Select alive only node when fallbackTag is given

- Apply to random and roundrobin strategy
- Require observatory config

Co-authored-by: Mark Ma <38940419+mkmark@users.noreply.github.com>

app/router/balancing.go

@@ -4,6 +4,9 @@ import (
 	"context"
 	sync "sync"
 
+	"github.com/xtls/xray-core/app/observatory"
+	"github.com/xtls/xray-core/common"
+	"github.com/xtls/xray-core/core"
 	"github.com/xtls/xray-core/features/extension"
 	"github.com/xtls/xray-core/features/outbound"
 )
@@ -17,14 +20,58 @@ type BalancingPrincipleTarget interface {
 }
 
 type RoundRobinStrategy struct {
+	FallbackTag string
+
+	ctx         context.Context
+	observatory extension.Observatory
 	mu    sync.Mutex
 	index int
 }
 
+func (s *RoundRobinStrategy) InjectContext(ctx context.Context) {
+	s.ctx = ctx
+}
+
+func (s *RoundRobinStrategy) GetPrincipleTarget(strings []string) []string {
+	return strings
+}
+
 func (s *RoundRobinStrategy) PickOutbound(tags []string) string {
+	if len(s.FallbackTag) > 0 && s.observatory == nil {
+		common.Must(core.RequireFeatures(s.ctx, func(observatory extension.Observatory) error {
+			s.observatory = observatory
+			return nil
+		}))
+	}
+	if s.observatory != nil {
+		observeReport, err := s.observatory.GetObservation(s.ctx)
+		if err == nil {
+			aliveTags := make([]string, 0)
+			if result, ok := observeReport.(*observatory.ObservationResult); ok {
+				status := result.Status
+				statusMap := make(map[string]*observatory.OutboundStatus)
+				for _, outboundStatus := range status {
+					statusMap[outboundStatus.OutboundTag] = outboundStatus
+				}
+				for _, candidate := range tags {
+					if outboundStatus, found := statusMap[candidate]; found {
+						if outboundStatus.Alive {
+							aliveTags = append(aliveTags, candidate)
+						}
+					} else {
+						// unfound candidate is considered alive
+						aliveTags = append(aliveTags, candidate)
+					}
+				}
+				tags = aliveTags
+			}
+		}
+	}
+
 	n := len(tags)
 	if n == 0 {
-		panic("0 tags")
+		// goes to fallbackTag
+		return ""
 	}
 
 	s.mu.Lock()