class DatabaseBackend

class DatabaseBackend implements CacheBackendInterface {
    
    /**
     * The default maximum number of rows that this cache bin table can store.
     *
     * This maximum is introduced to ensure that the database is not filled with
     * hundred of thousand of cache entries with gigabytes in size.
     *
     * Read about how to change it in the @link cache Cache API topic. @endlink
     */
    const DEFAULT_MAX_ROWS = 5000;
    
    /**
     * -1 means infinite allows numbers of rows for the cache backend.
     */
    const MAXIMUM_NONE = -1;
    
    /**
     * The maximum number of rows that this cache bin table is allowed to store.
     *
     * @see ::MAXIMUM_NONE
     *
     * @var int
     */
    protected $maxRows;
    
    /**
     * @var string
     */
    protected $bin;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * The cache tags checksum provider.
     *
     * @var \Drupal\Core\Cache\CacheTagsChecksumInterface
     */
    protected $checksumProvider;
    
    /**
     * Constructs a DatabaseBackend object.
     *
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection.
     * @param \Drupal\Core\Cache\CacheTagsChecksumInterface $checksum_provider
     *   The cache tags checksum provider.
     * @param string $bin
     *   The cache bin for which the object is created.
     * @param int $max_rows
     *   (optional) The maximum number of rows that are allowed in this cache bin
     *   table.
     */
    public function __construct(Connection $connection, CacheTagsChecksumInterface $checksum_provider, $bin, $max_rows = NULL) {
        // All cache tables should be prefixed with 'cache_'.
        $bin = 'cache_' . $bin;
        $this->bin = $bin;
        $this->connection = $connection;
        $this->checksumProvider = $checksum_provider;
        $this->maxRows = $max_rows === NULL ? static::DEFAULT_MAX_ROWS : $max_rows;
    }
    
    /**
     * {@inheritdoc}
     */
    public function get($cid, $allow_invalid = FALSE) {
        $cids = [
            $cid,
        ];
        $cache = $this->getMultiple($cids, $allow_invalid);
        return reset($cache);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getMultiple(&$cids, $allow_invalid = FALSE) {
        $cid_mapping = [];
        foreach ($cids as $cid) {
            $cid_mapping[$this->normalizeCid($cid)] = $cid;
        }
        // When serving cached pages, the overhead of using ::select() was found
        // to add around 30% overhead to the request. Since $this->bin is a
        // variable, this means the call to ::query() here uses a concatenated
        // string. This is highly discouraged under any other circumstances, and
        // is used here only due to the performance overhead we would incur
        // otherwise. When serving an uncached page, the overhead of using
        // ::select() is a much smaller proportion of the request.
        $result = [];
        try {
            $result = $this->connection
                ->query('SELECT [cid], [data], [created], [expire], [serialized], [tags], [checksum] FROM {' . $this->connection
                ->escapeTable($this->bin) . '} WHERE [cid] IN ( :cids[] ) ORDER BY [cid]', [
                ':cids[]' => array_keys($cid_mapping),
            ]);
        } catch (\Exception $e) {
            // Nothing to do.
        }
        $cache = [];
        foreach ($result as $item) {
            // Map the cache ID back to the original.
            $item->cid = $cid_mapping[$item->cid];
            $item = $this->prepareItem($item, $allow_invalid);
            if ($item) {
                $cache[$item->cid] = $item;
            }
        }
        $cids = array_diff($cids, array_keys($cache));
        return $cache;
    }
    
    /**
     * Prepares a cached item.
     *
     * Checks that items are either permanent or did not expire, and unserializes
     * data as appropriate.
     *
     * @param object $cache
     *   An item loaded from self::get() or self::getMultiple().
     * @param bool $allow_invalid
     *   If FALSE, the method returns FALSE if the cache item is not valid.
     *
     * @return mixed|false
     *   The item with data unserialized as appropriate and a property indicating
     *   whether the item is valid, or FALSE if there is no valid item to load.
     */
    protected function prepareItem($cache, $allow_invalid) {
        if (!isset($cache->data)) {
            return FALSE;
        }
        $cache->tags = $cache->tags ? explode(' ', $cache->tags) : [];
        // Check expire time.
        $cache->valid = $cache->expire == Cache::PERMANENT || $cache->expire >= REQUEST_TIME;
        // Check if invalidateTags() has been called with any of the item's tags.
        if (!$this->checksumProvider
            ->isValid($cache->checksum, $cache->tags)) {
            $cache->valid = FALSE;
        }
        if (!$allow_invalid && !$cache->valid) {
            return FALSE;
        }
        // Unserialize and return the cached data.
        if ($cache->serialized) {
            $cache->data = unserialize($cache->data);
        }
        return $cache;
    }
    
