class DrupalSqlBase

abstract class DrupalSqlBase extends SqlBase implements DependentPluginInterface {
    use DependencyTrait;
    
    /**
     * The contents of the system table.
     *
     * @var array
     */
    protected $systemData;
    
    /**
     * If the source provider is missing.
     *
     * @var bool
     */
    protected $requirements = TRUE;
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * {@inheritdoc}
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, StateInterface $state, EntityTypeManagerInterface $entity_type_manager) {
        parent::__construct($configuration, $plugin_id, $plugin_definition, $migration, $state);
        $this->entityTypeManager = $entity_type_manager;
    }
    
    /**
     * Retrieves all system data information from the source Drupal database.
     *
     * @return array
     *   List of system table information keyed by type and name.
     */
    public function getSystemData() {
        if (!isset($this->systemData)) {
            $this->systemData = [];
            try {
                $results = $this->select('system', 's')
                    ->fields('s')
                    ->execute();
                foreach ($results as $result) {
                    $this->systemData[$result['type']][$result['name']] = $result;
                }
            } catch (\Exception $e) {
                // The table might not exist for example in tests.
            }
        }
        return $this->systemData;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
        return new static($configuration, $plugin_id, $plugin_definition, $migration, $container->get('state'), $container->get('entity_type.manager'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function checkRequirements() {
        parent::checkRequirements();
        if ($this->pluginDefinition['requirements_met'] === TRUE) {
            if ($source_module = $this->getSourceModule()) {
                if ($this->moduleExists($source_module)) {
                    if (isset($this->pluginDefinition['minimum_version'])) {
                        $minimum_version = (int) $this->pluginDefinition['minimum_version'];
                        $installed_version = (int) $this->getModuleSchemaVersion($source_module);
                        if ($minimum_version > $installed_version) {
                            throw new RequirementsException('Required minimum version ' . $this->pluginDefinition['minimum_version'], [
                                'minimum_version' => $this->pluginDefinition['minimum_version'],
                            ]);
                        }
                    }
                }
                else {
                    throw new RequirementsException('The module ' . $source_module . ' is not enabled in the source site.', [
                        'source_module' => $source_module,
                    ]);
                }
            }
        }
    }
    
    /**
     * Retrieves a module schema_version from the source Drupal database.
     *
     * @param string $module
     *   Name of module.
     *
     * @return mixed
     *   The current module schema version on the origin system table or FALSE if
     *   not found.
     */
    protected function getModuleSchemaVersion($module) {
        $system_data = $this->getSystemData();
        return $system_data['module'][$module]['schema_version'] ?? FALSE;
    }
    
    /**
     * Checks if a given module is enabled in the source Drupal database.
     *
     * @param string $module
     *   Name of module to check.
     *
     * @return bool
     *   TRUE if module is enabled on the origin system, FALSE if not.
     */
    protected function moduleExists($module) {
        $system_data = $this->getSystemData();
        return !empty($system_data['module'][$module]['status']);
    }
    
    /**
     * Reads a variable from a source Drupal database.
     *
     * @param $name
     *   Name of the variable.
     * @param $default
     *   The default value.
     *
     * @return mixed
     */
    protected function variableGet($name, $default) {
        try {
            $result = $this->select('variable', 'v')
                ->fields('v', [
                'value',
            ])
                ->condition('name', $name)
                ->execute()
                ->fetchField();
        } catch (\Exception $e) {
            $result = FALSE;
        }
        return $result !== FALSE ? unserialize($result) : $default;
    }
    
    /**
     * {@inheritdoc}
     */
    public function calculateDependencies() {
        // Generic handling for Drupal source plugin constants.
        if (isset($this->configuration['constants']['entity_type'])) {
            $this->addDependency('module', $this->entityTypeManager
                ->getDefinition($this->configuration['constants']['entity_type'])
                ->getProvider());
        }
        if (isset($this->configuration['constants']['module'])) {
            $this->addDependency('module', $this->configuration['constants']['module']);
        }
        return $this->dependencies;
    }

}