DatabaseLockBackend.php

<?php

namespace Drupal\Core\Lock;

use Drupal\Component\Utility\Crypt;
use Drupal\Core\Database\Connection;
use Drupal\Core\Database\DatabaseException;
use Drupal\Core\Database\IntegrityConstraintViolationException;

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

}