les: do not disconnect another server (#20453)

les/peer.go

@@ -113,7 +113,7 @@ type peer struct {
 	fcParams flowcontrol.ServerParams
 	fcCosts  requestCostTable
 
-	trusted                 bool
+	trusted, server         bool
 	onlyAnnounce            bool
 	chainSince, chainRecent uint64
 	stateSince, stateRecent uint64
@@ -675,11 +675,16 @@ func (p *peer) Handshake(td *big.Int, head common.Hash, headNum uint64, genesis
 	}
 
 	if server != nil {
+		p.server = recv.get("flowControl/MRR", nil) == nil
+		if p.server {
+			p.announceType = announceTypeNone // connected to another server, send no messages
+		} else {
 			if recv.get("announceType", &p.announceType) != nil {
 				// set default announceType on server side
 				p.announceType = announceTypeSimple
 			}
 			p.fcClient = flowcontrol.NewClientNode(server.fcManager, server.defParams)
+		}
 	} else {
 		if recv.get("serveChainSince", &p.chainSince) != nil {
 			p.onlyAnnounce = true
@@ -726,6 +731,7 @@ func (p *peer) Handshake(td *big.Int, head common.Hash, headNum uint64, genesis
 				}
 			}
 		}
+		p.server = true
 	}
 	p.headInfo = &announceData{Td: rTd, Hash: rHash, Number: rNum}
 	return nil

les/server_handler.go

@@ -108,10 +108,6 @@ func (h *serverHandler) runPeer(version uint, p *p2p.Peer, rw p2p.MsgReadWriter)
 }
 
 func (h *serverHandler) handle(p *peer) error {
-	// Reject light clients if server is not synced.
-	if !h.synced() {
-		return p2p.DiscRequested
-	}
 	p.Log().Debug("Light Ethereum peer connected", "name", p.Name())
 
 	// Execute the LES handshake
@@ -125,6 +121,15 @@ func (h *serverHandler) handle(p *peer) error {
 		p.Log().Debug("Light Ethereum handshake failed", "err", err)
 		return err
 	}
+	if p.server {
+		// connected to another server, no messages expected, just wait for disconnection
+		_, err := p.rw.ReadMsg()
+		return err
+	}
+	// Reject light clients if server is not synced.
+	if !h.synced() {
+		return p2p.DiscRequested
+	}
 	defer p.fcClient.Disconnect()
 
 	// Disconnect the inbound peer if it's rejected by clientPool