Chris@0: <?php
Chris@0: 
Chris@0: namespace Drupal\Core\Lock;
Chris@0: 
Chris@0: use Drupal\Component\Utility\Crypt;
Chris@0: use Drupal\Core\Database\Connection;
Chris@0: use Drupal\Core\Database\IntegrityConstraintViolationException;
Chris@0: use Drupal\Core\Database\SchemaObjectExistsException;
Chris@0: 
Chris@0: /**
Chris@0:  * Defines the database lock backend. This is the default backend in Drupal.
Chris@0:  *
Chris@0:  * @ingroup lock
Chris@0:  */
Chris@0: class DatabaseLockBackend extends LockBackendAbstract {
Chris@0: 
Chris@0:   /**
Chris@0:    * The database table name.
Chris@0:    */
Chris@0:   const TABLE_NAME = 'semaphore';
Chris@0: 
Chris@0:   /**
Chris@0:    * The database connection.
Chris@0:    *
Chris@0:    * @var \Drupal\Core\Database\Connection
Chris@0:    */
Chris@0:   protected $database;
Chris@0: 
Chris@0:   /**
Chris@0:    * Constructs a new DatabaseLockBackend.
Chris@0:    *
Chris@0:    * @param \Drupal\Core\Database\Connection $database
Chris@0:    *   The database connection.
Chris@0:    */
Chris@0:   public function __construct(Connection $database) {
Chris@0:     // __destruct() is causing problems with garbage collections, register a
Chris@0:     // shutdown function instead.
Chris@0:     drupal_register_shutdown_function([$this, 'releaseAll']);
Chris@0:     $this->database = $database;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function acquire($name, $timeout = 30.0) {
Chris@0:     $name = $this->normalizeName($name);
Chris@0: 
Chris@0:     // Insure that the timeout is at least 1 ms.
Chris@0:     $timeout = max($timeout, 0.001);
Chris@0:     $expire = microtime(TRUE) + $timeout;
Chris@0:     if (isset($this->locks[$name])) {
Chris@0:       // Try to extend the expiration of a lock we already acquired.
Chris@0:       $success = (bool) $this->database->update('semaphore')
Chris@0:         ->fields(['expire' => $expire])
Chris@0:         ->condition('name', $name)
Chris@0:         ->condition('value', $this->getLockId())
Chris@0:         ->execute();
Chris@0:       if (!$success) {
Chris@0:         // The lock was broken.
Chris@0:         unset($this->locks[$name]);
Chris@0:       }
Chris@0:       return $success;
Chris@0:     }
Chris@0:     else {
Chris@0:       // Optimistically try to acquire the lock, then retry once if it fails.
Chris@0:       // The first time through the loop cannot be a retry.
Chris@0:       $retry = FALSE;
Chris@0:       // We always want to do this code at least once.
Chris@0:       do {
Chris@0:         try {
Chris@0:           $this->database->insert('semaphore')
Chris@0:             ->fields([
Chris@0:               'name' => $name,
Chris@0:               'value' => $this->getLockId(),
Chris@0:               'expire' => $expire,
Chris@0:             ])
Chris@0:             ->execute();
Chris@0:           // We track all acquired locks in the global variable.
Chris@0:           $this->locks[$name] = TRUE;
Chris@0:           // We never need to try again.
Chris@0:           $retry = FALSE;
Chris@0:         }
Chris@0:         catch (IntegrityConstraintViolationException $e) {
Chris@0:           // Suppress the error. If this is our first pass through the loop,
Chris@0:           // then $retry is FALSE. In this case, the insert failed because some
Chris@0:           // other request acquired the lock but did not release it. We decide
Chris@0:           // whether to retry by checking lockMayBeAvailable(). This will clear
Chris@0:           // the offending row from the database table in case it has expired.
Chris@0:           $retry = $retry ? FALSE : $this->lockMayBeAvailable($name);
Chris@0:         }
Chris@0:         catch (\Exception $e) {
Chris@0:           // Create the semaphore table if it does not exist and retry.
Chris@0:           if ($this->ensureTableExists()) {
Chris@0:             // Retry only once.
Chris@0:             $retry = !$retry;
Chris@0:           }
Chris@0:           else {
Chris@0:             throw $e;
Chris@0:           }
Chris@0:         }
Chris@0:         // We only retry in case the first attempt failed, but we then broke
Chris@0:         // an expired lock.
Chris@0:       } while ($retry);
Chris@0:     }
Chris@0:     return isset($this->locks[$name]);
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function lockMayBeAvailable($name) {
Chris@0:     $name = $this->normalizeName($name);
Chris@0: 
Chris@0:     try {
Chris@0:       $lock = $this->database->query('SELECT expire, value FROM {semaphore} WHERE name = :name', [':name' => $name])->fetchAssoc();
Chris@0:     }
Chris@0:     catch (\Exception $e) {
Chris@0:       $this->catchException($e);
Chris@0:       // If the table does not exist yet then the lock may be available.
Chris@0:       $lock = FALSE;
Chris@0:     }
Chris@0:     if (!$lock) {
Chris@0:       return TRUE;
Chris@0:     }
Chris@0:     $expire = (float) $lock['expire'];
Chris@0:     $now = microtime(TRUE);
Chris@0:     if ($now > $expire) {
Chris@0:       // We check two conditions to prevent a race condition where another
Chris@0:       // request acquired the lock and set a new expire time. We add a small
Chris@0:       // number to $expire to avoid errors with float to string conversion.
