UpdateCompilerPass.php

<?php

namespace Drupal\Core\Update;

use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\DependencyInjection\Reference;

/**
 * Removes services with unmet dependencies.
 *
 * Updates can install new modules that add services that existing services now
 * depend on. This compiler pass allows the update system to work in such cases.
 */
class UpdateCompilerPass implements CompilerPassInterface {
  
  /**
   * {@inheritdoc}
   */
  public function process(ContainerBuilder $container) {
    $process_aliases = FALSE;
    // Loop over the defined services and remove any with unmet dependencies.
    // The kernel cannot be booted if the container has such services. This
    // allows modules to run their update hooks to enable newly added
    // dependencies.
    do {
      $has_changed = FALSE;
      foreach ($container->getDefinitions() as $key => $definition) {
        // Ensure all the definition's arguments are valid.
        foreach ($definition->getArguments() as $argument) {
          if ($this->isArgumentMissingService($argument, $container)) {
            $container->removeDefinition($key);
            $container->log($this, sprintf('Removed service "%s"; reason: depends on non-existent service "%s".', $key, (string) $argument));
            $has_changed = TRUE;
            $process_aliases = TRUE;
            // Process the next definition.
            continue 2;
          }
        }
        // Ensure all the method call arguments are valid.
        foreach ($definition->getMethodCalls() as $call) {
          foreach ($call[1] as $argument) {
            if ($this->isArgumentMissingService($argument, $container)) {
              $container->removeDefinition($key);
              $container->log($this, sprintf('Removed service "%s"; reason: method call "%s" depends on non-existent service "%s".', $key, $call[0], (string) $argument));
              $has_changed = TRUE;
              $process_aliases = TRUE;
              // Process the next definition.
              continue 3;
            }
          }
        }
      }
      // Repeat if services have been removed.
    } while ($has_changed);
    // Remove aliases to services that have been removed. This does not need to
    // be part of the loop above because references to aliases have already been
    // resolved by Symfony's ResolveReferencesToAliasesPass.
    if ($process_aliases) {
      foreach ($container->getAliases() as $key => $alias) {
        $id = (string) $alias;
        if (!$container->has($id)) {
          $container->removeAlias($key);
          $container->log($this, sprintf('Removed alias "%s"; reason: alias to non-existent service "%s".', $key, $id));
        }
      }
    }
  }
  
  /**
   * Checks if a reference argument is to a missing service.
   *
   * @param mixed $argument
   *   The argument to check.
   * @param \Symfony\Component\DependencyInjection\ContainerBuilder $container
   *   The container.
   *
   * @return bool
   *   TRUE if the argument is a reference to a service that is missing from the
   *   container and the reference is required, FALSE if not.
   */
  private function isArgumentMissingService($argument, ContainerBuilder $container) {
    if ($argument instanceof Reference) {
      $argument_id = (string) $argument;
      if (!$container->has($argument_id) && $argument->getInvalidBehavior() === ContainerInterface::EXCEPTION_ON_INVALID_REFERENCE) {
        return TRUE;
      }
    }
    return FALSE;
  }

}