vppn/peer/peerstates.go

package peer

import (
	"fmt"
	"log"
	"net/netip"
	"strings"
	"time"
	"vppn/m"

	"git.crumpington.com/lib/go/ratelimiter"
)

type peerState interface {
	OnPeerUpdate(*m.Peer) peerState
	OnSyn(controlMsg[packetSyn]) peerState
	OnAck(controlMsg[packetAck])
	OnProbe(controlMsg[packetProbe]) peerState
	OnLocalDiscovery(controlMsg[packetLocalDiscovery])
	OnPingTimer() peerState
}

// ----------------------------------------------------------------------------

type pState struct {
	// Output.
	publish           func(remotePeer)
	sendControlPacket func(remotePeer, marshaller)

	// Immutable data.
	localIP   byte
	remoteIP  byte
	privKey   []byte
	localAddr netip.AddrPort // If valid, then local peer is publicly accessible.

	pubAddrs *pubAddrStore

	// The purpose of this state machine is to manage the RemotePeer object,
	// publishing it as necessary.
	staged remotePeer // Local copy of shared data. See publish().

	// Mutable peer data.
	peer *m.Peer

	// We rate limit per remote endpoint because if we don't we tend to lose
	// packets.
	limiter *ratelimiter.Limiter
}

func (s *pState) OnPeerUpdate(peer *m.Peer) peerState {
	defer func() {
		// Don't defer directly otherwise s.staged will be evaluated immediately
		// and won't reflect changes made in the function.
		s.publish(s.staged)
	}()

	s.peer = peer
	s.staged.localIP = s.localIP
	s.staged.Up = false
	s.staged.Relay = false
	s.staged.Direct = false
	s.staged.DirectAddr = netip.AddrPort{}
	s.staged.PubSignKey = nil
	s.staged.ControlCipher = nil
	s.staged.DataCipher = nil

	if peer == nil {
		return enterStateDisconnected(s)
	}

	s.staged.IP = peer.PeerIP
	s.staged.PubSignKey = peer.PubSignKey
	log.Printf("New cipher: %x, %x", s.privKey, peer.PubKey)
	s.staged.ControlCipher = newControlCipher(s.privKey, peer.PubKey)
	s.staged.DataCipher = newDataCipher()

	if ip, isValid := netip.AddrFromSlice(peer.PublicIP); isValid {
		s.staged.Relay = peer.Relay
		s.staged.Direct = true
		s.staged.DirectAddr = netip.AddrPortFrom(ip, peer.Port)

		if s.localAddr.IsValid() && s.localIP < s.remoteIP {
			return enterStateServer(s)
		}

		return enterStateClientDirect(s)
	}

	if s.localAddr.IsValid() {
		s.staged.Direct = true
		return enterStateServer(s)
	}

	if s.localIP < s.remoteIP {
		return enterStateServer(s)
	}

	return enterStateClientRelayed(s)
}

func (s *pState) logf(format string, args ...any) {
	b := strings.Builder{}
	name := ""
	if s.peer != nil {
		name = s.peer.Name
	}
	b.WriteString(fmt.Sprintf("%03d", s.remoteIP))

	b.WriteString(fmt.Sprintf("%30s: ", name))

	if s.staged.Direct {
		b.WriteString("DIRECT  | ")
	} else {
		b.WriteString("RELAYED | ")
	}

	if s.staged.Up {
		b.WriteString("UP   | ")
	} else {
		b.WriteString("DOWN | ")
	}

	log.Printf(b.String()+format, args...)
}

// ----------------------------------------------------------------------------

func (s *pState) SendTo(pkt marshaller, addr netip.AddrPort) {
	if !addr.IsValid() {
		return
	}
	route := s.staged
	route.Direct = true
	route.DirectAddr = addr
	s.Send(route, pkt)
}

func (s *pState) Send(peer remotePeer, pkt marshaller) {
	if err := s.limiter.Limit(); err != nil {
		s.logf("Rate limited.")
		return
	}
	s.sendControlPacket(peer, pkt)
}

// ----------------------------------------------------------------------------

type stateDisconnected struct{ *pState }

func enterStateDisconnected(s *pState) peerState {
	return &stateDisconnected{pState: s}
}

func (s *stateDisconnected) OnSyn(controlMsg[packetSyn]) peerState             { return s }
func (s *stateDisconnected) OnAck(controlMsg[packetAck])                       {}
func (s *stateDisconnected) OnProbe(controlMsg[packetProbe]) peerState         { return s }
func (s *stateDisconnected) OnLocalDiscovery(controlMsg[packetLocalDiscovery]) {}
func (s *stateDisconnected) OnPingTimer() peerState                            { return s }

// ----------------------------------------------------------------------------

type stateServer struct {
	*stateDisconnected
	lastSeen   time.Time
	synTraceID uint64
}

func enterStateServer(s *pState) peerState {
	s.logf("==> Server")
	return &stateServer{stateDisconnected: &stateDisconnected{pState: s}}
}

func (s *stateServer) OnSyn(msg controlMsg[packetSyn]) peerState {
	s.lastSeen = time.Now()
	p := msg.Packet

