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;
    }

}