public function LockBackendAbstract::wait

public LockBackendAbstract::wait($name, $delay = 30)

Waits a short amount of time before a second lock acquire attempt.

While this method is subject to have a generic implementation in abstract backend implementation, some backends may provide non blocking or less I/O intensive wait mechanism: this is why this method remains on the backend interface.

Parameters

string $name: Lock name currently being locked.

int $delay: Milliseconds to wait for. Defaults to 30.

Return value

bool TRUE if the lock holds, FALSE if it may be available. You still need to acquire the lock manually and it may fail again.

Overrides LockBackendInterface::wait

File

core/lib/Drupal/Core/Lock/LockBackendAbstract.php, line 29

Class

LockBackendAbstract

Non backend related common methods implementation for lock backends.

Namespace

Drupal\Core\Lock

Code

public function 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, $delay);
    if ($this->lockMayBeAvailable($name)) {
      // No longer need to wait.
      return FALSE;
    }
  }
  // The caller must still wait longer to get the lock.
  return TRUE;
}