10189747109

Committed 01 Aug 2024 12:31AM UTC coverage: 58.641% (+0.2%) from 58.459%

Build # 10189747109

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push

github

Committed by

web-flow

Commit Message

Merge pull request #8949 from ProofOfKeags/fn/req

[MICRO]: fn: Add new Req type to abstract the pattern of remote processing.

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82.6

/htlcswitch/switch.go

package htlcswitch

import (
        "bytes"
        "errors"
        "fmt"
        "math/rand"
        "sync"
        "sync/atomic"
        "time"

        "github.com/btcsuite/btcd/btcec/v2/ecdsa"
        "github.com/btcsuite/btcd/btcutil"
        "github.com/btcsuite/btcd/wire"
        "github.com/davecgh/go-spew/spew"
        "github.com/lightningnetwork/lnd/chainntnfs"
        "github.com/lightningnetwork/lnd/channeldb"
        "github.com/lightningnetwork/lnd/channeldb/models"
        "github.com/lightningnetwork/lnd/clock"
        "github.com/lightningnetwork/lnd/contractcourt"
        "github.com/lightningnetwork/lnd/fn"
        "github.com/lightningnetwork/lnd/htlcswitch/hop"
        "github.com/lightningnetwork/lnd/kvdb"
        "github.com/lightningnetwork/lnd/lntypes"
        "github.com/lightningnetwork/lnd/lnutils"
        "github.com/lightningnetwork/lnd/lnwallet"
        "github.com/lightningnetwork/lnd/lnwallet/chainfee"
        "github.com/lightningnetwork/lnd/lnwire"
        "github.com/lightningnetwork/lnd/ticker"
)

const (
        // DefaultFwdEventInterval is the duration between attempts to flush
        // pending forwarding events to disk.
        DefaultFwdEventInterval = 15 * time.Second

        // DefaultLogInterval is the duration between attempts to log statistics
        // about forwarding events.
        DefaultLogInterval = 10 * time.Second

        // DefaultAckInterval is the duration between attempts to ack any settle
        // fails in a forwarding package.
        DefaultAckInterval = 15 * time.Second

        // DefaultMailboxDeliveryTimeout is the duration after which Adds will
        // be cancelled if they could not get added to an outgoing commitment.
        DefaultMailboxDeliveryTimeout = time.Minute
)

var (
        // ErrChannelLinkNotFound is used when channel link hasn't been found.
        ErrChannelLinkNotFound = errors.New("channel link not found")

        // ErrDuplicateAdd signals that the ADD htlc was already forwarded
        // through the switch and is locked into another commitment txn.
        ErrDuplicateAdd = errors.New("duplicate add HTLC detected")

        // ErrUnknownErrorDecryptor signals that we were unable to locate the
        // error decryptor for this payment. This is likely due to restarting
        // the daemon.
        ErrUnknownErrorDecryptor = errors.New("unknown error decryptor")

        // ErrSwitchExiting signaled when the switch has received a shutdown
        // request.
        ErrSwitchExiting = errors.New("htlcswitch shutting down")

        // ErrNoLinksFound is an error returned when we attempt to retrieve the
        // active links in the switch for a specific destination.
        ErrNoLinksFound = errors.New("no channel links found")

        // ErrUnreadableFailureMessage is returned when the failure message
        // cannot be decrypted.
        ErrUnreadableFailureMessage = errors.New("unreadable failure message")

        // ErrLocalAddFailed signals that the ADD htlc for a local payment
        // failed to be processed.
        ErrLocalAddFailed = errors.New("local add HTLC failed")

        // errFeeExposureExceeded is only surfaced to callers of SendHTLC and
        // signals that sending the HTLC would exceed the outgoing link's fee
        // exposure threshold.
        errFeeExposureExceeded = errors.New("fee exposure exceeded")

        // DefaultMaxFeeExposure is the default threshold after which we'll
        // fail payments if they increase our fee exposure. This is currently
        // set to 500m msats.
        DefaultMaxFeeExposure = lnwire.MilliSatoshi(500_000_000)
)

