9111774206

package htlcswitch

import (
        "bytes"
        "container/list"
        "errors"
        "fmt"
        "sync"
        "time"

        "github.com/lightningnetwork/lnd/clock"
        "github.com/lightningnetwork/lnd/lnwallet/chainfee"
        "github.com/lightningnetwork/lnd/lnwire"
)

var (
        // ErrMailBoxShuttingDown is returned when the mailbox is interrupted by
        // a shutdown request.
        ErrMailBoxShuttingDown = errors.New("mailbox is shutting down")

        // ErrPacketAlreadyExists signals that an attempt to add a packet failed
        // because it already exists in the mailbox.
        ErrPacketAlreadyExists = errors.New("mailbox already has packet")
)

// MailBox is an interface which represents a concurrent-safe, in-order
// delivery queue for messages from the network and also from the main switch.
// This struct serves as a buffer between incoming messages, and messages to
// the handled by the link. Each of the mutating methods within this interface
// should be implemented in a non-blocking manner.
type MailBox interface {
        // AddMessage appends a new message to the end of the message queue.
        AddMessage(msg lnwire.Message) error

        // AddPacket appends a new message to the end of the packet queue.
        AddPacket(pkt *htlcPacket) error

        // HasPacket queries the packets for a circuit key, this is used to drop
        // packets bound for the switch that already have a queued response.
        HasPacket(CircuitKey) bool

        // AckPacket removes a packet from the mailboxes in-memory replay
        // buffer. This will prevent a packet from being delivered after a link
        // restarts if the switch has remained online. The returned boolean
        // indicates whether or not a packet with the passed incoming circuit
        // key was removed.
        AckPacket(CircuitKey) bool

        // FailAdd fails an UpdateAddHTLC that exists within the mailbox,
        // removing it from the in-memory replay buffer. This will prevent the
        // packet from being delivered after the link restarts if the switch has
        // remained online. The generated LinkError will show an
        // OutgoingFailureDownstreamHtlcAdd FailureDetail.
        FailAdd(pkt *htlcPacket)

        // MessageOutBox returns a channel that any new messages ready for
        // delivery will be sent on.
        MessageOutBox() chan lnwire.Message

        // PacketOutBox returns a channel that any new packets ready for
        // delivery will be sent on.
        PacketOutBox() chan *htlcPacket

        // Clears any pending wire messages from the inbox.
        ResetMessages() error

        // Reset the packet head to point at the first element in the list.
        ResetPackets() error

        // SetDustClosure takes in a closure that is used to evaluate whether
        // mailbox HTLC's are dust.
        SetDustClosure(isDust dustClosure)

        // SetFeeRate sets the feerate to be used when evaluating dust.
        SetFeeRate(feerate chainfee.SatPerKWeight)

        // DustPackets returns the dust sum for Adds in the mailbox for the
        // local and remote commitments.
        DustPackets() (lnwire.MilliSatoshi, lnwire.MilliSatoshi)

        // Start starts the mailbox and any goroutines it needs to operate
        // properly.
        Start()

        // Stop signals the mailbox and its goroutines for a graceful shutdown.
        Stop()
}

type mailBoxConfig struct {
        // shortChanID is the short channel id of the channel this mailbox
        // belongs to.
        shortChanID lnwire.ShortChannelID

        // forwardPackets send a varidic number of htlcPackets to the switch to
        // be routed. A quit channel should be provided so that the call can
        // properly exit during shutdown.
        forwardPackets func(chan struct{}, ...*htlcPacket) error

        // clock is a time source for the mailbox.
        clock clock.Clock

        // expiry is the interval after which Adds will be cancelled if they
        // have not been yet been delivered. The computed deadline will expiry
        // this long after the Adds are added via AddPacket.
        expiry time.Duration

        // failMailboxUpdate is used to fail an expired HTLC and use the
        // correct SCID if the underlying channel uses aliases.
        failMailboxUpdate func(outScid,
                mailboxScid lnwire.ShortChannelID) lnwire.FailureMessage
}

