[release-v2.0] Attempt cxns to >=3 mixing-capable peers

Similar to dcrd, we want SPV wallets to maintain a minimum number of peers
that support the mixing protocol version.  This will be especially important
after dcrwallet is updated to a mixclient branch that submits PR messages over
the entire duration before the epoch occurs, as any errors for failed message
sends due to not having a single mixing-capable peer connected would be
delayed far into the future.

Backport of 62c10d5.

spv/sync.go

@@ -34,6 +34,15 @@ import (
 // they do not provide each of these services.
 const reqSvcs = wire.SFNodeNetwork
 
+const (
+	targetPeerCount = 8
+
+	// Require a subset of all connected outbounded peers to meet some
+	// minimum protocol version.
+	minVersion       = wire.MixVersion
+	minVersionTarget = 3
+)
+
 // Syncer implements wallet synchronization services by over the Decred wire
 // protocol using Simplified Payment Verification (SPV) with compact filters.
 type Syncer struct {
@@ -451,9 +460,12 @@ func (s *Syncer) peerCandidate(svcs wire.ServiceFlag) (*addrmgr.NetAddress, erro
 	return nil, errors.New("no addresses")
 }
 
+var errBreaksMinVersionTarget = errors.New("peer uses too low version to satisify " +
+	"minimum target version count")
+
 // connectAndRunPeer connects to and runs the syncing process with the specified
 // peer. It blocks until the peer disconnects and logs any errors.
-func (s *Syncer) connectAndRunPeer(ctx context.Context, raddr string) {
+func (s *Syncer) connectAndRunPeer(ctx context.Context, raddr string, persistent bool) {
 	// Attempt connection to peer.
 	rp, err := s.lp.ConnectOutbound(ctx, raddr, reqSvcs)
 	if err != nil {
@@ -467,18 +479,24 @@ func (s *Syncer) connectAndRunPeer(ctx context.Context, raddr string) {
 		}
 		return
 	}
-	log.Infof("New peer %v %v %v", raddr, rp.UA(), rp.Services())
 
 	// Track peer as running as opposed to attempting connection.
 	s.remotesMu.Lock()
 	delete(s.connectingRemotes, raddr)
+	if !persistent && s.breaksMinVersionTarget(rp) {
+		s.remotesMu.Unlock()
+		log.Debugf("Disconnecting %v: %v", raddr, errBreaksMinVersionTarget)
+		rp.Disconnect(errBreaksMinVersionTarget)
+		return
+	}
 	s.remotes[raddr] = rp
 	n := len(s.remotes)
 	if s.remoteAvailable != nil {
 		close(s.remoteAvailable)
 		s.remoteAvailable = nil
 	}
 	s.remotesMu.Unlock()
+	log.Infof("New peer %v %v version=%d %v", raddr, rp.UA(), rp.Pver(), rp.Services())
 	s.peerConnected(n, raddr)
 
 	// Alert disconnection once this peer is done.
@@ -514,9 +532,29 @@ func (s *Syncer) connectAndRunPeer(ctx context.Context, raddr string) {
 	}
 }
 
+func (s *Syncer) breaksMinVersionTarget(rp *p2p.RemotePeer) bool {
+	if rp.Pver() >= minVersion {
+		return false
+	}
+	n := len(s.remotes)
+	if n < targetPeerCount-minVersionTarget {
+		return false
+	}
+	var meetsMin int
+	for _, rp := range s.remotes {
+		if rp.Pver() >= minVersion {
+			meetsMin++
+			if meetsMin == minVersionTarget {
+				return false
+			}
+		}
+	}
+	return true
+}
+
 func (s *Syncer) connectToPersistent(ctx context.Context, raddr string) error {
 	for {
-		s.connectAndRunPeer(ctx, raddr)
+		s.connectAndRunPeer(ctx, raddr, true)
 
 		// Retry persistent peer after 5 seconds.
 		select {
@@ -531,7 +569,7 @@ func (s *Syncer) connectToCandidates(ctx context.Context) error {
 	var wg sync.WaitGroup
 	defer wg.Wait()
 
-	sem := make(chan struct{}, 8)
+	sem := make(chan struct{}, targetPeerCount)
 	for {
 		if ctx.Err() != nil {
 			return ctx.Err()
@@ -555,7 +593,7 @@ func (s *Syncer) connectToCandidates(ctx context.Context) error {
 		wg.Add(1)
 		go func() {
 			raddr := na.String()
-			s.connectAndRunPeer(ctx, raddr)
+			s.connectAndRunPeer(ctx, raddr, false)
 			wg.Done()
 			<-sem
 		}()