PEX: impede full-meching in tracker-less swarms by adding a cooldown minute

pex.go

@@ -3,6 +3,7 @@ package torrent
 import (
 	"net"
 	"sync"
+	"time"
 
 	"github.com/anacrolix/dht/v2/krpc"
 	pp "github.com/anacrolix/torrent/peer_protocol"
@@ -179,6 +180,7 @@ func shortestIP(ip net.IP) net.IP {
 type pexState struct {
 	ev        []pexEvent    // event feed, append-only
 	hold      []pexEvent    // delayed drops
+	rest      time.Time     // cooldown deadline on inbound
 	nc        int           // net number of alive conns
 	initCache pexMsgFactory // last generated initial message
 	initSeq   int           // number of events which went into initCache
@@ -190,6 +192,7 @@ func (s *pexState) Reset() {
 	s.ev = nil
 	s.hold = nil
 	s.nc = 0
+	s.rest = time.Time{}
 	s.initLock.Lock()
 	s.initCache = pexMsgFactory{}
 	s.initSeq = 0

pexconn.go

@@ -88,6 +88,15 @@ func (s *pexConnState) Share(postfn messageWriter) bool {
 
 // Recv is called from the reader goroutine
 func (s *pexConnState) Recv(payload []byte) error {
+	if !s.torrent.wantPeers() {
+		s.dbg.Printf("peer reserve ok, incoming PEX discarded")
+		return nil
+	}
+	if time.Now().Before(s.torrent.pex.rest) {
+		s.dbg.Printf("in cooldown period, incoming PEX discarded")
+		return nil
+	}
+
 	rx, err := pp.LoadPexMsg(payload)
 	if err != nil {
 		return fmt.Errorf("error unmarshalling PEX message: %s", err)
@@ -100,8 +109,10 @@ func (s *pexConnState) Recv(payload []byte) error {
 	peers.AppendFromPex(rx.Added6, rx.Added6Flags)
 	peers.AppendFromPex(rx.Added, rx.AddedFlags)
 	s.dbg.Printf("adding %d peers from PEX", len(peers))
-	s.torrent.addPeers(peers)
+	if len(peers) > 0 {
-	// s.dbg.Print("known swarm now:", s.torrent.KnownSwarm())
+		s.torrent.pex.rest = time.Now().Add(pexInterval)
+		s.torrent.addPeers(peers)
+	}
 
 	// one day we may also want to:
 	// - check if the peer is not flooding us with PEX updates