// memoryMailBox is an implementation of the MailBox struct backed by purely
// in-memory queues.
//
// TODO(morehouse): use typed lists instead of list.Lists to avoid type asserts.
type memoryMailBox struct {
        started sync.Once
        stopped sync.Once

        cfg *mailBoxConfig

        wireMessages *list.List
        wireMtx      sync.Mutex
        wireCond     *sync.Cond

        messageOutbox chan lnwire.Message
        msgReset      chan chan struct{}

        // repPkts is a queue for reply packets, e.g. Settles and Fails.
        repPkts  *list.List
        repIndex map[CircuitKey]*list.Element
        repHead  *list.Element

        // addPkts is a dedicated queue for Adds.
        addPkts  *list.List
        addIndex map[CircuitKey]*list.Element
        addHead  *list.Element

        pktMtx  sync.Mutex
        pktCond *sync.Cond

        pktOutbox chan *htlcPacket
        pktReset  chan chan struct{}

        wireShutdown chan struct{}
        pktShutdown  chan struct{}
        quit         chan struct{}

        // feeRate is set when the link receives or sends out fee updates. It
        // is refreshed when AttachMailBox is called in case a fee update did
        // not get committed. In some cases it may be out of sync with the
        // channel's feerate, but it should eventually get back in sync.
        feeRate chainfee.SatPerKWeight

        // isDust is set when AttachMailBox is called and serves to evaluate
        // the outstanding dust in the memoryMailBox given the current set
        // feeRate.
        isDust dustClosure
}

// newMemoryMailBox creates a new instance of the memoryMailBox.
func newMemoryMailBox(cfg *mailBoxConfig) *memoryMailBox {
        box := &memoryMailBox{
                cfg:           cfg,
                wireMessages:  list.New(),
                repPkts:       list.New(),
                addPkts:       list.New(),
                messageOutbox: make(chan lnwire.Message),
                pktOutbox:     make(chan *htlcPacket),
                msgReset:      make(chan chan struct{}, 1),
                pktReset:      make(chan chan struct{}, 1),
                repIndex:      make(map[CircuitKey]*list.Element),
                addIndex:      make(map[CircuitKey]*list.Element),
                wireShutdown:  make(chan struct{}),
                pktShutdown:   make(chan struct{}),
                quit:          make(chan struct{}),
        }
        box.wireCond = sync.NewCond(&box.wireMtx)
        box.pktCond = sync.NewCond(&box.pktMtx)

        return box
}

// A compile time assertion to ensure that memoryMailBox meets the MailBox
// interface.
var _ MailBox = (*memoryMailBox)(nil)

// courierType is an enum that reflects the distinct types of messages a
// MailBox can handle. Each type will be placed in an isolated mail box and
// will have a dedicated goroutine for delivering the messages.
type courierType uint8

const (
        // wireCourier is a type of courier that handles wire messages.
        wireCourier courierType = iota

        // pktCourier is a type of courier that handles htlc packets.
        pktCourier
)

// Start starts the mailbox and any goroutines it needs to operate properly.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) Start() {
        m.started.Do(func() {
                go m.wireMailCourier()
                go m.pktMailCourier()
        })
}

// ResetMessages blocks until all buffered wire messages are cleared.
func (m *memoryMailBox) ResetMessages() error {
        msgDone := make(chan struct{})
        select {
        case m.msgReset <- msgDone:
                return m.signalUntilReset(wireCourier, msgDone)
        case <-m.quit:
                return ErrMailBoxShuttingDown
        }
}

// ResetPackets blocks until the head of packets buffer is reset, causing the
// packets to be redelivered in order.
func (m *memoryMailBox) ResetPackets() error {
        pktDone := make(chan struct{})
        select {
        case m.pktReset <- pktDone:
                return m.signalUntilReset(pktCourier, pktDone)
        case <-m.quit:
                return ErrMailBoxShuttingDown
        }
}

// signalUntilReset strobes the condition variable for the specified inbox type
// until receiving a response that the mailbox has processed a reset.
func (m *memoryMailBox) signalUntilReset(cType courierType,
        done chan struct{}) error {

        for {

                switch cType {
                case wireCourier:
                        m.wireCond.Signal()
                case pktCourier:
                        m.pktCond.Signal()
                }

                select {
                case <-time.After(time.Millisecond):
                        continue
                case <-done:
                        return nil
                case <-m.quit:
                        return ErrMailBoxShuttingDown
                }
        }
}