    /**
     * {@inheritdoc}
     */
    public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = []) {
        $this->setMultiple([
            $cid => [
                'data' => $data,
                'expire' => $expire,
                'tags' => $tags,
            ],
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function setMultiple(array $items) {
        $try_again = FALSE;
        try {
            // The bin might not yet exist.
            $this->doSetMultiple($items);
        } catch (\Exception $e) {
            // If there was an exception, try to create the bins.
            if (!($try_again = $this->ensureBinExists())) {
                // If the exception happened for other reason than the missing bin
                // table, propagate the exception.
                throw $e;
            }
        }
        // Now that the bin has been created, try again if necessary.
        if ($try_again) {
            $this->doSetMultiple($items);
        }
    }
    
    /**
     * Stores multiple items in the persistent cache.
     *
     * @param array $items
     *   An array of cache items, keyed by cid.
     *
     * @see \Drupal\Core\Cache\CacheBackendInterface::setMultiple()
     */
    protected function doSetMultiple(array $items) {
        $values = [];
        foreach ($items as $cid => $item) {
            $item += [
                'expire' => CacheBackendInterface::CACHE_PERMANENT,
                'tags' => [],
            ];
            assert(Inspector::assertAllStrings($item['tags']), 'Cache Tags must be strings.');
            $item['tags'] = array_unique($item['tags']);
            // Sort the cache tags so that they are stored consistently in the DB.
            sort($item['tags']);
            $fields = [
                'cid' => $this->normalizeCid($cid),
                'expire' => $item['expire'],
                'created' => round(microtime(TRUE), 3),
                'tags' => implode(' ', $item['tags']),
                'checksum' => $this->checksumProvider
                    ->getCurrentChecksum($item['tags']),
            ];
            // Avoid useless writes.
            if ($fields['checksum'] === CacheTagsChecksumInterface::INVALID_CHECKSUM_WHILE_IN_TRANSACTION) {
                continue;
            }
            if (!is_string($item['data'])) {
                $fields['data'] = serialize($item['data']);
                $fields['serialized'] = 1;
            }
            else {
                $fields['data'] = $item['data'];
                $fields['serialized'] = 0;
            }
            $values[] = $fields;
        }
        // If all $items were useless writes, we may end up with zero writes.
        if (empty($values)) {
            return;
        }
        // Use an upsert query which is atomic and optimized for multiple-row
        // merges.
        $query = $this->connection
            ->upsert($this->bin)
            ->key('cid')
            ->fields([
            'cid',
            'expire',
            'created',
            'tags',
            'checksum',
            'data',
            'serialized',
        ]);
        foreach ($values as $fields) {
            // Only pass the values since the order of $fields matches the order of
            // the insert fields. This is a performance optimization to avoid
            // unnecessary loops within the method.
            $query->values(array_values($fields));
        }
        $query->execute();
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete($cid) {
        $this->deleteMultiple([
            $cid,
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteMultiple(array $cids) {
        $cids = array_values(array_map([
            $this,
            'normalizeCid',
        ], $cids));
        try {
            // Delete in chunks when a large array is passed.
            foreach (array_chunk($cids, 1000) as $cids_chunk) {
                $this->connection
                    ->delete($this->bin)
                    ->condition('cid', $cids_chunk, 'IN')
                    ->execute();
            }
        } catch (\Exception $e) {
            // Create the cache table, which will be empty. This fixes cases during
            // core install where a cache table is cleared before it is set
            // with {cache_render} and {cache_data}.
            if (!$this->ensureBinExists()) {
                $this->catchException($e);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteAll() {
        try {
            $this->connection
                ->truncate($this->bin)
                ->execute();
        } catch (\Exception $e) {
            // Create the cache table, which will be empty. This fixes cases during
            // core install where a cache table is cleared before it is set
            // with {cache_render} and {cache_data}.
            if (!$this->ensureBinExists()) {
                $this->catchException($e);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidate($cid) {
        $this->invalidateMultiple([
            $cid,
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateMultiple(array $cids) {
        $cids = array_values(array_map([
            $this,
            'normalizeCid',
        ], $cids));
        try {
            // Update in chunks when a large array is passed.
            foreach (array_chunk($cids, 1000) as $cids_chunk) {
                $this->connection
                    ->update($this->bin)
                    ->fields([
                    'expire' => REQUEST_TIME - 1,
                ])
                    ->condition('cid', $cids_chunk, 'IN')
                    ->execute();
            }
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function invalidateAll() {
        try {
            $this->connection
                ->update($this->bin)
                ->fields([
                'expire' => REQUEST_TIME - 1,
            ])
                ->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function garbageCollection() {
        try {
            // Bounded size cache bin, using FIFO.
            if ($this->maxRows !== static::MAXIMUM_NONE) {
                $first_invalid_create_time = $this->connection
                    ->select($this->bin)
                    ->fields($this->bin, [
                    'created',
                ])
                    ->orderBy("{$this->bin}.created", 'DESC')
                    ->range($this->maxRows, 1)
                    ->execute()
                    ->fetchField();
                if ($first_invalid_create_time) {
                    $this->connection
                        ->delete($this->bin)
                        ->condition('created', $first_invalid_create_time, '<=')
                        ->execute();
                }
            }
            $this->connection
                ->delete($this->bin)
                ->condition('expire', Cache::PERMANENT, '<>')
                ->condition('expire', REQUEST_TIME, '<')
                ->execute();
        } catch (\Exception $e) {
            // If the table does not exist, it surely does not have garbage in it.
            // If the table exists, the next garbage collection will clean up.
            // There is nothing to do.
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function removeBin() {
        try {
            $this->connection
                ->schema()
                ->dropTable($this->bin);
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * Check if the cache bin exists and create it if not.
     */
    protected function ensureBinExists() {
        try {
            $database_schema = $this->connection
                ->schema();
            if (!$database_schema->tableExists($this->bin)) {
                $schema_definition = $this->schemaDefinition();
                $database_schema->createTable($this->bin, $schema_definition);
                return TRUE;
            }
        } catch (DatabaseException $e) {
            return TRUE;
        }
        return FALSE;
    }
    