	// Before we can respond to this packet, we need to make sure the
	// route is setup properly.
	//
	// The client will update the syn's TraceID whenever there's a change.
	// The server will follow the client's request.
	if p.TraceID != s.synTraceID || !s.staged.Up {
		s.synTraceID = p.TraceID
		s.staged.Up = true
		s.staged.Direct = p.Direct
		s.staged.DataCipher = newDataCipherFromKey(p.SharedKey)
		s.staged.DirectAddr = msg.SrcAddr
		s.publish(s.staged)
		s.logf("Got SYN.")
	}

	// Always respond.
	ack := packetAck{
		TraceID:       p.TraceID,
		ToAddr:        s.staged.DirectAddr,
		PossibleAddrs: s.pubAddrs.Get(),
	}
	s.Send(s.staged, ack)

	if p.Direct {
		return s
	}

	for _, addr := range msg.Packet.PossibleAddrs {
		if !addr.IsValid() {
			break
		}
		s.SendTo(packetProbe{TraceID: newTraceID()}, addr)
	}

	return s
}

func (s *stateServer) OnProbe(msg controlMsg[packetProbe]) peerState {
	if msg.SrcAddr.IsValid() {
		s.SendTo(packetProbe{TraceID: msg.Packet.TraceID}, msg.SrcAddr)
	}
	return s
}

func (s *stateServer) OnPingTimer() peerState {
	if time.Since(s.lastSeen) > timeoutInterval && s.staged.Up {
		s.staged.Up = false
		s.publish(s.staged)
		s.logf("Timeout.")
	}
	return s
}

// ----------------------------------------------------------------------------

type stateClientDirect struct {
	*stateDisconnected
	lastSeen time.Time
	syn      packetSyn
}

func enterStateClientDirect(s *pState) peerState {
	s.logf("==> ClientDirect")
	return newStateClientDirect(s)
}

func newStateClientDirect(s *pState) *stateClientDirect {
	state := &stateClientDirect{
		stateDisconnected: &stateDisconnected{s},
		lastSeen:          time.Now(), // Avoid immediate timeout.
	}

	state.syn = packetSyn{
		TraceID:       newTraceID(),
		SharedKey:     s.staged.DataCipher.Key(),
		Direct:        s.staged.Direct,
		PossibleAddrs: s.pubAddrs.Get(),
	}
	state.Send(s.staged, state.syn)
	return state
}

func (s *stateClientDirect) OnAck(msg controlMsg[packetAck]) {
	if msg.Packet.TraceID != s.syn.TraceID {
		return
	}

	s.lastSeen = time.Now()

	if !s.staged.Up {
		s.staged.Up = true
		s.publish(s.staged)
		s.logf("Got ACK.")
	}

	s.pubAddrs.Store(msg.Packet.ToAddr)
}

func (s *stateClientDirect) OnPingTimer() peerState {
	if next := s.onPingTimer(); next != nil {
		return next
	}
	return s
}

func (s *stateClientDirect) onPingTimer() peerState {
	if time.Since(s.lastSeen) > timeoutInterval {
		if s.staged.Up {
			s.staged.Up = false
			s.publish(s.staged)
			s.logf("Timeout.")
		}
		return s.OnPeerUpdate(s.peer)
	}

	s.Send(s.staged, s.syn)
	return nil
}

// ----------------------------------------------------------------------------

type stateClientRelayed struct {
	*stateClientDirect
	ack                packetAck
	probes             map[uint64]netip.AddrPort
	localDiscoveryAddr netip.AddrPort
}

func enterStateClientRelayed(s *pState) peerState {
	s.logf("==> ClientRelayed")
	return &stateClientRelayed{
		stateClientDirect: newStateClientDirect(s),
		probes:            map[uint64]netip.AddrPort{},
	}
}

func (s *stateClientRelayed) OnAck(msg controlMsg[packetAck]) {
	s.ack = msg.Packet
	s.stateClientDirect.OnAck(msg)
}

func (s *stateClientRelayed) OnProbe(msg controlMsg[packetProbe]) peerState {
	addr, ok := s.probes[msg.Packet.TraceID]
	if !ok {
		return s
	}

	s.staged.DirectAddr = addr
	s.staged.Direct = true
	s.publish(s.staged)
	return enterStateClientDirect(s.stateClientDirect.pState)
}

func (s *stateClientRelayed) OnLocalDiscovery(msg controlMsg[packetLocalDiscovery]) {
	// The source port will be the multicast port, so we'll have to
	// construct the correct address using the peer's listed port.
	s.localDiscoveryAddr = netip.AddrPortFrom(msg.SrcAddr.Addr(), s.peer.Port)
}

func (s *stateClientRelayed) OnPingTimer() peerState {
	if next := s.stateClientDirect.onPingTimer(); next != nil {
		return next
	}

	clear(s.probes)
	for _, addr := range s.ack.PossibleAddrs {
		if !addr.IsValid() {
			break
		}
		s.sendProbeTo(addr)
	}

	if s.localDiscoveryAddr.IsValid() {
		s.sendProbeTo(s.localDiscoveryAddr)
		s.localDiscoveryAddr = netip.AddrPort{}
	}

	return s
}

func (s *stateClientRelayed) sendProbeTo(addr netip.AddrPort) {
	probe := packetProbe{TraceID: newTraceID()}
	s.probes[probe.TraceID] = addr
	s.SendTo(probe, addr)
}