class DatabaseCacheTagsChecksum

class DatabaseCacheTagsChecksum implements CacheTagsChecksumInterface, CacheTagsInvalidatorInterface {
    use CacheTagsChecksumTrait;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * Constructs a DatabaseCacheTagsChecksum object.
     *
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection.
     */
    public function __construct(Connection $connection) {
        $this->connection = $connection;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function doInvalidateTags(array $tags) {
        try {
            foreach ($tags as $tag) {
                $this->connection
                    ->merge('cachetags')
                    ->insertFields([
                    'invalidations' => 1,
                ])
                    ->expression('invalidations', 'invalidations + 1')
                    ->key('tag', $tag)
                    ->execute();
            }
        } catch (\Exception $e) {
            // Create the cache table, which will be empty. This fixes cases during
            // core install where cache tags are invalidated before the table is
            // created.
            if (!$this->ensureTableExists()) {
                $this->catchException($e);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getTagInvalidationCounts(array $tags) {
        try {
            return $this->connection
                ->query('SELECT tag, invalidations FROM {cachetags} WHERE tag IN ( :tags[] )', [
                ':tags[]' => $tags,
            ])
                ->fetchAllKeyed();
        } catch (\Exception $e) {
            // If the table does not exist yet, create.
            if (!$this->ensureTableExists()) {
                $this->catchException($e);
            }
        }
        return [];
    }
    
    /**
     * Check if the cache tags table exists and create it if not.
     */
    protected function ensureTableExists() {
        try {
            $database_schema = $this->connection
                ->schema();
            // Create the cache tags table if it does not exist.
            if (!$database_schema->tableExists('cachetags')) {
                $schema_definition = $this->schemaDefinition();
                $database_schema->createTable('cachetags', $schema_definition);
                return TRUE;
            }
        } catch (DatabaseException $e) {
            return TRUE;
        }
        return FALSE;
    }
    
    /**
     * Defines the schema for the {cachetags} table.
     *
     * @internal
     */
    public function schemaDefinition() {
        $schema = [
            'description' => 'Cache table for tracking cache tag invalidations.',
            'fields' => [
                'tag' => [
                    'description' => 'Namespace-prefixed tag string.',
                    'type' => 'varchar_ascii',
                    'length' => 255,
                    'not null' => TRUE,
                    'default' => '',
                ],
                'invalidations' => [
                    'description' => 'Number incremented when the tag is invalidated.',
                    'type' => 'int',
                    'not null' => TRUE,
                    'default' => 0,
                ],
            ],
            'primary key' => [
                'tag',
            ],
        ];
        return $schema;
    }
    
    /**
     * Act on an exception when cache might be stale.
     *
     * If the {cachetags} table does not yet exist, that's fine but if the table
     * exists and yet the query failed, then the cache is stale and the
     * exception needs to propagate.
     *
     * @param \Exception $e
     *   The exception.
     *
     * @throws \Exception
     */
    protected function catchException(\Exception $e) {
        if ($this->connection
            ->schema()
            ->tableExists('cachetags')) {
            throw $e;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDatabaseConnection() {
        return $this->connection;
    }

}