function lock_acquire

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

Parameters

$name: The name of the lock. Limit of name's length is 255 characters.

$timeout: A number of seconds (float) before the lock expires (minimum of 0.001).

Return value

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

Related topics

File

includes/lock.inc, line 121

Code

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]);
}