// AckPacket removes the packet identified by it's incoming circuit key from the
// queue of packets to be delivered. The returned boolean indicates whether or
// not a packet with the passed incoming circuit key was removed.
//
// NOTE: It is safe to call this method multiple times for the same circuit key.
func (m *memoryMailBox) AckPacket(inKey CircuitKey) bool {
        m.pktCond.L.Lock()
        defer m.pktCond.L.Unlock()

        if entry, ok := m.repIndex[inKey]; ok {
                // Check whether we are removing the head of the queue. If so,
                // we must advance the head to the next packet before removing.
                // It's possible that the courier has already advanced the
                // repHead, so this check prevents the repHead from getting
                // desynchronized.
                if entry == m.repHead {
                        m.repHead = entry.Next()
                }
                m.repPkts.Remove(entry)
                delete(m.repIndex, inKey)

                return true
        }

        if entry, ok := m.addIndex[inKey]; ok {
                // Check whether we are removing the head of the queue. If so,
                // we must advance the head to the next add before removing.
                // It's possible that the courier has already advanced the
                // addHead, so this check prevents the addHead from getting
                // desynchronized.
                //
                // NOTE: While this event is rare for Settles or Fails, it could
                // be very common for Adds since the mailbox has the ability to
                // cancel Adds before they are delivered. When that occurs, the
                // head of addPkts has only been peeked and we expect to be
                // removing the head of the queue.
                if entry == m.addHead {
                        m.addHead = entry.Next()
                }

                m.addPkts.Remove(entry)
                delete(m.addIndex, inKey)

                return true
        }

        return false
}

// HasPacket queries the packets for a circuit key, this is used to drop packets
// bound for the switch that already have a queued response.
func (m *memoryMailBox) HasPacket(inKey CircuitKey) bool {
        m.pktCond.L.Lock()
        _, ok := m.repIndex[inKey]
        m.pktCond.L.Unlock()

        return ok
}

// Stop signals the mailbox and its goroutines for a graceful shutdown.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) Stop() {
        m.stopped.Do(func() {
                close(m.quit)

                m.signalUntilShutdown(wireCourier)
                m.signalUntilShutdown(pktCourier)
        })
}

// signalUntilShutdown strobes the condition variable of the passed courier
// type, blocking until the worker has exited.
func (m *memoryMailBox) signalUntilShutdown(cType courierType) {
        var (
                cond     *sync.Cond
                shutdown chan struct{}
        )

        switch cType {
        case wireCourier:
                cond = m.wireCond
                shutdown = m.wireShutdown
        case pktCourier:
                cond = m.pktCond
                shutdown = m.pktShutdown
        }

        for {
                select {
                case <-time.After(time.Millisecond):
                        cond.Signal()
                case <-shutdown:
                        return
                }
        }
}

// pktWithExpiry wraps an incoming packet and records the time at which it it
// should be canceled from the mailbox. This will be used to detect if it gets
// stuck in the mailbox and inform when to cancel back.
type pktWithExpiry struct {
        pkt    *htlcPacket
        expiry time.Time
}

func (p *pktWithExpiry) deadline(clock clock.Clock) <-chan time.Time {
        return clock.TickAfter(p.expiry.Sub(clock.Now()))
}

// wireMailCourier is a dedicated goroutine whose job is to reliably deliver
// wire messages.
func (m *memoryMailBox) wireMailCourier() {
        defer close(m.wireShutdown)

        for {
                // First, we'll check our condition. If our mailbox is empty,
                // then we'll wait until a new item is added.
                m.wireCond.L.Lock()
                for m.wireMessages.Front() == nil {
                        m.wireCond.Wait()

                        select {
                        case msgDone := <-m.msgReset:
                                m.wireMessages.Init()
                                close(msgDone)
                        case <-m.quit:
                                m.wireCond.L.Unlock()
                                return
                        default:
                        }
                }