// plexPacket encapsulates switch packet and adds error channel to receive
// error from request handler.
type plexPacket struct {
        pkt *htlcPacket
        err chan error
}

// ChanClose represents a request which close a particular channel specified by
// its id.
type ChanClose struct {
        // CloseType is a variable which signals the type of channel closure the
        // peer should execute.
        CloseType contractcourt.ChannelCloseType

        // ChanPoint represent the id of the channel which should be closed.
        ChanPoint *wire.OutPoint

        // TargetFeePerKw is the ideal fee that was specified by the caller.
        // This value is only utilized if the closure type is CloseRegular.
        // This will be the starting offered fee when the fee negotiation
        // process for the cooperative closure transaction kicks off.
        TargetFeePerKw chainfee.SatPerKWeight

        // MaxFee is the highest fee the caller is willing to pay.
        //
        // NOTE: This field is only respected if the caller is the initiator of
        // the channel.
        MaxFee chainfee.SatPerKWeight

        // DeliveryScript is an optional delivery script to pay funds out to.
        DeliveryScript lnwire.DeliveryAddress

        // Updates is used by request creator to receive the notifications about
        // execution of the close channel request.
        Updates chan interface{}

        // Err is used by request creator to receive request execution error.
        Err chan error
}

// Config defines the configuration for the service. ALL elements within the
// configuration MUST be non-nil for the service to carry out its duties.
type Config struct {
        // FwdingLog is an interface that will be used by the switch to log
        // forwarding events. A forwarding event happens each time a payment
        // circuit is successfully completed. So when we forward an HTLC, and a
        // settle is eventually received.
        FwdingLog ForwardingLog

        // LocalChannelClose kicks-off the workflow to execute a cooperative or
        // forced unilateral closure of the channel initiated by a local
        // subsystem.
        LocalChannelClose func(pubKey []byte, request *ChanClose)

        // DB is the database backend that will be used to back the switch's
        // persistent circuit map.
        DB kvdb.Backend

        // FetchAllOpenChannels is a function that fetches all currently open
        // channels from the channel database.
        FetchAllOpenChannels func() ([]*channeldb.OpenChannel, error)

        // FetchAllChannels is a function that fetches all pending open, open,
        // and waiting close channels from the database.
        FetchAllChannels func() ([]*channeldb.OpenChannel, error)

        // FetchClosedChannels is a function that fetches all closed channels
        // from the channel database.
        FetchClosedChannels func(
                pendingOnly bool) ([]*channeldb.ChannelCloseSummary, error)

        // SwitchPackager provides access to the forwarding packages of all
        // active channels. This gives the switch the ability to read arbitrary
        // forwarding packages, and ack settles and fails contained within them.
        SwitchPackager channeldb.FwdOperator

        // ExtractErrorEncrypter is an interface allowing switch to reextract
        // error encrypters stored in the circuit map on restarts, since they
        // are not stored directly within the database.
        ExtractErrorEncrypter hop.ErrorEncrypterExtracter

        // FetchLastChannelUpdate retrieves the latest routing policy for a
        // target channel. This channel will typically be the outgoing channel
        // specified when we receive an incoming HTLC.  This will be used to
        // provide payment senders our latest policy when sending encrypted
        // error messages.
        FetchLastChannelUpdate func(lnwire.ShortChannelID) (*lnwire.ChannelUpdate, error)

        // Notifier is an instance of a chain notifier that we'll use to signal
        // the switch when a new block has arrived.
        Notifier chainntnfs.ChainNotifier

        // HtlcNotifier is an instance of a htlcNotifier which we will pipe htlc
        // events through.
        HtlcNotifier htlcNotifier

        // FwdEventTicker is a signal that instructs the htlcswitch to flush any
        // pending forwarding events.
        FwdEventTicker ticker.Ticker

