eth: reserve peer slot for usable snap peer (#35180)

This PR improves the slot reservation logic in the context of snap/2.

Geth has the mechanism to reserve roughly half the peer slots for peers 
supporting the snap protocol if snap syncing is needed by local node.

With the context of snap/2, this mechanism should be changed that:
we reserve the slot for the "usable snap peer", not blindly for peer
with snap extension enabled (such as legacy snap/1, which can't serve
the snap/2).

eth/downloader/downloader.go

@@ -1132,11 +1132,23 @@ func (d *Downloader) DeliverSnapPacket(peer *snap.Peer, packet snap.Packet) erro
 // snap/2 syncer skips snap/1-only peers, which cannot answer its BAL requests.
 func (d *Downloader) RegisterSnapPeer(p *snap.Peer) error {
 	if p.Version() < d.snapSyncer.Version() {
+		// The peer speaks an older snap version than the active syncer needs
+		// (e.g. snap/1 while we sync via snap/2). We still serve it, but it
+		// cannot answer our requests, so it is not registered for syncing.
+		// Surface it so an operator can tell a stalled sync from a quiet one.
+		snapPeerSkipMeter.Mark(1)
+		log.Debug("Skipping snap peer below syncer version", "peer", p.ID(), "version", p.Version(), "required", d.snapSyncer.Version())
 		return nil
 	}
 	return d.snapSyncer.Register(p)
 }
 
+// SnapSyncVersion returns the snap protocol version of the active state syncer.
+// Peers negotiating a lower version cannot serve its requests.
+func (d *Downloader) SnapSyncVersion() uint {
+	return d.snapSyncer.Version()
+}
+
 // UnregisterSnapPeer removes a snap peer from the active state syncer. It mirrors
 // RegisterSnapPeer's version gate: a peer below the active syncer's version was
 // never registered, so there is nothing to remove.

eth/downloader/metrics.go

@@ -38,4 +38,8 @@ var (
 	receiptTimeoutMeter = metrics.NewRegisteredMeter("eth/downloader/receipts/timeout", nil)
 
 	throttleCounter = metrics.NewRegisteredCounter("eth/downloader/throttle", nil)
+
+	// snapPeerSkipMeter tracks snap peers skipped by the state syncer because
+	// they negotiated a version below the one the syncer requires.
+	snapPeerSkipMeter = metrics.NewRegisteredMeter("eth/downloader/snap/peerskip", nil)
 )

eth/handler.go

@@ -245,11 +245,17 @@ func (h *handler) runEthPeer(peer *eth.Peer, handler eth.Handler) error {
 	}
 	reject := false // reserved peer slots
 	if h.downloader.ConfigSyncMode() == ethconfig.SnapSync {
-		if snap == nil {
-			// If we are running snap-sync, we want to reserve roughly half the peer
-			// slots for peers supporting the snap protocol.
-			// The logic here is; we only allow up to 5 more non-snap peers than snap-peers.
-			if all, snp := h.peers.len(), h.peers.snapLen(); all-snp > snp+5 {
+		// A peer is useful to the active state syncer only if it offers the snap
+		// extension at (or above) the syncer's version. Non-snap peers AND peers
+		// stuck on an older snap version (e.g. snap/1 while we sync via snap/2)
+		// are both "non-usable": the node still serves them, but they must not be
+		// allowed to fill the slots reserved for peers that can serve the sync.
+		minVersion := h.downloader.SnapSyncVersion()
+		usable := snap != nil && snap.Version() >= minVersion
+		if !usable {
+			// Reserve roughly half the slots for usable peers: only allow up to 5
+			// more non-usable peers (non-snap or below-version snap) than usable ones.
+			if all, snp := h.peers.len(), h.peers.snapLen(minVersion); all-snp > snp+5 {
 				reject = true
 			}
 		}

eth/peerset.go

@@ -49,8 +49,7 @@ var (
 // peerSet represents the collection of active peers currently participating in
 // the `eth` protocol, with or without the `snap` extension.
 type peerSet struct {
-	peers     map[string]*ethPeer // Peers connected on the `eth` protocol
-	snapPeers int                 // Number of `snap` compatible peers for connection prioritization
+	peers map[string]*ethPeer // Peers connected on the `eth` protocol
 
 	snapWait map[string]chan *snap.Peer // Peers connected on `eth` waiting for their snap extension
 	snapPend map[string]*snap.Peer      // Peers connected on the `snap` protocol, but not yet on `eth`
@@ -161,7 +160,6 @@ func (ps *peerSet) registerPeer(peer *eth.Peer, ext *snap.Peer) error {
 	}
 	if ext != nil {
 		eth.snapExt = &snapPeer{ext}
-		ps.snapPeers++
 	}
 	ps.peers[id] = eth
 	return nil
@@ -173,14 +171,10 @@ func (ps *peerSet) unregisterPeer(id string) error {
 	ps.lock.Lock()
 	defer ps.lock.Unlock()
 
-	peer, ok := ps.peers[id]
-	if !ok {
+	if _, ok := ps.peers[id]; !ok {
 		return errPeerNotRegistered
 	}
 	delete(ps.peers, id)
-	if peer.snapExt != nil {
-		ps.snapPeers--
-	}
 	return nil
 }
 
@@ -210,12 +204,21 @@ func (ps *peerSet) len() int {
 	return len(ps.peers)
 }
 
-// snapLen returns if the current number of `snap` peers in the set.
-func (ps *peerSet) snapLen() int {
+// snapLen returns the number of `snap` peers whose negotiated version is at
+// least minVersion — i.e. peers usable by a state syncer running that version.
+// Lower-version snap peers (which the node still serves but cannot sync from)
+// are excluded, so they don't get counted toward the reserved snap-peer slots.
+func (ps *peerSet) snapLen(minVersion uint) int {
 	ps.lock.RLock()
 	defer ps.lock.RUnlock()
 
-	return ps.snapPeers
+	var n int
+	for _, p := range ps.peers {
+		if p.snapExt != nil && p.snapExt.Version() >= minVersion {
+			n++
+		}
+	}
+	return n
 }
 
 // close disconnects all peers.