                // Grab the datum off the front of the queue, shifting the
                // slice's reference down one in order to remove the datum from
                // the queue.
                entry := m.wireMessages.Front()

                //nolint:forcetypeassert
                nextMsg := m.wireMessages.Remove(entry).(lnwire.Message)

                // Now that we're done with the condition, we can unlock it to
                // allow any callers to append to the end of our target queue.
                m.wireCond.L.Unlock()

                // With the next message obtained, we'll now select to attempt
                // to deliver the message. If we receive a kill signal, then
                // we'll bail out.
                select {
                case m.messageOutbox <- nextMsg:
                case msgDone := <-m.msgReset:
                        m.wireCond.L.Lock()
                        m.wireMessages.Init()
                        m.wireCond.L.Unlock()

                        close(msgDone)
                case <-m.quit:
                        return
                }
        }
}

// pktMailCourier is a dedicated goroutine whose job is to reliably deliver
// packet messages.
func (m *memoryMailBox) pktMailCourier() {
        defer close(m.pktShutdown)

        for {
                // First, we'll check our condition. If our mailbox is empty,
                // then we'll wait until a new item is added.
                m.pktCond.L.Lock()
                for m.repHead == nil && m.addHead == nil {
                        m.pktCond.Wait()

                        select {
                        // Resetting the packet queue means just moving our
                        // pointer to the front. This ensures that any un-ACK'd
                        // messages are re-delivered upon reconnect.
                        case pktDone := <-m.pktReset:
                                m.repHead = m.repPkts.Front()
                                m.addHead = m.addPkts.Front()

                                close(pktDone)

                        case <-m.quit:
                                m.pktCond.L.Unlock()
                                return
                        default:
                        }
                }

                var (
                        nextRep   *htlcPacket
                        nextRepEl *list.Element
                        nextAdd   *pktWithExpiry
                        nextAddEl *list.Element
                )
                // For packets, we actually never remove an item until it has
                // been ACK'd by the link. This ensures that if a read packet
                // doesn't make it into a commitment, then it'll be
                // re-delivered once the link comes back online.

                // Peek at the head of the Settle/Fails and Add queues. We peak
                // both even if there is a Settle/Fail present because we need
                // to set a deadline for the next pending Add if it's present.
                // Due to clock monotonicity, we know that the head of the Adds
                // is the next to expire.
                if m.repHead != nil {
                        //nolint:forcetypeassert
                        nextRep = m.repHead.Value.(*htlcPacket)
                        nextRepEl = m.repHead
                }
                if m.addHead != nil {
                        //nolint:forcetypeassert
                        nextAdd = m.addHead.Value.(*pktWithExpiry)
                        nextAddEl = m.addHead
                }

                // Now that we're done with the condition, we can unlock it to
                // allow any callers to append to the end of our target queue.
                m.pktCond.L.Unlock()

                var (
                        pktOutbox chan *htlcPacket
                        addOutbox chan *htlcPacket
                        add       *htlcPacket
                        deadline  <-chan time.Time
                )

                // Prioritize delivery of Settle/Fail packets over Adds. This
                // ensures that we actively clear the commitment of existing
                // HTLCs before trying to add new ones. This can help to improve
                // forwarding performance since the time to sign a commitment is
                // linear in the number of HTLCs manifested on the commitments.
                //
                // NOTE: Both types are eventually delivered over the same
                // channel, but we can control which is delivered by exclusively
                // making one nil and the other non-nil. We know from our loop
                // condition that at least one nextRep and nextAdd are non-nil.
                if nextRep != nil {
                        pktOutbox = m.pktOutbox
                } else {
                        addOutbox = m.pktOutbox
                }

