Feature: support relay (proxy chains) (#539)

adapters/outboundgroup/relay.go

@@ -0,0 +1,84 @@
+package outboundgroup
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+
+	"github.com/Dreamacro/clash/adapters/outbound"
+	"github.com/Dreamacro/clash/adapters/provider"
+	"github.com/Dreamacro/clash/common/singledo"
+	"github.com/Dreamacro/clash/component/dialer"
+	C "github.com/Dreamacro/clash/constant"
+)
+
+type Relay struct {
+	*outbound.Base
+	single    *singledo.Single
+	providers []provider.ProxyProvider
+}
+
+func (r *Relay) DialContext(ctx context.Context, metadata *C.Metadata) (C.Conn, error) {
+	proxies := r.proxies()
+	if len(proxies) == 0 {
+		return nil, errors.New("Proxy does not exist")
+	}
+	first := proxies[0]
+	last := proxies[len(proxies)-1]
+
+	c, err := dialer.DialContext(ctx, "tcp", first.Addr())
+	if err != nil {
+		return nil, fmt.Errorf("%s connect error: %w", first.Addr(), err)
+	}
+	tcpKeepAlive(c)
+
+	var currentMeta *C.Metadata
+	for _, proxy := range proxies[1:] {
+		currentMeta, err = addrToMetadata(proxy.Addr())
+		if err != nil {
+			return nil, err
+		}
+
+		c, err = first.StreamConn(c, currentMeta)
+		if err != nil {
+			return nil, fmt.Errorf("%s connect error: %w", first.Addr(), err)
+		}
+
+		first = proxy
+	}
+
+	c, err = last.StreamConn(c, metadata)
+	if err != nil {
+		return nil, fmt.Errorf("%s connect error: %w", last.Addr(), err)
+	}
+
+	return outbound.NewConn(c, r), nil
+}
+
+func (r *Relay) MarshalJSON() ([]byte, error) {
+	var all []string
+	for _, proxy := range r.proxies() {
+		all = append(all, proxy.Name())
+	}
+	return json.Marshal(map[string]interface{}{
+		"type": r.Type().String(),
+		"all":  all,
+	})
+}
+
+func (r *Relay) proxies() []C.Proxy {
+	elm, _, _ := r.single.Do(func() (interface{}, error) {
+		return getProvidersProxies(r.providers), nil
+	})
+
+	return elm.([]C.Proxy)
+}
+
+func NewRelay(name string, providers []provider.ProxyProvider) *Relay {
+	return &Relay{
+		Base:      outbound.NewBase(name, "", C.Relay, false),
+		single:    singledo.NewSingle(defaultGetProxiesDuration),
+		providers: providers,
+	}
+}