eth/protocols/snap: add stateless peer cooldown for partial state mode

The statelessPeers map permanently blacklists peers that return empty
responses for the entire Sync() cycle. In partial state mode, the faster
account advancement (due to skipping storage/code for non-tracked
contracts) creates bursty request patterns that can trigger transient
empty responses. Combined with the permanent blacklist, this causes a
cascade where all peers get banned and sync stalls permanently.

Replace the permanent map[string]struct{} with map[string]time.Time to
track when each peer was marked. For partial state mode, peers are given
a 30-second cooldown instead of permanent banishment. After the cooldown
expires, the peer is eligible for task assignment again. Full sync mode
behavior is unchanged (permanent blacklist preserved).

eth/protocols/snap/sync.go

@@ -97,6 +97,11 @@ const (
 
 	// batchSizeThreshold is the maximum size allowed for gentrie batch.
 	batchSizeThreshold = 8 * 1024 * 1024
+
+	// statelessCooldown is how long a peer that returned empty responses is
+	// excluded from task assignment in partial state mode. In full sync mode,
+	// stateless marking remains permanent (cooldown is not checked).
+	statelessCooldown = 30 * time.Second
 )
 
 var (
@@ -463,7 +468,7 @@ type Syncer struct {
 	rates    *msgrate.Trackers   // Message throughput rates for peers
 
 	// Request tracking during syncing phase
-	statelessPeers map[string]struct{} // Peers that failed to deliver state data
+	statelessPeers map[string]time.Time // Peers that failed to deliver state data (value = when marked)
 	accountIdlers  map[string]struct{} // Peers that aren't serving account requests
 	bytecodeIdlers map[string]struct{} // Peers that aren't serving bytecode requests
 	storageIdlers  map[string]struct{} // Peers that aren't serving storage requests
@@ -645,7 +650,7 @@ func (s *Syncer) Sync(root common.Hash, cancel chan struct{}) error {
 		trieTasks: make(map[string]common.Hash),
 		codeTasks: make(map[common.Hash]struct{}),
 	}
-	s.statelessPeers = make(map[string]struct{})
+	s.statelessPeers = make(map[string]time.Time)
 	s.lock.Unlock()
 