                // If we have a pending Add, we'll also construct the deadline
                // so we can fail it back if we are unable to deliver any
                // message in time. We also dereference the nextAdd's packet,
                // since we will need access to it in the case we are delivering
                // it and/or if the deadline expires.
                //
                // NOTE: It's possible after this point for add to be nil, but
                // this can only occur when addOutbox is also nil, hence we
                // won't accidentally deliver a nil packet.
                if nextAdd != nil {
                        add = nextAdd.pkt
                        deadline = nextAdd.deadline(m.cfg.clock)
                }

                select {
                case pktOutbox <- nextRep:
                        m.pktCond.L.Lock()
                        // Only advance the repHead if this Settle or Fail is
                        // still at the head of the queue.
                        if m.repHead != nil && m.repHead == nextRepEl {
                                m.repHead = m.repHead.Next()
                        }
                        m.pktCond.L.Unlock()

                case addOutbox <- add:
                        m.pktCond.L.Lock()
                        // Only advance the addHead if this Add is still at the
                        // head of the queue.
                        if m.addHead != nil && m.addHead == nextAddEl {
                                m.addHead = m.addHead.Next()
                        }
                        m.pktCond.L.Unlock()

                case <-deadline:
                        log.Debugf("Expiring add htlc with "+
                                "keystone=%v", add.keystone())
                        m.FailAdd(add)

                case pktDone := <-m.pktReset:
                        m.pktCond.L.Lock()
                        m.repHead = m.repPkts.Front()
                        m.addHead = m.addPkts.Front()
                        m.pktCond.L.Unlock()

                        close(pktDone)

                case <-m.quit:
                        return
                }
        }
}

// AddMessage appends a new message to the end of the message queue.
//
// NOTE: This method is safe for concrete use and part of the MailBox
// interface.
func (m *memoryMailBox) AddMessage(msg lnwire.Message) error {
        // First, we'll lock the condition, and add the message to the end of
        // the wire message inbox.
        m.wireCond.L.Lock()
        m.wireMessages.PushBack(msg)
        m.wireCond.L.Unlock()

        // With the message added, we signal to the mailCourier that there are
        // additional messages to deliver.
        m.wireCond.Signal()

        return nil
}

// AddPacket appends a new message to the end of the packet queue.
//
// NOTE: This method is safe for concrete use and part of the MailBox
// interface.
func (m *memoryMailBox) AddPacket(pkt *htlcPacket) error {
        m.pktCond.L.Lock()
        switch htlc := pkt.htlc.(type) {
        // Split off Settle/Fail packets into the repPkts queue.
        case *lnwire.UpdateFulfillHTLC, *lnwire.UpdateFailHTLC:
                if _, ok := m.repIndex[pkt.inKey()]; ok {
                        m.pktCond.L.Unlock()
                        return ErrPacketAlreadyExists
                }

                entry := m.repPkts.PushBack(pkt)
                m.repIndex[pkt.inKey()] = entry
                if m.repHead == nil {
                        m.repHead = entry
                }

        // Split off Add packets into the addPkts queue.
        case *lnwire.UpdateAddHTLC:
                if _, ok := m.addIndex[pkt.inKey()]; ok {
                        m.pktCond.L.Unlock()
                        return ErrPacketAlreadyExists
                }

                entry := m.addPkts.PushBack(&pktWithExpiry{
                        pkt:    pkt,
                        expiry: m.cfg.clock.Now().Add(m.cfg.expiry),
                })
                m.addIndex[pkt.inKey()] = entry
                if m.addHead == nil {
                        m.addHead = entry
                }

        default:
                m.pktCond.L.Unlock()
                return fmt.Errorf("unknown htlc type: %T", htlc)
        }
        m.pktCond.L.Unlock()

        // With the packet added, we signal to the mailCourier that there are
        // additional packets to consume.
        m.pktCond.Signal()

        return nil
}

// SetFeeRate sets the memoryMailBox's feerate for use in DustPackets.
func (m *memoryMailBox) SetFeeRate(feeRate chainfee.SatPerKWeight) {
        m.pktCond.L.Lock()
        defer m.pktCond.L.Unlock()

        m.feeRate = feeRate
}

// SetDustClosure sets the memoryMailBox's dustClosure for use in DustPackets.
func (m *memoryMailBox) SetDustClosure(isDust dustClosure) {
        m.pktCond.L.Lock()
        defer m.pktCond.L.Unlock()

        m.isDust = isDust
}

// DustPackets returns the dust sum for add packets in the mailbox. The first
// return value is the local dust sum and the second is the remote dust sum.
// This will keep track of a given dust HTLC from the time it is added via
// AddPacket until it is removed via AckPacket.
func (m *memoryMailBox) DustPackets() (lnwire.MilliSatoshi,
        lnwire.MilliSatoshi) {

        m.pktCond.L.Lock()
        defer m.pktCond.L.Unlock()

        var (
                localDustSum  lnwire.MilliSatoshi
                remoteDustSum lnwire.MilliSatoshi
        )

