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<?php
namespace Amp\Sync;
use Amp\Coroutine;
use Amp\Delayed;
use Amp\Promise;
/**
* A cross-platform mutex that uses exclusive files as the lock mechanism.
*
* This mutex implementation is not always atomic and depends on the operating
* system's implementation of file creation operations. Use this implementation
* only if no other mutex types are available.
*
* This implementation avoids using [flock()](https://www.php.net/flock)
* because flock() is known to have some atomicity issues on some systems. In
* addition, flock() does not work as expected when trying to lock a file
* multiple times in the same process on Linux. Instead, exclusive file creation
* is used to create a lock file, which is atomic on most systems.
*
* @see https://www.php.net/fopen
*/
class FileMutex implements Mutex
{
public const LATENCY_TIMEOUT = 10;
/** @var string The full path to the lock file. */
private $fileName;
/**
* @param string $fileName Name of temporary file to use as a mutex.
*/
public function __construct(string $fileName)
{
$this->fileName = $fileName;
}
/**
* {@inheritdoc}
*/
public function acquire(): Promise
{
return new Coroutine($this->doAcquire());
}
/**
* @coroutine
*
* @return \Generator
*/
private function doAcquire(): \Generator
{
// Try to create the lock file. If the file already exists, someone else
// has the lock, so set an asynchronous timer and try again.
while (($handle = @\fopen($this->fileName, 'x')) === false) {
yield new Delayed(self::LATENCY_TIMEOUT);
}
// Return a lock object that can be used to release the lock on the mutex.
$lock = new Lock(0, function (): void {
$this->release();
});
\fclose($handle);
return $lock;
}
/**
* Releases the lock on the mutex.
*
* @throws SyncException If the unlock operation failed.
*/
protected function release()
{
$success = @\unlink($this->fileName);
if (!$success) {
throw new SyncException('Failed to unlock the mutex file.');
}
}
}