[release-v2.0] server: Prefer min one mix capable outbound peer.

This adds logic to attempt to help maintain at least one mix capable
outbound peer to ensure better connectivity among the mix capable peers.

server.go

@@ -529,6 +529,11 @@ type server struct {
 	bytesSent     atomic.Uint64 // Total bytes sent by all peers since start.
 	shutdown      atomic.Bool
 
+	// targetOutbound is the calculated number of target outbound peers to
+	// maintain.  It is set at creation time and never modified afterwards, so
+	// it does not need to be protected for concurrent access.
+	targetOutbound uint32
+
 	// minKnownWork houses the minimum known work from the associated network
 	// params converted to a uint256 so the conversion only needs to be
 	// performed once when the server is initialized.  Ideally, the chain params
@@ -1010,6 +1015,28 @@ func (sp *serverPeer) OnVersion(_ *peer.Peer, msg *wire.MsgVersion) {
 		return
 	}
 
+	// Maintain at least one outbound peer capable of supporting p2p mixing.
+	if !isInbound && msg.ProtocolVersion < int32(wire.MixVersion) {
+		var hasMixCapableOutbound bool
+		var numOutbound uint32
+		peerState := &sp.server.peerState
+		peerState.Lock()
+		peerState.forAllOutboundPeers(func(sp *serverPeer) {
+			if sp.ProtocolVersion() >= wire.MixVersion {
+				hasMixCapableOutbound = true
+			}
+			numOutbound++
+		})
+		peerState.Unlock()
+
+		if !hasMixCapableOutbound && numOutbound+1 == sp.server.targetOutbound {
+			srvrLog.Debugf("Rejecting outbound peer %s with protocol version "+
+				"%d in favor of a peer with minimum version %d", sp,
+				msg.ProtocolVersion, wire.MixVersion)
+			sp.Disconnect()
+		}
+	}
+
 	// Reject outbound peers that are not full nodes.
 	wantServices := wire.SFNodeNetwork
 	if !isInbound && !hasServices(msg.Services, wantServices) {
@@ -3686,6 +3713,7 @@ func newServer(ctx context.Context, listenAddrs []string, db database.DB,
 	}
 
 	s := server{
+		targetOutbound:       defaultTargetOutbound,
 		chainParams:          chainParams,
 		addrManager:          amgr,
 		peerState:            makePeerState(),
@@ -4039,15 +4067,14 @@ func newServer(ctx context.Context, listenAddrs []string, db database.DB,
 	}
 
 	// Create a connection manager.
-	targetOutbound := defaultTargetOutbound
-	if cfg.MaxPeers < targetOutbound {
-		targetOutbound = cfg.MaxPeers
+	if uint32(cfg.MaxPeers) < s.targetOutbound {
+		s.targetOutbound = uint32(cfg.MaxPeers)
 	}
 	cmgr, err := connmgr.New(&connmgr.Config{
 		Listeners:      listeners,
 		OnAccept:       s.inboundPeerConnected,
 		RetryDuration:  connectionRetryInterval,
-		TargetOutbound: uint32(targetOutbound),
+		TargetOutbound: s.targetOutbound,
 		Dial:           s.attemptDcrdDial,
 		Timeout:        cfg.DialTimeout,
 		OnConnection:   s.outboundPeerConnected,