        // LogEventTicker is a signal instructing the htlcswitch to log
        // aggregate stats about it's forwarding during the last interval.
        LogEventTicker ticker.Ticker

        // AckEventTicker is a signal instructing the htlcswitch to ack any settle
        // fails in forwarding packages.
        AckEventTicker ticker.Ticker

        // AllowCircularRoute is true if the user has configured their node to
        // allow forwards that arrive and depart our node over the same channel.
        AllowCircularRoute bool

        // RejectHTLC is a flag that instructs the htlcswitch to reject any
        // HTLCs that are not from the source hop.
        RejectHTLC bool

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

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

        // MaxFeeExposure is the threshold in milli-satoshis after which we'll
        // fail incoming or outgoing payments for a particular channel.
        MaxFeeExposure lnwire.MilliSatoshi

        // SignAliasUpdate is used when sending FailureMessages backwards for
        // option_scid_alias channels. This avoids a potential privacy leak by
        // replacing the public, confirmed SCID with the alias in the
        // ChannelUpdate.
        SignAliasUpdate func(u *lnwire.ChannelUpdate) (*ecdsa.Signature,
                error)

        // IsAlias returns whether or not a given SCID is an alias.
        IsAlias func(scid lnwire.ShortChannelID) bool
}

// Switch is the central messaging bus for all incoming/outgoing HTLCs.
// Connected peers with active channels are treated as named interfaces which
// refer to active channels as links. A link is the switch's message
// communication point with the goroutine that manages an active channel. New
// links are registered each time a channel is created, and unregistered once
// the channel is closed. The switch manages the hand-off process for multi-hop
// HTLCs, forwarding HTLCs initiated from within the daemon, and finally
// notifies users local-systems concerning their outstanding payment requests.
type Switch struct {
        started  int32 // To be used atomically.
        shutdown int32 // To be used atomically.

        // bestHeight is the best known height of the main chain. The links will
        // be used this information to govern decisions based on HTLC timeouts.
        // This will be retrieved by the registered links atomically.
        bestHeight uint32

        wg   sync.WaitGroup
        quit chan struct{}

        // cfg is a copy of the configuration struct that the htlc switch
        // service was initialized with.
        cfg *Config

        // networkResults stores the results of payments initiated by the user.
        // The store is used to later look up the payments and notify the
        // user of the result when they are complete. Each payment attempt
        // should be given a unique integer ID when it is created, otherwise
        // results might be overwritten.
        networkResults *networkResultStore

        // circuits is storage for payment circuits which are used to
        // forward the settle/fail htlc updates back to the add htlc initiator.
        circuits CircuitMap

        // mailOrchestrator manages the lifecycle of mailboxes used throughout
        // the switch, and facilitates delayed delivery of packets to links that
        // later come online.
        mailOrchestrator *mailOrchestrator

        // indexMtx is a read/write mutex that protects the set of indexes
        // below.
        indexMtx sync.RWMutex

        // pendingLinkIndex holds links that have not had their final, live
        // short_chan_id assigned.
        pendingLinkIndex map[lnwire.ChannelID]ChannelLink

        // links is a map of channel id and channel link which manages
        // this channel.
        linkIndex map[lnwire.ChannelID]ChannelLink

        // forwardingIndex is an index which is consulted by the switch when it
        // needs to locate the next hop to forward an incoming/outgoing HTLC
        // update to/from.
        //
        // TODO(roasbeef): eventually add a NetworkHop mapping before the
        // ChannelLink
        forwardingIndex map[lnwire.ShortChannelID]ChannelLink

        // interfaceIndex maps the compressed public key of a peer to all the
        // channels that the switch maintains with that peer.
        interfaceIndex map[[33]byte]map[lnwire.ChannelID]ChannelLink

        // linkStopIndex stores the currently stopping ChannelLinks,
        // represented by their ChannelID. The key is the link's ChannelID and
        // the value is a chan that is closed when the link has fully stopped.
        // This map is only added to if RemoveLink is called and is not added
        // to when the Switch is shutting down and calls Stop() on each link.
        //
        // MUST be used with the indexMtx.
        linkStopIndex map[lnwire.ChannelID]chan struct{}

