feat(gost): add debug logging for failover mechanism analysis

Add debug logs to trace failover behavior:
- FailFilter.Filter(): log node name, fail count, maxFails, timeSince, failTimeout
- hop.Select(): log excludeNodes list, node selection results
- handler retry loop: log maxRetries, selected nodes, dial failures

This helps diagnose issues where failover between multiple target nodes
is not working as expected.

go-gost/x/handler/forward/local/handler.go

@@ -192,6 +192,8 @@ func (h *forwardHandler) Handle(ctx context.Context, conn net.Conn, opts ...hand
 	var lastErr error
 	var cc net.Conn
 
+	h.options.Logger.Debugf("[handler.retry] starting retry loop: maxRetries=%d", maxRetries)
+
 	for attempt := 0; attempt < maxRetries; attempt++ {
 		// Select a target node, excluding previously tried nodes
 		selectCtx := ctxvalue.ContextWithExcludeNodes(ctx, triedNodes)
@@ -202,12 +204,15 @@ func (h *forwardHandler) Handle(ctx context.Context, conn net.Conn, opts ...hand
 			)
 		}
 		if target == nil {
+			h.options.Logger.Debugf("[handler.retry] attempt=%d target=nil, triedNodes=%v", attempt, triedNodes)
 			if lastErr != nil {
 				return lastErr
 			}
 			return errors.New("node not available")
 		}
 
+		h.options.Logger.Debugf("[handler.retry] attempt=%d selected node=%s addr=%s", attempt, target.Name, target.Addr)
+
 		// Track this node as tried
 		triedNodes = append(triedNodes, target.Addr)
 
@@ -233,6 +238,8 @@ func (h *forwardHandler) Handle(ctx context.Context, conn net.Conn, opts ...hand
 			// Mark node as failed for future selections
 			if marker := target.Marker(); marker != nil {
 				marker.Mark()
+				h.options.Logger.Debugf("[handler.retry] attempt=%d dial failed, marked node=%s count=%d err=%v",
+					attempt, target.Addr, marker.Count(), err)
 			}
 			lastErr = err
 			// Try next node

go-gost/x/hop/hop.go

@@ -149,6 +149,9 @@ func (p *chainHop) Select(ctx context.Context, opts ...hop.SelectOption) *chain.
 		excludeSet[addr] = true
 	}
 
+	// Debug logging for failover analysis
+	log.Debugf("[hop.Select] excludeNodes=%v, totalNodes=%d", excludeNodes, len(p.Nodes()))
+
 	var nodes []*chain.Node
 	for _, node := range p.Nodes() {
 		if node == nil {
@@ -213,7 +216,9 @@ func (p *chainHop) Select(ctx context.Context, opts ...hop.SelectOption) *chain.
 	// FailFilter will exclude recently-failed nodes, allowing traffic to
 	// be routed to healthy alternatives.
 	if s := p.options.selector; s != nil {
+		log.Debugf("[hop.Select] calling selector.Select with %d nodes", len(nodes))
 		if node := s.Select(ctx, nodes...); node != nil {
+			log.Debugf("[hop.Select] selected node=%s addr=%s", node.Name, node.Addr)
 			return node
 		}
 		// All nodes filtered out by FailFilter - all are marked as failed.

go-gost/x/selector/filter.go

@@ -2,8 +2,10 @@ package selector
 
 import (
 	"context"
+	"fmt"
 	"time"
 
+	"github.com/go-gost/core/chain"
 	"github.com/go-gost/core/metadata"
 	"github.com/go-gost/core/selector"
 	mdutil "github.com/go-gost/x/metadata/util"
@@ -53,8 +55,21 @@ func (f *failFilter[T]) Filter(ctx context.Context, vs ...T) []T {
 
 		if mi, _ := any(v).(selector.Markable); mi != nil {
 			if marker := mi.Marker(); marker != nil {
-				if marker.Count() < int64(maxFails) ||
-					time.Since(marker.Time()) >= failTimeout {
+				count := marker.Count()
+				timeSince := time.Since(marker.Time())
+				passed := count < int64(maxFails) || timeSince >= failTimeout
+
+				// Debug logging for failover analysis
+				nodeName := "unknown"
+				nodeAddr := "unknown"
+				if node, ok := any(v).(*chain.Node); ok {
+					nodeName = node.Name
+					nodeAddr = node.Addr
+				}
+				fmt.Printf("[FailFilter] node=%s addr=%s count=%d maxFails=%d timeSince=%v failTimeout=%v passed=%v\n",
+					nodeName, nodeAddr, count, maxFails, timeSince, failTimeout, passed)
+
+				if passed {
 					l = append(l, v)
 				}
 				continue