p2p: resolve incomplete dial targets

This change makes it possible to add peers without providing their IP
address. The endpoint of the target node is resolved using the discovery
protocol.

p2p/dial.go

@@ -36,6 +36,10 @@ const (
 	// Discovery lookups are throttled and can only run
 	// once every few seconds.
 	lookupInterval = 4 * time.Second
+
+	// Endpoint resolution is throttled with bounded backoff.
+	initialResolveDelay = 60 * time.Second
+	maxResolveDelay     = time.Hour
 )
 
 // dialstate schedules dials and discovery lookups.
@@ -46,17 +50,17 @@ type dialstate struct {
 	ntab        discoverTable
 
 	lookupRunning bool
-
-	dialing     map[discover.NodeID]connFlag
-	lookupBuf   []*discover.Node // current discovery lookup results
-	randomNodes []*discover.Node // filled from Table
-	static      map[discover.NodeID]*discover.Node
-	hist        *dialHistory
+	dialing       map[discover.NodeID]connFlag
+	lookupBuf     []*discover.Node // current discovery lookup results
+	randomNodes   []*discover.Node // filled from Table
+	static        map[discover.NodeID]*dialTask
+	hist          *dialHistory
 }
 
 type discoverTable interface {
 	Self() *discover.Node
 	Close()
+	Resolve(target discover.NodeID) *discover.Node
 	Lookup(target discover.NodeID) []*discover.Node
 	ReadRandomNodes([]*discover.Node) int
 }
@@ -74,10 +78,13 @@ type task interface {
 	Do(*Server)
 }
 
-// A dialTask is generated for each node that is dialed.
+// A dialTask is generated for each node that is dialed. Its
+// fields cannot be accessed while the task is running.
 type dialTask struct {
-	flags connFlag
-	dest  *discover.Node
+	flags        connFlag
+	dest         *discover.Node
+	lastResolved time.Time
+	resolveDelay time.Duration
 }
 
 // discoverTask runs discovery table operations.
@@ -97,26 +104,31 @@ func newDialState(static []*discover.Node, ntab discoverTable, maxdyn int) *dial
 	s := &dialstate{
 		maxDynDials: maxdyn,
 		ntab:        ntab,
-		static:      make(map[discover.NodeID]*discover.Node),
+		static:      make(map[discover.NodeID]*dialTask),
 		dialing:     make(map[discover.NodeID]connFlag),
 		randomNodes: make([]*discover.Node, maxdyn/2),
 		hist:        new(dialHistory),
 	}
 	for _, n := range static {
-		s.static[n.ID] = n
+		s.addStatic(n)
 	}
 	return s
 }
 
 func (s *dialstate) addStatic(n *discover.Node) {
-	s.static[n.ID] = n
+	// This overwites the task instead of updating an existing
+	// entry, giving users the opportunity to force a resolve operation.
+	s.static[n.ID] = &dialTask{flags: staticDialedConn, dest: n}
 }
 
 func (s *dialstate) newTasks(nRunning int, peers map[discover.NodeID]*Peer, now time.Time) []task {
 	var newtasks []task
+	isDialing := func(id discover.NodeID) bool {
+		_, found := s.dialing[id]
+		return found || peers[id] != nil || s.hist.contains(id)
+	}
 	addDial := func(flag connFlag, n *discover.Node) bool {
-		_, dialing := s.dialing[n.ID]
-		if dialing || peers[n.ID] != nil || s.hist.contains(n.ID) {
+		if isDialing(n.ID) {
 			return false
 		}
 		s.dialing[n.ID] = flag
@@ -141,8 +153,11 @@ func (s *dialstate) newTasks(nRunning int, peers map[discover.NodeID]*Peer, now
 	s.hist.expire(now)
 
 	// Create dials for static nodes if they are not connected.
-	for _, n := range s.static {
-		addDial(staticDialedConn, n)
+	for id, t := range s.static {
+		if !isDialing(id) {
+			s.dialing[id] = t.flags
+			newtasks = append(newtasks, t)
+		}
 	}
 
 	// Use random nodes from the table for half of the necessary
@@ -194,17 +209,68 @@ func (s *dialstate) taskDone(t task, now time.Time) {
 }
 
 func (t *dialTask) Do(srv *Server) {
-	addr := &net.TCPAddr{IP: t.dest.IP, Port: int(t.dest.TCP)}
-	glog.V(logger.Debug).Infof("dialing %v\n", t.dest)
+	if t.dest.Incomplete() {
+		if !t.resolve(srv) {
+			return
+		}
+	}
+	success := t.dial(srv, t.dest)
+	// Try resolving the ID of static nodes if dialing failed.
+	if !success && t.flags&staticDialedConn != 0 {
+		if t.resolve(srv) {
+			t.dial(srv, t.dest)
+		}
+	}
+}
+
+// resolve attempts to find the current endpoint for the destination
+// using discovery.
+//
+// Resolve operations are throttled with backoff to avoid flooding the
+// discovery network with useless queries for nodes that don't exist.
+// The backoff delay resets when the node is found.
+func (t *dialTask) resolve(srv *Server) bool {
+	if srv.ntab == nil {
+		glog.V(logger.Debug).Infof("can't resolve node %x: discovery is disabled", t.dest.ID[:6])
+		return false
+	}
+	if t.resolveDelay == 0 {
+		t.resolveDelay = initialResolveDelay
+	}
+	if time.Since(t.lastResolved) < t.resolveDelay {
+		return false
+	}
+	resolved := srv.ntab.Resolve(t.dest.ID)
+	t.lastResolved = time.Now()
+	if resolved == nil {
+		t.resolveDelay *= 2
+		if t.resolveDelay > maxResolveDelay {
+			t.resolveDelay = maxResolveDelay
+		}
+		glog.V(logger.Debug).Infof("resolving node %x failed (new delay: %v)", t.dest.ID[:6], t.resolveDelay)
+		return false
+	}
+	// The node was found.
+	t.resolveDelay = initialResolveDelay
+	t.dest = resolved
+	glog.V(logger.Debug).Infof("resolved node %x: %v:%d", t.dest.ID[:6], t.dest.IP, t.dest.TCP)
+	return true
+}
+
+// dial performs the actual connection attempt.
+func (t *dialTask) dial(srv *Server, dest *discover.Node) bool {
+	addr := &net.TCPAddr{IP: dest.IP, Port: int(dest.TCP)}
+	glog.V(logger.Debug).Infof("dial tcp %v (%x)\n", addr, dest.ID[:6])
 	fd, err := srv.Dialer.Dial("tcp", addr.String())
 	if err != nil {
-		glog.V(logger.Detail).Infof("dial error: %v", err)
-		return
+		glog.V(logger.Detail).Infof("%v", err)
+		return false
 	}
 	mfd := newMeteredConn(fd, false)
-
-	srv.setupConn(mfd, t.flags, t.dest)
+	srv.setupConn(mfd, t.flags, dest)
+	return true
 }
+
 func (t *dialTask) String() string {
 	return fmt.Sprintf("%v %x %v:%d", t.flags, t.dest.ID[:8], t.dest.IP, t.dest.TCP)
 }