Add best exit selection (#486)

* docs: add best exit selection design

* feat: add best exit selection

go-gost/x/registry/chain.go

@@ -12,10 +12,25 @@ type chainRegistry struct {
 	registry[chain.Chainer]
 }
 
+func ReplaceChain(name string, v chain.Chainer) error {
+	if name == "" {
+		return nil
+	}
+	if r, ok := chainReg.(*chainRegistry); ok {
+		r.replace(name, v)
+		return nil
+	}
+	return chainReg.Register(name, v)
+}
+
 func (r *chainRegistry) Register(name string, v chain.Chainer) error {
 	return r.registry.Register(name, v)
 }
 
+func (r *chainRegistry) replace(name string, v chain.Chainer) {
+	r.m.Store(name, v)
+}
+
 func (r *chainRegistry) Get(name string) chain.Chainer {
 	if name != "" {
 		return &chainWrapper{name: name, r: r}

go-gost/x/registry/chain_test.go

@@ -0,0 +1,51 @@
+package registry
+
+import (
+	"context"
+	"net"
+	"testing"
+
+	"github.com/go-gost/core/chain"
+)
+
+type testChainer struct {
+	route chain.Route
+}
+
+func (c testChainer) Route(context.Context, string, string, ...chain.RouteOption) chain.Route {
+	return c.route
+}
+
+type testRoute struct {
+	nodes []*chain.Node
+}
+
+func (r testRoute) Dial(context.Context, string, string, ...chain.DialOption) (net.Conn, error) {
+	return nil, nil
+}
+
+func (r testRoute) Bind(context.Context, string, string, ...chain.BindOption) (net.Listener, error) {
+	return nil, nil
+}
+
+func (r testRoute) Nodes() []*chain.Node {
+	return r.nodes
+}
+
+func TestReplaceChainOverwritesExistingRegistration(t *testing.T) {
+	name := "replace_chain_tdd"
+	ChainRegistry().Unregister(name)
+	defer ChainRegistry().Unregister(name)
+
+	if err := ChainRegistry().Register(name, testChainer{route: testRoute{nodes: []*chain.Node{{Name: "old"}}}}); err != nil {
+		t.Fatalf("register old chain: %v", err)
+	}
+	if err := ReplaceChain(name, testChainer{route: testRoute{nodes: []*chain.Node{{Name: "new"}}}}); err != nil {
+		t.Fatalf("replace chain: %v", err)
+	}
+
+	route := ChainRegistry().Get(name).Route(context.Background(), "tcp", "example.com:443")
+	if route == nil || len(route.Nodes()) != 1 || route.Nodes()[0].Name != "new" {
+		t.Fatalf("expected replacement chain route, got %#v", route)
+	}
+}