Chris@0:       return (bool) $this->database->delete('semaphore')
Chris@0:         ->condition('name', $name)
Chris@0:         ->condition('value', $lock['value'])
Chris@0:         ->condition('expire', 0.0001 + $expire, '<=')
Chris@0:         ->execute();
Chris@0:     }
Chris@0:     return FALSE;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function release($name) {
Chris@0:     $name = $this->normalizeName($name);
Chris@0: 
Chris@0:     unset($this->locks[$name]);
Chris@0:     try {
Chris@0:       $this->database->delete('semaphore')
Chris@0:         ->condition('name', $name)
Chris@0:         ->condition('value', $this->getLockId())
Chris@0:         ->execute();
Chris@0:     }
Chris@0:     catch (\Exception $e) {
Chris@0:       $this->catchException($e);
Chris@0:     }
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * {@inheritdoc}
Chris@0:    */
Chris@0:   public function releaseAll($lock_id = NULL) {
Chris@0:     // Only attempt to release locks if any were acquired.
Chris@0:     if (!empty($this->locks)) {
Chris@0:       $this->locks = [];
Chris@0:       if (empty($lock_id)) {
Chris@0:         $lock_id = $this->getLockId();
Chris@0:       }
Chris@0:       $this->database->delete('semaphore')
Chris@0:         ->condition('value', $lock_id)
Chris@0:         ->execute();
Chris@0:     }
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * Check if the semaphore table exists and create it if not.
Chris@0:    */
Chris@0:   protected function ensureTableExists() {
Chris@0:     try {
Chris@0:       $database_schema = $this->database->schema();
Chris@0:       if (!$database_schema->tableExists(static::TABLE_NAME)) {
Chris@0:         $schema_definition = $this->schemaDefinition();
Chris@0:         $database_schema->createTable(static::TABLE_NAME, $schema_definition);
Chris@0:         return TRUE;
Chris@0:       }
Chris@0:     }
Chris@0:     // If another process has already created the semaphore table, attempting to
Chris@0:     // recreate it will throw an exception. In this case just catch the
Chris@0:     // exception and do nothing.
Chris@0:     catch (SchemaObjectExistsException $e) {
Chris@0:       return TRUE;
Chris@0:     }
Chris@0:     return FALSE;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * Act on an exception when semaphore might be stale.
Chris@0:    *
Chris@0:    * If the table does not yet exist, that's fine, but if the table exists and
Chris@0:    * yet the query failed, then the semaphore is stale and the exception needs
Chris@0:    * to propagate.
Chris@0:    *
Chris@0:    * @param $e
Chris@0:    *   The exception.
Chris@0:    *
Chris@0:    * @throws \Exception
Chris@0:    */
Chris@0:   protected function catchException(\Exception $e) {
Chris@0:     if ($this->database->schema()->tableExists(static::TABLE_NAME)) {
Chris@0:       throw $e;
Chris@0:     }
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * Normalizes a lock name in order to comply with database limitations.
Chris@0:    *
Chris@0:    * @param string $name
Chris@0:    *   The passed in lock name.
Chris@0:    *
Chris@0:    * @return string
Chris@0:    *   An ASCII-encoded lock name that is at most 255 characters long.
Chris@0:    */
Chris@0:   protected function normalizeName($name) {
Chris@0:     // Nothing to do if the name is a US ASCII string of 255 characters or less.
Chris@0:     $name_is_ascii = mb_check_encoding($name, 'ASCII');
Chris@0: 
Chris@0:     if (strlen($name) <= 255 && $name_is_ascii) {
Chris@0:       return $name;
Chris@0:     }
Chris@0:     // Return a string that uses as much as possible of the original name with
Chris@0:     // the hash appended.
Chris@0:     $hash = Crypt::hashBase64($name);
Chris@0: 
Chris@0:     if (!$name_is_ascii) {
Chris@0:       return $hash;
Chris@0:     }
Chris@0: 
Chris@0:     return substr($name, 0, 255 - strlen($hash)) . $hash;
Chris@0:   }
Chris@0: 
Chris@0:   /**
Chris@0:    * Defines the schema for the semaphore table.
Chris@0:    *
Chris@0:    * @internal
Chris@0:    */
Chris@0:   public function schemaDefinition() {
Chris@0:     return [
Chris@0:       'description' => 'Table for holding semaphores, locks, flags, etc. that cannot be stored as state since they must not be cached.',
Chris@0:       'fields' => [
Chris@0:         'name' => [
Chris@0:           'description' => 'Primary Key: Unique name.',
Chris@0:           'type' => 'varchar_ascii',
Chris@0:           'length' => 255,
Chris@0:           'not null' => TRUE,
Chris@17:           'default' => '',
Chris@0:         ],
Chris@0:         'value' => [
Chris@0:           'description' => 'A value for the semaphore.',
Chris@0:           'type' => 'varchar_ascii',
Chris@0:           'length' => 255,
Chris@0:           'not null' => TRUE,
Chris@17:           'default' => '',
Chris@0:         ],
Chris@0:         'expire' => [
Chris@0:           'description' => 'A Unix timestamp with microseconds indicating when the semaphore should expire.',
Chris@0:           'type' => 'float',
Chris@0:           'size' => 'big',
Chris@17:           'not null' => TRUE,
Chris@0:         ],
Chris@0:       ],
Chris@0:       'indexes' => [
Chris@0:         'value' => ['value'],
Chris@0:         'expire' => ['expire'],
Chris@0:       ],
Chris@0:       'primary key' => ['name'],
Chris@0:     ];
Chris@0:   }
Chris@0: 
Chris@0: }