eth: add logic to drop peers randomly when saturated (#31476)

As of now, Geth disconnects peers only on protocol error or timeout,
meaning once connection slots are filled, the peerset is largely fixed.

As mentioned in https://github.com/ethereum/go-ethereum/issues/31321,
Geth should occasionally disconnect peers to ensure some churn.
What/when to disconnect could depend on:
- the state of geth (e.g. sync or not)
- current number of peers
- peer level metrics

This PR adds a very slow churn using a random drop.

---------

Signed-off-by: Csaba Kiraly <csaba.kiraly@gmail.com>
Co-authored-by: Felix Lange <fjl@twurst.com>

eth/backend.go

@@ -76,6 +76,7 @@ type Ethereum struct {
 
 	handler *handler
 	discmix *enode.FairMix
+	dropper *dropper
 
 	// DB interfaces
 	chainDb ethdb.Database // Block chain database
@@ -300,6 +301,8 @@ func New(stack *node.Node, config *ethconfig.Config) (*Ethereum, error) {
 		return nil, err
 	}
 
+	eth.dropper = newDropper(eth.p2pServer.MaxDialedConns(), eth.p2pServer.MaxInboundConns())
+
 	eth.miner = miner.New(eth, config.Miner, eth.engine)
 	eth.miner.SetExtra(makeExtraData(config.Miner.ExtraData))
 	eth.miner.SetPrioAddresses(config.TxPool.Locals)
@@ -410,6 +413,9 @@ func (s *Ethereum) Start() error {
 	// Start the networking layer
 	s.handler.Start(s.p2pServer.MaxPeers)
 
+	// Start the connection manager
+	s.dropper.Start(s.p2pServer, func() bool { return !s.Synced() })
+
 	// start log indexer
 	s.filterMaps.Start()
 	go s.updateFilterMapsHeads()
@@ -511,6 +517,7 @@ func (s *Ethereum) setupDiscovery() error {
 func (s *Ethereum) Stop() error {
 	// Stop all the peer-related stuff first.
 	s.discmix.Close()
+	s.dropper.Stop()
 	s.handler.Stop()
 
 	// Then stop everything else.

eth/dropper.go

@@ -0,0 +1,167 @@
+// Copyright 2025 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package eth
+
+import (
+	mrand "math/rand"
+	"slices"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/mclock"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/metrics"
+	"github.com/ethereum/go-ethereum/p2p"
+)
+
+const (
+	// Interval between peer drop events (uniform between min and max)
+	peerDropIntervalMin = 3 * time.Minute
+	// Interval between peer drop events (uniform between min and max)
+	peerDropIntervalMax = 7 * time.Minute
+	// Avoid dropping peers for some time after connection
+	doNotDropBefore = 10 * time.Minute
+	// How close to max should we initiate the drop timer. O should be fine,
+	// dropping when no more peers can be added. Larger numbers result in more
+	// aggressive drop behavior.
+	peerDropThreshold = 0
+)
+
+var (
+	// droppedInbound is the number of inbound peers dropped
+	droppedInbound = metrics.NewRegisteredMeter("eth/dropper/inbound", nil)
+	// droppedOutbound is the number of outbound peers dropped
+	droppedOutbound = metrics.NewRegisteredMeter("eth/dropper/outbound", nil)
+)
+
+// dropper monitors the state of the peer pool and makes changes as follows:
+//   - during sync the Downloader handles peer connections, so dropper is disabled
+//   - if not syncing and the peer count is close to the limit, it drops peers
+//     randomly every peerDropInterval to make space for new peers
+//   - peers are dropped separately from the inboud pool and from the dialed pool
+type dropper struct {
+	maxDialPeers    int // maximum number of dialed peers
+	maxInboundPeers int // maximum number of inbound peers
+	peersFunc       getPeersFunc
+	syncingFunc     getSyncingFunc
+
+	// peerDropTimer introduces churn if we are close to limit capacity.
+	// We handle Dialed and Inbound connections separately
+	peerDropTimer *time.Timer
+
+	wg         sync.WaitGroup // wg for graceful shutdown
+	shutdownCh chan struct{}
+}
+
+// Callback type to get the list of connected peers.
+type getPeersFunc func() []*p2p.Peer
+
+// Callback type to get syncing status.
+// Returns true while syncing, false when synced.
+type getSyncingFunc func() bool
+
+func newDropper(maxDialPeers, maxInboundPeers int) *dropper {
+	cm := &dropper{
+		maxDialPeers:    maxDialPeers,
+		maxInboundPeers: maxInboundPeers,
+		peerDropTimer:   time.NewTimer(randomDuration(peerDropIntervalMin, peerDropIntervalMax)),
+		shutdownCh:      make(chan struct{}),
+	}
+	if peerDropIntervalMin > peerDropIntervalMax {
+		panic("peerDropIntervalMin duration must be less than or equal to peerDropIntervalMax duration")
+	}
+	return cm
+}
+
+// Start the dropper.
+func (cm *dropper) Start(srv *p2p.Server, syncingFunc getSyncingFunc) {
+	cm.peersFunc = srv.Peers
+	cm.syncingFunc = syncingFunc
+	cm.wg.Add(1)
+	go cm.loop()
+}
+
+// Stop the dropper.
+func (cm *dropper) Stop() {
+	cm.peerDropTimer.Stop()
+	close(cm.shutdownCh)
+	cm.wg.Wait()
+}
+
+// dropRandomPeer selects one of the peers randomly and drops it from the peer pool.
+func (cm *dropper) dropRandomPeer() bool {
+	peers := cm.peersFunc()
+	var numInbound int
+	for _, p := range peers {
+		if p.Inbound() {
+			numInbound++
+		}
+	}
+	numDialed := len(peers) - numInbound
+
+	selectDoNotDrop := func(p *p2p.Peer) bool {
+		// Avoid dropping trusted and static peers, or recent peers.
+		// Only drop peers if their respective category (dialed/inbound)
+		// is close to limit capacity.
+		return p.Trusted() || p.StaticDialed() ||
+			p.Lifetime() < mclock.AbsTime(doNotDropBefore) ||
+			(p.DynDialed() && cm.maxDialPeers-numDialed > peerDropThreshold) ||
+			(p.Inbound() && cm.maxInboundPeers-numInbound > peerDropThreshold)
+	}
+
+	droppable := slices.DeleteFunc(peers, selectDoNotDrop)
+	if len(droppable) > 0 {
+		p := droppable[mrand.Intn(len(droppable))]
+		log.Debug("Dropping random peer", "inbound", p.Inbound(),
+			"id", p.ID(), "duration", common.PrettyDuration(p.Lifetime()), "peercountbefore", len(peers))
+		p.Disconnect(p2p.DiscUselessPeer)
+		if p.Inbound() {
+			droppedInbound.Mark(1)
+		} else {
+			droppedOutbound.Mark(1)
+		}
+		return true
+	}
+	return false
+}
+
+// randomDuration generates a random duration between min and max.
+func randomDuration(min, max time.Duration) time.Duration {
+	if min > max {
+		panic("min duration must be less than or equal to max duration")
+	}
+	return time.Duration(mrand.Int63n(int64(max-min)) + int64(min))
+}
+
+// loop is the main loop of the connection dropper.
+func (cm *dropper) loop() {
+	defer cm.wg.Done()
+
+	for {
+		select {
+		case <-cm.peerDropTimer.C:
+			// Drop a random peer if we are not syncing and the peer count is close to the limit.
+			if !cm.syncingFunc() {
+				cm.dropRandomPeer()
+			}
+			cm.peerDropTimer.Reset(randomDuration(peerDropIntervalMin, peerDropIntervalMax))
+		case <-cm.shutdownCh:
+			return
+		}
+	}
+}

