SqlBase source plugin class now implements RequirementsInterface, throws RequirementsException

SqlBase::getDatabase() falls back to using a database connection keyed as 'migrate' if no explicit configuration, or fallback_state_key, is provided. If it fell back to the 'migrate' key, and no such database connection was defined, it would formerly throw ConnectionNotDefinedException. This was particularly problematic with the migrate_drupal module enabled - the node derivers would cause this exception to be thrown on a general plugin manager getDefinitions() or createInstances(), preventing either API from returning even valid migrations if the derivers were in play.

SqlBase::getDatabase() will now throw RequirementsException when there is no explicit database connection configured and no implicit 'migrate' connection is present. In addition, SqlBase now implements RequirementsInterface, and provides a default checkRequirements() which calls getDatabase() and thus will throw RequirementsException if there is no 'migrate' connection. In situations where you may be invoking a SqlBase-based source plugin and cannot guarantee it has a valid connection, you should catch RequirementsException and proceed accordingly. For example, when invoking a source plugin in a deriver:

Before:

      foreach (static::getSourcePlugin('d7_node_type') as $row) {

After:

    $node_types = static::getSourcePlugin('d7_node_type');
    try {
      $node_types->checkRequirements();
    }
    catch (RequirementsException $e) {
      // If the d7_node_type requirements failed, that means we do not have a
      // Drupal source database configured - there is nothing to generate.
      return $this->derivatives;
    }
    foreach ($node_types as $row) {

If you have a source plugin derived from SqlBase in which you've implemented RequirementsInterface (and thus a checkRequirements() method), you should remove the dependency on RequirementsInterface and have your checkRequirements() method call its parent.

Before:

abstract class DrupalSqlBase extends SqlBase implements ContainerFactoryPluginInterface, RequirementsInterface, DependentPluginInterface {
...
  public function checkRequirements() {
    // Our own requirements checking...
   }

After:

abstract class DrupalSqlBase extends SqlBase implements ContainerFactoryPluginInterface, DependentPluginInterface {
...
  public function checkRequirements() {
    // Our own requirements checking...
    parent::checkRequirements();
   }