        // Run through the map of HTLC's and determine the dust sum with calls
        // to the memoryMailBox's isDust closure. Note that all mailbox packets
        // are outgoing so the second argument to isDust will be false.
        for _, e := range m.addIndex {
                addPkt := e.Value.(*pktWithExpiry).pkt

                // Evaluate whether this HTLC is dust on the local commitment.
                if m.isDust(
                        m.feeRate, false, true, addPkt.amount.ToSatoshis(),
                ) {

                        localDustSum += addPkt.amount
                }

                // Evaluate whether this HTLC is dust on the remote commitment.
                if m.isDust(
                        m.feeRate, false, false, addPkt.amount.ToSatoshis(),
                ) {

                        remoteDustSum += addPkt.amount
                }
        }

        return localDustSum, remoteDustSum
}

// FailAdd fails an UpdateAddHTLC that exists within the mailbox, removing it
// from the in-memory replay buffer. This will prevent the packet from being
// delivered after the link restarts if the switch has remained online. The
// generated LinkError will show an OutgoingFailureDownstreamHtlcAdd
// FailureDetail.
func (m *memoryMailBox) FailAdd(pkt *htlcPacket) {
        // First, remove the packet from mailbox. If we didn't find the packet
        // because it has already been acked, we'll exit early to avoid sending
        // a duplicate fail message through the switch.
        if !m.AckPacket(pkt.inKey()) {
                return
        }

        var (
                localFailure = false
                reason       lnwire.OpaqueReason
        )

        // Create a temporary channel failure which we will send back to our
        // peer if this is a forward, or report to the user if the failed
        // payment was locally initiated.
        failure := m.cfg.failMailboxUpdate(
                pkt.originalOutgoingChanID, m.cfg.shortChanID,
        )

        // If the payment was locally initiated (which is indicated by a nil
        // obfuscator), we do not need to encrypt it back to the sender.
        if pkt.obfuscator == nil {
                var b bytes.Buffer
                err := lnwire.EncodeFailure(&b, failure, 0)
                if err != nil {
                        log.Errorf("Unable to encode failure: %v", err)
                        return
                }
                reason = lnwire.OpaqueReason(b.Bytes())
                localFailure = true
        } else {
                // If the packet is part of a forward, (identified by a non-nil
                // obfuscator) we need to encrypt the error back to the source.
                var err error
                reason, err = pkt.obfuscator.EncryptFirstHop(failure)
                if err != nil {
                        log.Errorf("Unable to obfuscate error: %v", err)
                        return
                }
        }

        // Create a link error containing the temporary channel failure and a
        // detail which indicates the we failed to add the htlc.
        linkError := NewDetailedLinkError(
                failure, OutgoingFailureDownstreamHtlcAdd,
        )

        failPkt := &htlcPacket{
                incomingChanID: pkt.incomingChanID,
                incomingHTLCID: pkt.incomingHTLCID,
                circuit:        pkt.circuit,
                sourceRef:      pkt.sourceRef,
                hasSource:      true,
                localFailure:   localFailure,
                obfuscator:     pkt.obfuscator,
                linkFailure:    linkError,
                htlc: &lnwire.UpdateFailHTLC{
                        Reason: reason,
                },
        }

        if err := m.cfg.forwardPackets(m.quit, failPkt); err != nil {
                log.Errorf("Unhandled error while reforwarding packets "+
                        "settle/fail over htlcswitch: %v", err)
        }
}

// MessageOutBox returns a channel that any new messages ready for delivery
// will be sent on.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) MessageOutBox() chan lnwire.Message {
        return m.messageOutbox
}