    /**
     * Act on an exception when cache 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 cache is stale and the exception needs to
     * propagate.
     *
     * @param $e
     *   The exception.
     * @param string|null $table_name
     *   The table name. Defaults to $this->bin.
     *
     * @throws \Exception
     */
    protected function catchException(\Exception $e, $table_name = NULL) {
        if ($this->connection
            ->schema()
            ->tableExists($table_name ?: $this->bin)) {
            throw $e;
        }
    }
    
    /**
     * Normalizes a cache ID in order to comply with database limitations.
     *
     * @param string $cid
     *   The passed in cache ID.
     *
     * @return string
     *   An ASCII-encoded cache ID that is at most 255 characters long.
     */
    protected function normalizeCid($cid) {
        // Nothing to do if the ID is a US ASCII string of 255 characters or less.
        $cid_is_ascii = mb_check_encoding($cid, 'ASCII');
        if (strlen($cid) <= 255 && $cid_is_ascii) {
            return $cid;
        }
        // Return a string that uses as much as possible of the original cache ID
        // with the hash appended.
        $hash = Crypt::hashBase64($cid);
        if (!$cid_is_ascii) {
            return $hash;
        }
        return substr($cid, 0, 255 - strlen($hash)) . $hash;
    }
    
    /**
     * Defines the schema for the {cache_*} bin tables.
     *
     * @internal
     */
    public function schemaDefinition() {
        $schema = [
            'description' => 'Storage for the cache API.',
            'fields' => [
                'cid' => [
                    'description' => 'Primary Key: Unique cache ID.',
                    'type' => 'varchar_ascii',
                    'length' => 255,
                    'not null' => TRUE,
                    'default' => '',
                    'binary' => TRUE,
                ],
                'data' => [
                    'description' => 'A collection of data to cache.',
                    'type' => 'blob',
                    'not null' => FALSE,
                    'size' => 'big',
                ],
                'expire' => [
                    'description' => 'A Unix timestamp indicating when the cache entry should expire, or ' . Cache::PERMANENT . ' for never.',
                    'type' => 'int',
                    'not null' => TRUE,
                    'default' => 0,
                ],
                'created' => [
                    'description' => 'A timestamp with millisecond precision indicating when the cache entry was created.',
                    'type' => 'numeric',
                    'precision' => 14,
                    'scale' => 3,
                    'not null' => TRUE,
                    'default' => 0,
                ],
                'serialized' => [
                    'description' => 'A flag to indicate whether content is serialized (1) or not (0).',
                    'type' => 'int',
                    'size' => 'small',
                    'not null' => TRUE,
                    'default' => 0,
                ],
                'tags' => [
                    'description' => 'Space-separated list of cache tags for this entry.',
                    'type' => 'text',
                    'size' => 'big',
                    'not null' => FALSE,
                ],
                'checksum' => [
                    'description' => 'The tag invalidation checksum when this entry was saved.',
                    'type' => 'varchar_ascii',
                    'length' => 255,
                    'not null' => TRUE,
                ],
            ],
            'indexes' => [
                'expire' => [
                    'expire',
                ],
                'created' => [
                    'created',
                ],
            ],
            'primary key' => [
                'cid',
            ],
        ];
        return $schema;
    }
    
    /**
     * The maximum number of rows that this cache bin table is allowed to store.
     *
     * @return int
     */
    public function getMaxRows() {
        return $this->maxRows;
    }

}