list($usec, $sec) = explode(' ', microtime());
- $now = (float)$usec + (float)$sec;
- if ($now > $lock['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.
- db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s' AND expire <= %f", $name, $lock['value'], 0.0001 + $expire);
- return (bool)db_affected_rows();
- }
- 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 during the sleep().
- *
- * @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, $delay = 30) {
-
- while ($delay--) {
- // 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.
- sleep(1);
- 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_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s'", $name, _lock_id());
- }
-
- /**
- * Release all previously acquired locks.
- */
- function lock_release_all($lock_id = NULL) {
- global $locks;
-
- $locks = array();
- if (empty($lock_id)) {
- $lock_id = _lock_id();
- }
-
- db_query("DELETE FROM {semaphore} WHERE value = '%s'", _lock_id());
- }
-
- /**
- * @} End of "defgroup locks".
- */