// PacketOutBox returns a channel that any new packets ready for delivery will
// be sent on.
//
// NOTE: This method is part of the MailBox interface.
func (m *memoryMailBox) PacketOutBox() chan *htlcPacket {
        return m.pktOutbox
}

// mailOrchestrator is responsible for coordinating the creation and lifecycle
// of mailboxes used within the switch. It supports the ability to create
// mailboxes, reassign their short channel id's, deliver htlc packets, and
// queue packets for mailboxes that have not been created due to a link's late
// registration.
type mailOrchestrator struct {
        mu sync.RWMutex

        cfg *mailOrchConfig

        // mailboxes caches exactly one mailbox for all known channels.
        mailboxes map[lnwire.ChannelID]MailBox

        // liveIndex maps a live short chan id to the primary mailbox key.
        // An index in liveIndex map is only entered under two conditions:
        //   1. A link has a non-zero short channel id at time of AddLink.
        //   2. A link receives a non-zero short channel via UpdateShortChanID.
        liveIndex map[lnwire.ShortChannelID]lnwire.ChannelID

        // TODO(conner): add another pair of indexes:
        //   chan_id -> short_chan_id
        //   short_chan_id -> mailbox
        // so that Deliver can lookup mailbox directly once live,
        // but still queryable by channel_id.

        // unclaimedPackets maps a live short chan id to queue of packets if no
        // mailbox has been created.
        unclaimedPackets map[lnwire.ShortChannelID][]*htlcPacket
}

type mailOrchConfig struct {
        // forwardPackets send a varidic number of htlcPackets to the switch to
        // be routed. A quit channel should be provided so that the call can
        // properly exit during shutdown.
        forwardPackets func(chan struct{}, ...*htlcPacket) error

        // clock is a time source for the generated mailboxes.
        clock clock.Clock

        // expiry is the interval after which Adds will be cancelled if they
        // have not been yet been delivered. The computed deadline will expiry
        // this long after the Adds are added to a mailbox via AddPacket.
        expiry time.Duration

        // failMailboxUpdate is used to fail an expired HTLC and use the
        // correct SCID if the underlying channel uses aliases.
        failMailboxUpdate func(outScid,
                mailboxScid lnwire.ShortChannelID) lnwire.FailureMessage
}

// newMailOrchestrator initializes a fresh mailOrchestrator.
func newMailOrchestrator(cfg *mailOrchConfig) *mailOrchestrator {
        return &mailOrchestrator{
                cfg:              cfg,
                mailboxes:        make(map[lnwire.ChannelID]MailBox),
                liveIndex:        make(map[lnwire.ShortChannelID]lnwire.ChannelID),
                unclaimedPackets: make(map[lnwire.ShortChannelID][]*htlcPacket),
        }
}

// Stop instructs the orchestrator to stop all active mailboxes.
func (mo *mailOrchestrator) Stop() {
        for _, mailbox := range mo.mailboxes {
                mailbox.Stop()
        }
}

// GetOrCreateMailBox returns an existing mailbox belonging to `chanID`, or
// creates and returns a new mailbox if none is found.
func (mo *mailOrchestrator) GetOrCreateMailBox(chanID lnwire.ChannelID,
        shortChanID lnwire.ShortChannelID) MailBox {

        // First, try lookup the mailbox directly using only the shared mutex.
        mo.mu.RLock()
        mailbox, ok := mo.mailboxes[chanID]
        if ok {
                mo.mu.RUnlock()
                return mailbox
        }
        mo.mu.RUnlock()

