Allow build command to work without pcntl

[kohenkatz]

PR #365 added a way to intercept cancellation of the build command and do some cleanup. PR #473 updated this to use Symfony's built-in SignalableCommandInterface, but also broke the command on some platforms. That code assumes that the pcntl extension is present when it references the SIGINT constant. This prevents the command from being run in Docker containers with a minimal set of extensions installed and on Windows.

Following the example of spatie/laravel-backup#1311, this PR checks for the presence of the SIGINT constant before attempting to use it.

[owenvoke]

I had assumed that Symfony itself handled the PCNTL check, but apparently not (I'm surprised). This looks like a good fix. I guess we could have alternatively checked for extension_loaded('pcntl'), but 🤷🏻

Thanks!

[kohenkatz]

I had assumed that Symfony itself handled the PCNTL check, but apparently not (I'm surprised).

I took a look through the Symfony code, and I found this:

class SignableCommand extends BaseSignableCommand implements SignalableCommandInterface
{
    public function getSubscribedSignals(): array
    {
        return SignalRegistry::isSupported() ? [\SIGUSR1] : [];
    }

    // ... [SNIP]
}

So it looks like they explicitly intend for users to check for support.

This looks like a good fix. I guess we could have alternatively checked for extension_loaded('pcntl'), but 🤷🏻

The definition of Symfony\Component\Console\SignalRegistry\SignalRegistry::isSupported() that is used in the Symfony test case is:

public static function isSupported(): bool
{
    return \function_exists('pcntl_signal');
}

In the end, there are probably several dozen equally valid ways to check it.

Thank you!