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PhabricatorGlobalLock.php
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Mon, Jul 8, 13:43

PhabricatorGlobalLock.php
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<?php
/**
* Global, MySQL-backed lock. This is a high-reliability, low-performance
* global lock.
*
* The lock is maintained by using GET_LOCK() in MySQL, and automatically
* released when the connection terminates. Thus, this lock can safely be used
* to control access to shared resources without implementing any sort of
* timeout or override logic: the lock can't normally be stuck in a locked state
* with no process actually holding the lock.
*
* However, acquiring the lock is moderately expensive (several network
* roundtrips). This makes it unsuitable for tasks where lock performance is
* important.
*
* $lock = PhabricatorGlobalLock::newLock('example');
* $lock->lock();
* do_contentious_things();
* $lock->unlock();
*
* NOTE: This lock is not completely global; it is namespaced to the active
* storage namespace so that unit tests running in separate table namespaces
* are isolated from one another.
*
* @task construct Constructing Locks
* @task impl Implementation
*/
final class PhabricatorGlobalLock extends PhutilLock {
private $conn;
private $isExternalConnection = false;
private static $pool = array();
/* -( Constructing Locks )------------------------------------------------- */
public static function newLock($name) {
$namespace = PhabricatorLiskDAO::getStorageNamespace();
$namespace = PhabricatorHash::digestToLength($namespace, 20);
$full_name = 'ph:'.$namespace.':'.$name;
$length_limit = 64;
if (strlen($full_name) > $length_limit) {
throw new Exception(
pht(
'Lock name "%s" is too long (full lock name is "%s"). The '.
'full lock name must not be longer than %s bytes.',
$name,
$full_name,
new PhutilNumber($length_limit)));
}
$lock = self::getLock($full_name);
if (!$lock) {
$lock = new PhabricatorGlobalLock($full_name);
self::registerLock($lock);
}
return $lock;
}
/**
* Use a specific database connection for locking.
*
* By default, `PhabricatorGlobalLock` will lock on the "repository" database
* (somewhat arbitrarily). In most cases this is fine, but this method can
* be used to lock on a specific connection.
*
* @param AphrontDatabaseConnection
* @return this
*/
public function useSpecificConnection(AphrontDatabaseConnection $conn) {
$this->conn = $conn;
$this->isExternalConnection = true;
return $this;
}
/* -( Implementation )----------------------------------------------------- */
protected function doLock($wait) {
$conn = $this->conn;
if (!$conn) {
// Try to reuse a connection from the connection pool.
$conn = array_pop(self::$pool);
}
if (!$conn) {
// NOTE: Using the 'repository' database somewhat arbitrarily, mostly
// because the first client of locks is the repository daemons. We must
// always use the same database for all locks, but don't access any
// tables so we could use any valid database. We could build a
// database-free connection instead, but that's kind of messy and we
// might forget about it in the future if we vertically partition the
// application.
$dao = new PhabricatorRepository();
// NOTE: Using "force_new" to make sure each lock is on its own
// connection.
$conn = $dao->establishConnection('w', $force_new = true);
}
// NOTE: Since MySQL will disconnect us if we're idle for too long, we set
// the wait_timeout to an enormous value, to allow us to hold the
// connection open indefinitely (or, at least, for 24 days).
$max_allowed_timeout = 2147483;
queryfx($conn, 'SET wait_timeout = %d', $max_allowed_timeout);
$lock_name = $this->getName();
$result = queryfx_one(
$conn,
'SELECT GET_LOCK(%s, %f)',
$lock_name,
$wait);
$ok = head($result);
if (!$ok) {
throw new PhutilLockException($lock_name);
}
$conn->rememberLock($lock_name);
$this->conn = $conn;
}
protected function doUnlock() {
$lock_name = $this->getName();
$conn = $this->conn;
try {
$result = queryfx_one(
$conn,
'SELECT RELEASE_LOCK(%s)',
$lock_name);
$conn->forgetLock($lock_name);
} catch (Exception $ex) {
$result = array(null);
}
$ok = head($result);
if (!$ok) {
// TODO: We could throw here, but then this lock doesn't get marked
// unlocked and we throw again later when exiting. It also doesn't
// particularly matter for any current applications. For now, just
// swallow the error.
}
$this->conn = null;
$this->isExternalConnection = false;
if (!$this->isExternalConnection) {
$conn->close();
self::$pool[] = $conn;
}
}
}

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