dissertation-2-code/udp/flow.go

package udp

import (
	"errors"
	"fmt"
	"log"
	"mpbl3p/proxy"
	"mpbl3p/shared"
	"net"
	"sync"
	"time"
)

type PacketWriter interface {
	Write(b []byte) (int, error)
	WriteToUDP(b []byte, addr *net.UDPAddr) (int, error)
	LocalAddr() net.Addr
}

type PacketConn interface {
	PacketWriter
	ReadFromUDP(b []byte) (int, *net.UDPAddr, error)
}

type InitiatedFlow struct {
	Local  func() string
	Remote string

	g         proxy.MacGenerator
	keepalive time.Duration

	mu sync.RWMutex
	Flow
}

func (f *InitiatedFlow) String() string {
	return fmt.Sprintf("UdpOutbound{%v -> %v}", f.Local(), f.Remote)
}

type Flow struct {
	writer PacketWriter
	raddr  *net.UDPAddr

	isAlive    bool
	startup    bool
	congestion Congestion

	v proxy.MacVerifier

	inboundDatagrams chan []byte
}

func (f Flow) String() string {
	return fmt.Sprintf("UdpInbound{%v -> %v}", f.raddr, f.writer.LocalAddr())
}

func InitiateFlow(
	local func() string,
	remote string,
	v proxy.MacVerifier,
	g proxy.MacGenerator,
	c Congestion,
	keepalive time.Duration,
) (*InitiatedFlow, error) {
	f := InitiatedFlow{
		Local:     local,
		Remote:    remote,
		Flow:      newFlow(c, v),
		g:         g,
		keepalive: keepalive,
	}

	return &f, nil
}

func newFlow(c Congestion, v proxy.MacVerifier) Flow {
	return Flow{
		inboundDatagrams: make(chan []byte),
		congestion:       c,
		v:                v,
	}
}

func (f *InitiatedFlow) Reconnect() error {
	f.mu.Lock()
	defer f.mu.Unlock()

	if f.isAlive {
		return nil
	}

	localAddr, err := net.ResolveUDPAddr("udp", f.Local())
	if err != nil {
		return err
	}

	remoteAddr, err := net.ResolveUDPAddr("udp", f.Remote)
	if err != nil {
		return err
	}

	conn, err := net.DialUDP("udp", localAddr, remoteAddr)
	if err != nil {
		return err
	}

	f.writer = conn
	f.startup = true

	// prod the connection once a second until we get an ack, then consider it alive
	go func() {
		seq := f.congestion.Sequence()

		for !f.isAlive {
			p := Packet{
				ack:  0,
				nack: 0,
				seq:  seq,
				data: proxy.SimplePacket(nil),
			}

			_ = f.sendPacket(p, f.g)
		}
	}()

	go func() {
		_, _ = f.produceInternal(f.v, false)
	}()
	go f.earlyUpdateLoop(f.g, f.keepalive)

	if err := f.acceptPacket(conn); err != nil {
		return err
	}

	f.isAlive = true
	f.startup = false

	go func() {
		lockedAccept := func() {
			f.mu.RLock()
			defer f.mu.RUnlock()

			if err := f.acceptPacket(conn); err != nil {
				log.Println(err)
			}
		}

		for f.isAlive {
			log.Println("alive and listening for packets")
			lockedAccept()
		}
		log.Println("no longer alive")
	}()

	return nil
}

func (f *InitiatedFlow) Consume(p proxy.Packet, g proxy.MacGenerator) error {
	f.mu.RLock()
	defer f.mu.RUnlock()

	return f.Flow.Consume(p, g)
}

func (f *InitiatedFlow) Produce(v proxy.MacVerifier) (proxy.Packet, error) {
	f.mu.RLock()
	defer f.mu.RUnlock()

	return f.Flow.Produce(v)
}

func (f *Flow) IsAlive() bool {
	return f.isAlive
}

func (f *Flow) Consume(pp proxy.Packet, g proxy.MacGenerator) error {
	if !f.isAlive {
		return shared.ErrDeadConnection
	}

	log.Println(f.congestion)

	// Sequence is the congestion controllers opportunity to block
	log.Println("awaiting sequence")
	p := Packet{
		seq:  f.congestion.Sequence(),
		data: pp,
	}
	log.Println("received sequence")

	// Choose up to date ACK/NACK even after blocking
	p.ack = f.congestion.NextAck()
	p.nack = f.congestion.NextNack()

	return f.sendPacket(p, g)
}

func (f *Flow) Produce(v proxy.MacVerifier) (proxy.Packet, error) {
	if !f.isAlive {
		return nil, shared.ErrDeadConnection
	}

	return f.produceInternal(v, true)
}

func (f *Flow) produceInternal(v proxy.MacVerifier, mustReturn bool) (proxy.Packet, error) {
	for once := true; mustReturn || once; once = false {
		log.Println(f.congestion)

		b, err := proxy.StripMac(<-f.inboundDatagrams, v)
		if err != nil {
			return nil, err
		}

		p, err := UnmarshalPacket(b)
		if err != nil {
			return nil, err
		}

		// schedule an ack for this sequence number
		if p.seq != 0 {
			f.congestion.ReceivedPacket(p.seq)
		}
		// adjust our sending congestion control based on their acks
		if p.ack != 0 {
			f.congestion.ReceivedAck(p.ack)
		}
		// adjust our sending congestion control based on their nacks
		if p.nack != 0 {
			f.congestion.ReceivedNack(p.nack)
		}

		// 12 bytes for header + the MAC + a timestamp
		if len(b) == 12+f.v.CodeLength()+8 {
			log.Println("handled keepalive/ack only packet")
			continue
		}

		return p, nil
	}

	return nil, nil
}

func (f *Flow) handleDatagram(p []byte) {
	f.inboundDatagrams <- p
}

func (f *Flow) sendPacket(p Packet, g proxy.MacGenerator) error {
	b := p.Marshal()
	b = proxy.AppendMac(b, g)

	if f.raddr == nil {
		_, err := f.writer.Write(b)
		return err
	} else {
		_, err := f.writer.WriteToUDP(b, f.raddr)
		return err
	}
}

func (f *Flow) earlyUpdateLoop(g proxy.MacGenerator, keepalive time.Duration) {
	var err error
	for !errors.Is(err, shared.ErrDeadConnection) {
		seq := f.congestion.AwaitEarlyUpdate(keepalive)
		p := Packet{
			ack:  f.congestion.NextAck(),
			nack: f.congestion.NextNack(),
			seq:  seq,
			data: proxy.SimplePacket(nil),
		}

		_ = f.sendPacket(p, g)
	}
}

func (f *Flow) acceptPacket(c PacketConn) error {
	buf := make([]byte, 6000)
	n, _, err := c.ReadFromUDP(buf)
	if err != nil {
		return err
	}

	f.handleDatagram(buf[:n])
	return nil
}