RFC: Invert DI Constructor Config Handling

[Stebalien]

The DI-based constructor currently has a bunch of options that take the config and then spit out some service.

I'd like to consider replacing these with a single super-option constructor that takes the config and then constructs the options necessary to start the node described in that config.

Rational:

• Ideally, applications using libp2p will configure libp2p through options instead of through a config file. This isn't currently possible
• I believe this will make it easier to reason about the config (centralizes everything).

---

Thoughts @magik6k?

[Stebalien]

For example, we'd replace options like:

func Pubsub(mctx MetricsCtx, lc fx.Lifecycle, host host.Host, cfg *config.Config) (service *pubsub.PubSub, err error) {
	var pubsubOptions []pubsub.Option
	if cfg.Pubsub.DisableSigning {
		pubsubOptions = append(pubsubOptions, pubsub.WithMessageSigning(false))
	}

	if cfg.Pubsub.StrictSignatureVerification {
		pubsubOptions = append(pubsubOptions, pubsub.WithStrictSignatureVerification(true))
	}

	switch cfg.Pubsub.Router {
	case "":
		fallthrough
	case "floodsub":
		service, err = pubsub.NewFloodSub(lifecycleCtx(mctx, lc), host, pubsubOptions...)

	case "gossipsub":
		service, err = pubsub.NewGossipSub(lifecycleCtx(mctx, lc), host, pubsubOptions...)

	default:
		err = fmt.Errorf("Unknown pubsub router %s", cfg.Pubsub.Router)
	}

	return service, err
}

With:

func FloodSub(pubsubOptions ...pubsub.Option) interface{} {
	return func (mctx MetricsCtx, lc fx.Lifecycle, host host.Host) (service *pubsub.PubSub, err error) {
		return pubsub.NewFloodSub(lifecycleCtx(mctx, lc), host, pubsubOptions...)
	}
}

func GossipSub(pubsubOptions ...pubsub.Option) interface{} {
	return func (mctx MetricsCtx, lc fx.Lifecycle, host host.Host) (service *pubsub.PubSub, err error) {
		return pubsub.NewGossipSub(lifecycleCtx(mctx, lc), host, pubsubOptions...)
	}
}

func FromConfig(cfg *config.Config) (fx.Option, error) {
	var opts []fx.Option

    // ...

	// Could live in a different function that _returns_ an `fx.Option` but it doesn't have to.
	var pubsubOptions []pubsub.Option
	if cfg.Pubsub.DisableSigning {
		pubsubOptions = append(pubsubOptions, pubsub.WithMessageSigning(false))
	}

	if cfg.Pubsub.StrictSignatureVerification {
		pubsubOptions = append(pubsubOptions, pubsub.WithStrictSignatureVerification(true))
	}


	switch cfg.Pubsub.Router {
	case "":
		fallthrough
	case "floodsub":
    	opts = append(opts, FloodSub(pubsubOptions))
	case "gossipsub":
    	opts = append(opts, GossipSub(pubsubOptions))
	default:
		return nil, fmt.Errorf("Unknown pubsub router %s", cfg.Pubsub.Router)
	}
    
    // ...

	return fx.Options(opts...), nil
}

That way we parse the config all at once but still start everything in the right order.