 	if s.startTime.IsZero() {
@@ -1061,8 +1066,11 @@ func (s *Syncer) assignAccountTasks(success chan *accountResponse, fail chan *ac
 	}
 	targetTTL := s.rates.TargetTimeout()
 	for id := range s.accountIdlers {
-		if _, ok := s.statelessPeers[id]; ok {
+		if markedAt, ok := s.statelessPeers[id]; ok {
-			continue
+			if !s.isPartialSync() || time.Since(markedAt) < statelessCooldown {
+				continue
+			}
+			delete(s.statelessPeers, id)
 		}
 		idlers.ids = append(idlers.ids, id)
 		idlers.caps = append(idlers.caps, s.rates.Capacity(id, AccountRangeMsg, targetTTL))
@@ -1159,8 +1167,11 @@ func (s *Syncer) assignBytecodeTasks(success chan *bytecodeResponse, fail chan *
 	}
 	targetTTL := s.rates.TargetTimeout()
 	for id := range s.bytecodeIdlers {
-		if _, ok := s.statelessPeers[id]; ok {
+		if markedAt, ok := s.statelessPeers[id]; ok {
-			continue
+			if !s.isPartialSync() || time.Since(markedAt) < statelessCooldown {
+				continue
+			}
+			delete(s.statelessPeers, id)
 		}
 		idlers.ids = append(idlers.ids, id)
 		idlers.caps = append(idlers.caps, s.rates.Capacity(id, ByteCodesMsg, targetTTL))
@@ -1262,8 +1273,11 @@ func (s *Syncer) assignStorageTasks(success chan *storageResponse, fail chan *st
 	}
 	targetTTL := s.rates.TargetTimeout()
 	for id := range s.storageIdlers {
-		if _, ok := s.statelessPeers[id]; ok {
+		if markedAt, ok := s.statelessPeers[id]; ok {
-			continue
+			if !s.isPartialSync() || time.Since(markedAt) < statelessCooldown {
+				continue
+			}
+			delete(s.statelessPeers, id)
 		}
 		idlers.ids = append(idlers.ids, id)
 		idlers.caps = append(idlers.caps, s.rates.Capacity(id, StorageRangesMsg, targetTTL))
@@ -1419,8 +1433,11 @@ func (s *Syncer) assignTrienodeHealTasks(success chan *trienodeHealResponse, fai
 	}
 	targetTTL := s.rates.TargetTimeout()
 	for id := range s.trienodeHealIdlers {
-		if _, ok := s.statelessPeers[id]; ok {
+		if markedAt, ok := s.statelessPeers[id]; ok {
-			continue
+			if !s.isPartialSync() || time.Since(markedAt) < statelessCooldown {
+				continue
+			}
+			delete(s.statelessPeers, id)
 		}
 		idlers.ids = append(idlers.ids, id)
 		idlers.caps = append(idlers.caps, s.rates.Capacity(id, TrieNodesMsg, targetTTL))
@@ -1547,8 +1564,11 @@ func (s *Syncer) assignBytecodeHealTasks(success chan *bytecodeHealResponse, fai
 	}
 	targetTTL := s.rates.TargetTimeout()
 	for id := range s.bytecodeHealIdlers {
-		if _, ok := s.statelessPeers[id]; ok {
+		if markedAt, ok := s.statelessPeers[id]; ok {
-			continue
+			if !s.isPartialSync() || time.Since(markedAt) < statelessCooldown {
+				continue
+			}
+			delete(s.statelessPeers, id)
 		}
 		idlers.ids = append(idlers.ids, id)
 		idlers.caps = append(idlers.caps, s.rates.Capacity(id, ByteCodesMsg, targetTTL))
@@ -2627,7 +2647,7 @@ func (s *Syncer) OnAccounts(peer SyncPeer, id uint64, hashes []common.Hash, acco
 	// synced to our head.
 	if len(hashes) == 0 && len(accounts) == 0 && len(proof) == 0 {
 		logger.Debug("Peer rejected account range request", "root", s.root)
-		s.statelessPeers[peer.ID()] = struct{}{}
+		s.statelessPeers[peer.ID()] = time.Now()
 		s.lock.Unlock()
 
 		// Signal this request as failed, and ready for rescheduling
@@ -2737,7 +2757,7 @@ func (s *Syncer) onByteCodes(peer SyncPeer, id uint64, bytecodes [][]byte) error
 	// yet synced.
 	if len(bytecodes) == 0 {
 		logger.Debug("Peer rejected bytecode request")
-		s.statelessPeers[peer.ID()] = struct{}{}
+		s.statelessPeers[peer.ID()] = time.Now()
 		s.lock.Unlock()
 
 		// Signal this request as failed, and ready for rescheduling
@@ -2865,7 +2885,7 @@ func (s *Syncer) OnStorage(peer SyncPeer, id uint64, hashes [][]common.Hash, slo
 	// synced to our head.
 	if len(hashes) == 0 && len(proof) == 0 {
 		logger.Debug("Peer rejected storage request")
-		s.statelessPeers[peer.ID()] = struct{}{}
+		s.statelessPeers[peer.ID()] = time.Now()
 		s.lock.Unlock()
 		s.scheduleRevertStorageRequest(req) // reschedule request
 		return nil
@@ -2984,7 +3004,7 @@ func (s *Syncer) OnTrieNodes(peer SyncPeer, id uint64, trienodes [][]byte) error
 	// yet synced.
 	if len(trienodes) == 0 {
 		logger.Debug("Peer rejected trienode heal request")
-		s.statelessPeers[peer.ID()] = struct{}{}
+		s.statelessPeers[peer.ID()] = time.Now()
 		s.lock.Unlock()
 
 		// Signal this request as failed, and ready for rescheduling
@@ -3091,7 +3111,7 @@ func (s *Syncer) onHealByteCodes(peer SyncPeer, id uint64, bytecodes [][]byte) e
 	// yet synced.
 	if len(bytecodes) == 0 {
 		logger.Debug("Peer rejected bytecode heal request")
-		s.statelessPeers[peer.ID()] = struct{}{}
+		s.statelessPeers[peer.ID()] = time.Now()
 		s.lock.Unlock()
 
 		// Signal this request as failed, and ready for rescheduling