p2p/peer.go

@@ -220,11 +220,35 @@ func (p *Peer) String() string {
 	return fmt.Sprintf("Peer %x %v", id[:8], p.RemoteAddr())
 }
 
-// Inbound returns true if the peer is an inbound connection
+// Inbound returns true if the peer is an inbound (not dialed) connection.
 func (p *Peer) Inbound() bool {
 	return p.rw.is(inboundConn)
 }
 
+// Trusted returns true if the peer is configured as trusted.
+// Trusted peers are accepted in above the MaxInboundConns limit.
+// The peer can be either inbound or dialed.
+func (p *Peer) Trusted() bool {
+	return p.rw.is(trustedConn)
+}
+
+// DynDialed returns true if the peer was dialed successfully (passed handshake) and
+// it is not configured as static.
+func (p *Peer) DynDialed() bool {
+	return p.rw.is(dynDialedConn)
+}
+
+// StaticDialed returns true if the peer was dialed successfully (passed handshake) and
+// it is configured as static.
+func (p *Peer) StaticDialed() bool {
+	return p.rw.is(staticDialedConn)
+}
+
+// Lifetime returns the time since peer creation.
+func (p *Peer) Lifetime() mclock.AbsTime {
+	return mclock.Now() - p.created
+}
+
 func newPeer(log log.Logger, conn *conn, protocols []Protocol) *Peer {
 	protomap := matchProtocols(protocols, conn.caps, conn)
 	p := &Peer{

p2p/server.go

@@ -508,7 +508,7 @@ func (srv *Server) setupDiscovery() error {
 func (srv *Server) setupDialScheduler() {
 	config := dialConfig{
 		self:           srv.localnode.ID(),
-		maxDialPeers:   srv.maxDialedConns(),
+		maxDialPeers:   srv.MaxDialedConns(),
 		maxActiveDials: srv.MaxPendingPeers,
 		log:            srv.Logger,
 		netRestrict:    srv.NetRestrict,
@@ -527,11 +527,11 @@ func (srv *Server) setupDialScheduler() {
 	}
 }
 
-func (srv *Server) maxInboundConns() int {
-	return srv.MaxPeers - srv.maxDialedConns()
+func (srv *Server) MaxInboundConns() int {
+	return srv.MaxPeers - srv.MaxDialedConns()
 }
 
-func (srv *Server) maxDialedConns() (limit int) {
+func (srv *Server) MaxDialedConns() (limit int) {
 	if srv.NoDial || srv.MaxPeers == 0 {
 		return 0
 	}
@@ -736,7 +736,7 @@ func (srv *Server) postHandshakeChecks(peers map[enode.ID]*Peer, inboundCount in
 	switch {
 	case !c.is(trustedConn) && len(peers) >= srv.MaxPeers:
 		return DiscTooManyPeers
-	case !c.is(trustedConn) && c.is(inboundConn) && inboundCount >= srv.maxInboundConns():
+	case !c.is(trustedConn) && c.is(inboundConn) && inboundCount >= srv.MaxInboundConns():
 		return DiscTooManyPeers
 	case peers[c.node.ID()] != nil:
 		return DiscAlreadyConnected