go-ethereumを読む(6) 補足 Service

gethではPluggableな実装になっており、serviceという単位でプログラム内では扱われています。
EthServiceがEthereumのコア機能をになっています。
v1.8.15ベースに解説します

具体例 サービスを登録しているプログラム

// cmd/utils/flags.go
// RegisterEthService adds an Ethereum client to the stack.
func RegisterEthService(stack *node.Node, cfg *eth.Config) {
	var err error
	if cfg.SyncMode == downloader.LightSync {
		err = stack.Register(func(ctx *node.ServiceContext) (node.Service, error) {
			return les.New(ctx, cfg)
		})
	} else {
		err = stack.Register(func(ctx *node.ServiceContext) (node.Service, error) {
			fullNode, err := eth.New(ctx, cfg)
			if fullNode != nil && cfg.LightServ > 0 {
				ls, _ := les.NewLesServer(fullNode, cfg)
				fullNode.AddLesServer(ls)
			}
			return fullNode, err
		})
	}
	if err != nil {
		Fatalf("Failed to register the Ethereum service: %v", err)
	}
}

// RegisterDashboardService adds a dashboard to the stack.
func RegisterDashboardService(stack *node.Node, cfg *dashboard.Config, commit string) {
	stack.Register(func(ctx *node.ServiceContext) (node.Service, error) {
		return dashboard.New(cfg, commit, ctx.ResolvePath("logs")), nil
	})
}

// RegisterShhService configures Whisper and adds it to the given node.
func RegisterShhService(stack *node.Node, cfg *whisper.Config) {
	if err := stack.Register(func(n *node.ServiceContext) (node.Service, error) {
		return whisper.New(cfg), nil
	}); err != nil {
		Fatalf("Failed to register the Whisper service: %v", err)
	}
}

ServiceContext

ServiceContextがservicesをmapで保持している

type ServiceContext struct {
	config         *Config
	services       map[reflect.Type]Service // Index of the already constructed services
	EventMux       *event.TypeMux           // Event multiplexer used for decoupled notifications
	AccountManager *accounts.Manager        // Account manager created by the node.
}

func (ctx *ServiceContext) Service(service interface{}) error {
	element := reflect.ValueOf(service).Elem()
	if running, ok := ctx.services[element.Type()]; ok {
		element.Set(reflect.ValueOf(running))
		return nil
	}
	return ErrServiceUnknown
}

type ServiceConstructor func(ctx *ServiceContext) (Service, error)

// Serviceに必要なinterface
type Service interface {
	// Protocols retrieves the P2P protocols the service wishes to start.
	Protocols() []p2p.Protocol

	// APIs retrieves the list of RPC descriptors the service provides
	APIs() []rpc.API

	// Start is called after all services have been constructed and the networking
	// layer was also initialized to spawn any goroutines required by the service.
	Start(server *p2p.Server) error

	// Stop terminates all goroutines belonging to the service, blocking until they
	// are all terminated.
	Stop() error
}