[9.x] Add support for lazy loading commands

[paras-malhotra]

Currently, all commands in Laravel have to be instantiated to be registered in the console application. That could be a problem for commands with big dependency graphs as all the dependencies are loaded in memory. See laravel/ideas#1399.

Symfony has had lazy loading support for commands since version 3.4. This PR extends Symfony's lazy loading to Laravel. To allow for lazy loading, Laravel package console commands and/or app console commands just need to set the static $defaultName property like so:

/**
 * The default command name for lazy loading.
 *
 * @var string|null
*/
protected static $defaultName = 'command:name';

Since the PR changes the return type of Application::resolve (adds a null return type), I'm sending this to master instead of 8x. If this PR is accepted, I can send across another PR to lazy load the commands shipped with Laravel.

[taylorotwell]

@paras-malhotra it seems like we could make the getDefaultName() method have a default implementation in our console command that gets $signature before the first white-space. That way it wouldn't typically need to be defined in commands? Is that a bad idea?

[paras-malhotra]

@taylorotwell the signature is a non-static variable and that's the real issue here (cannot be accessed from static methods). The getDefaultName is a static method and it's structured that way in Symfony so that there's no need to instantiate the commands to access the default name.

[taylorotwell]

Thanks. Does it make sense to add a defaultName to some of Laravel's own heavier commands?

[paras-malhotra]

Yes @taylorotwell, I'll send across a follow-up PR to add default names to the commands shipped with Laravel. That should be a nice performance boost.

[garygreen]

@paras-malhotra does this actually work? It still has to resolve all of the console commands in order to get the name:

            $this->artisan = (new Artisan($this->app, $this->events, $this->app->version()))
                                    ->resolveCommands($this->commands)
                                    ->setContainerCommandLoader();

... then when resolvedCommands is called:

    /**
     * Add a command, resolving through the application.
     *
     * @param  string  $command
     * @return \Symfony\Component\Console\Command\Command|null
     */
    public function resolve($command)
    {
        if (class_exists($command) && ($commandName = $command::getDefaultName())) {
            $this->commandMap[$commandName] = $command;

            return null;
        }

        return $this->add($this->laravel->make($command));
    }

It calls $command::getDefaultName() which then LOADS the class in memory. It doesn't matter that it's a static function or not, it still has to load up the class which then consumes memory?

[garygreen]

I just tested with a large class based class file and when making static calls to a class it does indeed load it in memory as can be observed with get_declared_classes and memory_get_usage shows it's using memory.

It's likely still much lower than actually resolving the classes from the container, but still room for optimization so the classes aren't loaded from disk at all. Though I'm not sure how this plays with OpCache as it may have those classes in memory already.