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<?php
namespace Amp\Sync\Internal;
use Amp\Delayed;
use Amp\Promise;
use Amp\Sync\Lock;
use function Amp\call;
/** @internal */
final class SemaphoreStorage extends \Threaded
{
public const LATENCY_TIMEOUT = 10;
/**
* Creates a new semaphore with a given number of locks.
*
* @param int $locks The maximum number of locks that can be acquired from the semaphore.
*/
public function __construct(int $locks)
{
foreach (\range(0, $locks - 1) as $lock) {
$this[] = $lock;
}
}
public function acquire(): Promise
{
/**
* Uses a double locking mechanism to acquire a lock without blocking. A
* synchronous mutex is used to make sure that the semaphore is queried one
* at a time to preserve the integrity of the semaphore itself. Then a lock
* count is used to check if a lock is available without blocking.
*
* If a lock is not available, we add the request to a queue and set a timer
* to check again in the future.
*/
return call(function (): \Generator {
$tsl = function (): ?int {
// If there are no locks available or the wait queue is not empty,
// we need to wait our turn to acquire a lock.
if (!$this->count()) {
return null;
}
return $this->shift();
};
while (!$this->count() || ($id = $this->synchronized($tsl)) === null) {
yield new Delayed(self::LATENCY_TIMEOUT);
}
return new Lock($id, function (Lock $lock): void {
$id = $lock->getId();
$this->synchronized(function () use ($id) {
$this[] = $id;
});
});
});
}
}