p2p/net/swarm: dial backoff

This commit introduces a backoff when failing to dial
peers. It makes everything much faster.

p2p/net/swarm/swarm.go

@@ -33,6 +33,7 @@ type Swarm struct {
 	peers peer.Peerstore
 	connh ConnHandler
 	dsync dialsync
+	backf dialbackoff
 
 	cg ctxgroup.ContextGroup
 }
@@ -50,10 +51,10 @@ func NewSwarm(ctx context.Context, listenAddrs []ma.Multiaddr,
 	}
 
 	s := &Swarm{
-		swarm:   ps.NewSwarm(PSTransport),
-		local:   local,
-		peers:   peers,
-		cg:      ctxgroup.WithContext(ctx),
+		swarm: ps.NewSwarm(PSTransport),
+		local: local,
+		peers: peers,
+		cg:    ctxgroup.WithContext(ctx),
 	}
 
 	// configure Swarm

p2p/net/swarm/swarm_dial.go

@@ -96,6 +96,71 @@ func (ds *dialsync) Unlock(dst peer.ID) {
 	ds.lock.Unlock()
 }
 
+// dialbackoff is a struct used to avoid over-dialing the same, dead peers.
+// Whenever we totally time out on a peer (all three attempts), we add them
+// to dialbackoff. Then, whenevers goroutines would _wait_ (dialsync), they
+// check dialbackoff. If it's there, they don't wait and exit promptly with
+// an error. (the single goroutine that is actually dialing continues to
+// dial). If a dial is successful, the peer is removed from backoff.
+// Example:
+//
+//  for {
+//  	if ok, wait := dialsync.Lock(p); !ok {
+//  		if backoff.Backoff(p) {
+//  			return errDialFailed
+//  		}
+//  		<-wait
+//  		continue
+//  	}
+//  	defer dialsync.Unlock(p)
+//  	c, err := actuallyDial(p)
+//  	if err != nil {
+//  		dialbackoff.AddBackoff(p)
+//  		continue
+//  	}
+//  	dialbackoff.Clear(p)
+//  }
+//
+type dialbackoff struct {
+	entries map[peer.ID]struct{}
+	lock    sync.RWMutex
+}
+
+func (db *dialbackoff) init() {
+	if db.entries == nil {
+		db.entries = make(map[peer.ID]struct{})
+	}
+}
+
+// Backoff returns whether the client should backoff from dialing
+// peeer p
+func (db *dialbackoff) Backoff(p peer.ID) bool {
+	db.lock.Lock()
+	db.init()
+	_, found := db.entries[p]
+	db.lock.Unlock()
+	return found
+}
+
+// AddBackoff lets other nodes know that we've entered backoff with
+// peer p, so dialers should not wait unnecessarily. We still will
+// attempt to dial with one goroutine, in case we get through.
+func (db *dialbackoff) AddBackoff(p peer.ID) {
+	db.lock.Lock()
+	db.init()
+	db.entries[p] = struct{}{}
+	db.lock.Unlock()
+}
+
+// Clear removes a backoff record. Clients should call this after a
+// successful Dial.
+func (db *dialbackoff) Clear(p peer.ID) {
+	db.lock.Lock()
+	db.init()
+	delete(db.entries, p)
+	db.lock.Unlock()
+}
+
 // Dial connects to a peer.
 //
 // The idea is that the client of Swarm does not need to know what network
@@ -103,6 +168,7 @@ func (ds *dialsync) Unlock(dst peer.ID) {
 // This allows us to use various transport protocols, do NAT traversal/relay,
 // etc. to achive connection.
 func (s *Swarm) Dial(ctx context.Context, p peer.ID) (*Conn, error) {
+	log := log.Prefix("swarm %s dialing %s", s.local, p)
 	if p == s.local {
 		return nil, errors.New("Attempted connection to self!")
 	}
@@ -126,7 +192,13 @@ func (s *Swarm) Dial(ctx context.Context, p peer.ID) (*Conn, error) {
 
 		// check if there's an ongoing dial to this peer
 		if ok, wait := s.dsync.Lock(p); !ok {
-			log.Debugf("swarm %s dialing %s -- waiting for ongoing dial", s.local, p)
+
+			if s.backf.Backoff(p) {
+				log.Debugf("backoff")
+				return nil, fmt.Errorf("%s failed to dial %s, backing off.", s.local, p)
+			}
+
+			log.Debugf("waiting for ongoing dial")
 			select {
 			case <-wait: // wait for that dial to finish.
 				continue // and see if it worked (loop), OR we got an incoming dial.
@@ -137,14 +209,17 @@ func (s *Swarm) Dial(ctx context.Context, p peer.ID) (*Conn, error) {
 
 		// ok, we have been charged to dial! let's do it.
 		// if it succeeds, dial will add the conn to the swarm itself.
-		log.Debugf("swarm %s dialing %s -- dial start", s.local, p)
+		log.Debugf("dial start")
 		ctxT, _ := context.WithTimeout(ctx, DialTimeout)
 		conn, err = s.dial(ctxT, p)
 		s.dsync.Unlock(p)
-		log.Debugf("swarm %s dialing %s -- dial end %s", s.local, p, conn)
+		log.Debugf("dial end %s", conn)
 		if err != nil {
+			s.backf.AddBackoff(p) // let others know to backoff
+
 			continue // ok, we failed. try again. (if loop is done, our error is output)
 		}
+		s.backf.Clear(p) // okay, no longer need to backoff
 		return conn, nil
 	}
 	if err == nil {