I updated the code to have a better design and DX:

• SignalRegistry::$handlingSignals is gone. This was a temporary data store to
  bind signal to symfony event. This does not belong to the SignalRegistry. It
  has been replaced by Application::$signalsToDispatchEvent. A method has been
  added to edit this list Application::setSignalsToDispatchEvent()

• The default value for Application::$signalsToDispatchEvent is SIGINT, SIGTERM, SIGUSR1, SIGUSR2. Theses defaults seems good enough for most of
  application. for recall:

  • SIGINT: CTRL+C
  • SIGTERM: kill PID, this is what occurs when stopping a process with SystemD, upstart & co
  • SIGUSR1 and SIGUSR2: Signal for user space

• Application::setSignalRegistry() is gone. Now the Application always owns a
  signal registry. Since it's a CLI, it's legit to always have support for
  signals. A method has been added for convenience, and to register signal
  handler manually from the command:

      $application->getSignalRegistry()->register(SIGINT, function ($signal) {
          dump("Signal ($signal) caught");
      });

• The interface SignalableCommandInterface has been added. When a command
  implements this interface, the command is automatically called when a
  registered signal has been caught

A note about the BC:

• If one register an handler before the Symfony ones, the Signal Registry keeps
  existing registered handlers. The BC is kept ✅
• If one register an handler after the Symfony ones, it overrides the Symfony
  behavior. Since the feature is new. the BC is kept ✅

So now, If one want to listen a signal, they have few options depending on the context:

• A global action is common to all commands (such as logging, enabling a
  profiler (👋 blackfire.io)): Use a listener. With autoconfigure, the following
  code is enough:

  class SignalSubscriber implements EventSubscriberInterface
  {
      private $logger;
  
      public function __construct(LoggerInterface $logger = null)
      {
          $this->logger = $logger ?: new NullLogger();
      }
  
      public function handleSignal(ConsoleSignalEvent $event)
      {
          $signal = $event->getHandlingSignal();
  
          $this->logger->debug('The application has been signaled', [
              'signal' => $signal,
          ]);
      }
  
      public static function getSubscribedEvents()
      {
          return [
              ConsoleEvents::SIGNAL => 'handleSignal',
          ];
      }
  }

• The command should react to a signal: Implements the interface:

  class SignalCommand extends Command implements SignalableCommandInterface
  {
      protected static $defaultName = 'signal';
  
      private $shouldStop = false;
  
      protected function execute(InputInterface $input, OutputInterface $output): int
      {
          $output->writeln('hello, I am '. getmypid());
  
          for ($i=0; $i < 60; $i++) {
              if ($this->shouldStop) {
                  break;
              }
  
              $output->write('.');
  
              sleep(1);
          }
          $output->writeln('');
  
          $output->writeln('bye');
  
          return 0;
      }
  
      public function handleSignal(int $signal)
      {
          dump([__METHOD__, $signal]);
  
          $this->shouldStop = true;
      }
  
      public function getSubscribedSignals(): array
      {
          return [SIGINT];
      }
  }

• The command should react differently to many event and/or one wants a full control on name of the method called

  class SignalCommand extends Command
  {
      protected static $defaultName = 'signal';
  
      private $shouldStop = false;
  
      protected function execute(InputInterface $input, OutputInterface $output): int
      {
          $this->getApplication()->getSignalRegistry()->register(SIGINT, [$this, 'stop']);
          $this->getApplication()->getSignalRegistry()->register(SIGUSR1, [$this, 'enableBlackfire']);
  
          $output->writeln('hello, I am '. getmypid());
  
          for ($i=0; $i < 60; $i++) {
              if ($this->shouldStop) {
                  break;
              }
  
              $output->write('.');
  
              sleep(1);
          }
          $output->writeln('');
  
          $output->writeln('bye');
  
          return 0;
      }
  
      public function stop()
      {
          $this->shouldStop = true;
      }
  
      public function enableBlackfire()
      {
          // ...
      }
  }

ping @marie