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function lock_may_be_available in Drupal 6

Same name in this branch
  1. 6 includes/lock.inc \lock_may_be_available()
  2. 6 includes/lock-install.inc \lock_may_be_available()
Same name and namespace in other branches
  1. 7 includes/lock.inc \lock_may_be_available()

Check if lock acquired by a different process may be available.

If an existing lock has expired, it is removed.

Parameters

$name: The name of the lock.

Return value

TRUE if there is no lock or it was removed, FALSE otherwise.

Related topics

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…
2 calls to lock_may_be_available()
lock_acquire in includes/lock.inc
Acquire (or renew) a lock, but do not block if it fails.
lock_wait in includes/lock.inc
Wait for a lock to be available.

File

includes/lock.inc, line 148
A database-mediated implementation of a locking mechanism.

Code

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 = (double) $lock['expire'];
  list($usec, $sec) = explode(' ', microtime());
  $now = (double) $usec + (double) $sec;
  if ($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;
}