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<?php
namespace Amp\Sync;
use Amp\Coroutine;
use Amp\Delayed;
use Amp\Promise;
/**
* A non-blocking, inter-process POSIX semaphore.
*
* Uses a POSIX message queue to store a queue of permits in a lock-free data structure. This semaphore implementation
* is preferred over other implementations when available, as it provides the best performance.
*
* Not compatible with Windows.
*/
class PosixSemaphore implements Semaphore
{
public const LATENCY_TIMEOUT = 10;
/** @var string */
private $id;
/** @var int The semaphore key. */
private $key;
/** @var int PID of the process that created the semaphore. */
private $initializer = 0;
/** @var resource A message queue of available locks. */
private $queue;
/**
* Creates a new semaphore with a given ID and number of locks.
*
* @param string $id The unique name for the new semaphore.
* @param int $maxLocks The maximum number of locks that can be acquired from the semaphore.
* @param int $permissions Permissions to access the semaphore. Use file permission format specified as 0xxx.
*
* @return PosixSemaphore
* @throws SyncException If the semaphore could not be created due to an internal error.
*/
public static function create(string $id, int $maxLocks, int $permissions = 0600): self
{
if ($maxLocks < 1) {
throw new \Error('Number of locks must be greater than 0');
}
$semaphore = new self($id);
$semaphore->init($maxLocks, $permissions);
return $semaphore;
}
/**
* @param string $id The unique name of the semaphore to use.
*
* @return \Amp\Sync\PosixSemaphore
*/
public static function use(string $id): self
{
$semaphore = new self($id);
$semaphore->open();
return $semaphore;
}
/**
* @param string $id
*
* @throws \Error If the sysvmsg extension is not loaded.
*/
private function __construct(string $id)
{
if (!\extension_loaded("sysvmsg")) {
throw new \Error(__CLASS__ . " requires the sysvmsg extension.");
}
$this->id = $id;
$this->key = self::makeKey($this->id);
}
/**
* Private method to prevent cloning.
*/
private function __clone()
{
}
/**
* Prevent serialization.
*/
public function __sleep()
{
throw new \Error('A semaphore cannot be serialized!');
}
public function getId(): string
{
return $this->id;
}
private function open(): void
{
if (!\msg_queue_exists($this->key)) {
throw new SyncException('No semaphore with that ID found');
}
$this->queue = \msg_get_queue($this->key);
if (!$this->queue) {
throw new SyncException('Failed to open the semaphore.');
}
}
/**
* @param int $maxLocks The maximum number of locks that can be acquired from the semaphore.
* @param int $permissions Permissions to access the semaphore.
*
* @throws SyncException If the semaphore could not be created due to an internal error.
*/
private function init(int $maxLocks, int $permissions): void
{
if (\msg_queue_exists($this->key)) {
throw new SyncException('A semaphore with that ID already exists');
}
$this->queue = \msg_get_queue($this->key, $permissions);
if (!$this->queue) {
throw new SyncException('Failed to create the semaphore.');
}
$this->initializer = \getmypid();
// Fill the semaphore with locks.
while (--$maxLocks >= 0) {
$this->release($maxLocks);
}
}
/**
* Gets the access permissions of the semaphore.
*
* @return int A permissions mode.
*/
public function getPermissions(): int
{
$stat = \msg_stat_queue($this->queue);
return $stat['msg_perm.mode'];
}
/**
* Sets the access permissions of the semaphore.
*
* The current user must have access to the semaphore in order to change the permissions.
*
* @param int $mode A permissions mode to set.
*
* @throws SyncException If the operation failed.
*/
public function setPermissions(int $mode)
{
if (!\msg_set_queue($this->queue, ['msg_perm.mode' => $mode])) {
throw new SyncException('Failed to change the semaphore permissions.');
}
}
public function acquire(): Promise
{
return new Coroutine($this->doAcquire());
}
/**
* {@inheritdoc}
*/
private function doAcquire(): \Generator
{
do {
// Attempt to acquire a lock from the semaphore.
if (@\msg_receive($this->queue, 0, $type, 1, $id, false, \MSG_IPC_NOWAIT, $errno)) {
// A free lock was found, so resolve with a lock object that can
// be used to release the lock.
return new Lock(\unpack("C", $id)[1], function (Lock $lock) {
$this->release($lock->getId());
});
}
// Check for unusual errors.
if ($errno !== \MSG_ENOMSG) {
throw new SyncException(\sprintf('Failed to acquire a lock; errno: %d', $errno));
}
} while (yield new Delayed(self::LATENCY_TIMEOUT, true));
}
/**
* Removes the semaphore if it still exists.
*
* @throws SyncException If the operation failed.
*/
public function __destruct()
{
if ($this->initializer === 0 || $this->initializer !== \getmypid()) {
return;
}
if (\PHP_VERSION_ID < 80000 && (!\is_resource($this->queue) || !\msg_queue_exists($this->key))) {
return;
}
if (\PHP_VERSION_ID >= 80000 && (!$this->queue instanceof \SysvMessageQueue || !\msg_queue_exists($this->key))) {
return;
}
\msg_remove_queue($this->queue);
}
/**
* Releases a lock from the semaphore.
*
* @param int $id Lock identifier.
*
* @throws SyncException If the operation failed.
*/
protected function release(int $id)
{
if (!$this->queue) {
return; // Queue already destroyed.
}
// Call send in non-blocking mode. If the call fails because the queue
// is full, then the number of locks configured is too large.
if (!@\msg_send($this->queue, 1, \pack("C", $id), false, false, $errno)) {
if ($errno === \MSG_EAGAIN) {
throw new SyncException('The semaphore size is larger than the system allows.');
}
throw new SyncException('Failed to release the lock.');
}
}
private static function makeKey(string $id): int
{
return \abs(\unpack("l", \md5($id, true))[1]);
}
}