        // Otherwise, we will try again with exclusive lock, creating a mailbox
        // if one still has not been created.
        mo.mu.Lock()
        mailbox = mo.exclusiveGetOrCreateMailBox(chanID, shortChanID)
        mo.mu.Unlock()

        return mailbox
}

// exclusiveGetOrCreateMailBox checks for the existence of a mailbox for the
// given channel id. If none is found, a new one is creates, started, and
// recorded.
//
// NOTE: This method MUST be invoked with the mailOrchestrator's exclusive lock.
func (mo *mailOrchestrator) exclusiveGetOrCreateMailBox(
        chanID lnwire.ChannelID, shortChanID lnwire.ShortChannelID) MailBox {

        mailbox, ok := mo.mailboxes[chanID]
        if !ok {
                mailbox = newMemoryMailBox(&mailBoxConfig{
                        shortChanID:       shortChanID,
                        forwardPackets:    mo.cfg.forwardPackets,
                        clock:             mo.cfg.clock,
                        expiry:            mo.cfg.expiry,
                        failMailboxUpdate: mo.cfg.failMailboxUpdate,
                })
                mailbox.Start()
                mo.mailboxes[chanID] = mailbox
        }

        return mailbox
}

// BindLiveShortChanID registers that messages bound for a particular short
// channel id should be forwarded to the mailbox corresponding to the given
// channel id. This method also checks to see if there are any unclaimed
// packets for this short_chan_id. If any are found, they are delivered to the
// mailbox and removed (marked as claimed).
func (mo *mailOrchestrator) BindLiveShortChanID(mailbox MailBox,
        cid lnwire.ChannelID, sid lnwire.ShortChannelID) {

        mo.mu.Lock()
        // Update the mapping from short channel id to mailbox's channel id.
        mo.liveIndex[sid] = cid

        // Retrieve any unclaimed packets destined for this mailbox.
        pkts := mo.unclaimedPackets[sid]
        delete(mo.unclaimedPackets, sid)
        mo.mu.Unlock()

        // Deliver the unclaimed packets.
        for _, pkt := range pkts {
                mailbox.AddPacket(pkt)
        }
}

// Deliver lookups the target mailbox using the live index from short_chan_id
// to channel_id. If the mailbox is found, the message is delivered directly.
// Otherwise the packet is recorded as unclaimed, and will be delivered to the
// mailbox upon the subsequent call to BindLiveShortChanID.
func (mo *mailOrchestrator) Deliver(
        sid lnwire.ShortChannelID, pkt *htlcPacket) error {

        var (
                mailbox MailBox
                found   bool
        )

        // First, try to find the channel id for the target short_chan_id. If
        // the link is live, we will also look up the created mailbox.
        mo.mu.RLock()
        chanID, isLive := mo.liveIndex[sid]
        if isLive {
                mailbox, found = mo.mailboxes[chanID]
        }
        mo.mu.RUnlock()

        // The link is live and target mailbox was found, deliver immediately.
        if isLive && found {
                return mailbox.AddPacket(pkt)
        }

        // If we detected that the link has not been made live, we will acquire
        // the exclusive lock preemptively in order to queue this packet in the
        // list of unclaimed packets.
        mo.mu.Lock()

        // Double check to see if the mailbox has been not made live since the
        // release of the shared lock.
        //
        // NOTE: Checking again with the exclusive lock held prevents a race
        // condition where BindLiveShortChanID is interleaved between the
        // release of the shared lock, and acquiring the exclusive lock. The
        // result would be stuck packets, as they wouldn't be redelivered until
        // the next call to BindLiveShortChanID, which is expected to occur
        // infrequently.
        chanID, isLive = mo.liveIndex[sid]
        if isLive {
                // Reaching this point indicates the mailbox is actually live.
                // We'll try to load the mailbox using the fresh channel id.
                //
                // NOTE: This should never create a new mailbox, as the live
                // index should only be set if the mailbox had been initialized
                // beforehand.  However, this does ensure that this case is
                // handled properly in the event that it could happen.
                mailbox = mo.exclusiveGetOrCreateMailBox(chanID, sid)
                mo.mu.Unlock()

                // Deliver the packet to the mailbox if it was found or created.
                return mailbox.AddPacket(pkt)
        }

        // Finally, if the channel id is still not found in the live index,
        // we'll add this to the list of unclaimed packets. These will be
        // delivered upon the next call to BindLiveShortChanID.
        mo.unclaimedPackets[sid] = append(mo.unclaimedPackets[sid], pkt)
        mo.mu.Unlock()

        return nil
}

lightningnetwork / lnd / 9111774206

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