        // htlcPlex is the channel which all connected links use to coordinate
        // the setup/teardown of Sphinx (onion routing) payment circuits.
        // Active links forward any add/settle messages over this channel each
        // state transition, sending new adds/settles which are fully locked
        // in.
        htlcPlex chan *plexPacket

        // chanCloseRequests is used to transfer the channel close request to
        // the channel close handler.
        chanCloseRequests chan *ChanClose

        // resolutionMsgs is the channel that all external contract resolution
        // messages will be sent over.
        resolutionMsgs chan *resolutionMsg

        // pendingFwdingEvents is the set of forwarding events which have been
        // collected during the current interval, but hasn't yet been written
        // to the forwarding log.
        fwdEventMtx         sync.Mutex
        pendingFwdingEvents []channeldb.ForwardingEvent

        // blockEpochStream is an active block epoch event stream backed by an
        // active ChainNotifier instance. This will be used to retrieve the
        // latest height of the chain.
        blockEpochStream *chainntnfs.BlockEpochEvent

        // pendingSettleFails is the set of settle/fail entries that we need to
        // ack in the forwarding package of the outgoing link. This was added to
        // make pipelining settles more efficient.
        pendingSettleFails []channeldb.SettleFailRef

        // resMsgStore is used to store the set of ResolutionMsg that come from
        // contractcourt. This is used so the Switch can properly forward them,
        // even on restarts.
        resMsgStore *resolutionStore

        // aliasToReal is a map used for option-scid-alias feature-bit links.
        // The alias SCID is the key and the real, confirmed SCID is the value.
        // If the channel is unconfirmed, there will not be a mapping for it.
        // Since channels can have multiple aliases, this map is essentially a
        // N->1 mapping for a channel. This MUST be accessed with the indexMtx.
        aliasToReal map[lnwire.ShortChannelID]lnwire.ShortChannelID

        // baseIndex is a map used for option-scid-alias feature-bit links.
        // The value is the SCID of the link's ShortChannelID. This value may
        // be an alias for zero-conf channels or a confirmed SCID for
        // non-zero-conf channels with the option-scid-alias feature-bit. The
        // key includes the value itself and also any other aliases. This MUST
        // be accessed with the indexMtx.
        baseIndex map[lnwire.ShortChannelID]lnwire.ShortChannelID
}

// New creates the new instance of htlc switch.
func New(cfg Config, currentHeight uint32) (*Switch, error) {
        resStore := newResolutionStore(cfg.DB)

        circuitMap, err := NewCircuitMap(&CircuitMapConfig{
                DB:                    cfg.DB,
                FetchAllOpenChannels:  cfg.FetchAllOpenChannels,
                FetchClosedChannels:   cfg.FetchClosedChannels,
                ExtractErrorEncrypter: cfg.ExtractErrorEncrypter,
                CheckResolutionMsg:    resStore.checkResolutionMsg,
        })
        if err != nil {
                return nil, err
        }

        s := &Switch{
                bestHeight:        currentHeight,
                cfg:               &cfg,
                circuits:          circuitMap,
                linkIndex:         make(map[lnwire.ChannelID]ChannelLink),
                forwardingIndex:   make(map[lnwire.ShortChannelID]ChannelLink),
                interfaceIndex:    make(map[[33]byte]map[lnwire.ChannelID]ChannelLink),
                pendingLinkIndex:  make(map[lnwire.ChannelID]ChannelLink),
                linkStopIndex:     make(map[lnwire.ChannelID]chan struct{}),
                networkResults:    newNetworkResultStore(cfg.DB),
                htlcPlex:          make(chan *plexPacket),
                chanCloseRequests: make(chan *ChanClose),
                resolutionMsgs:    make(chan *resolutionMsg),
                resMsgStore:       resStore,
                quit:              make(chan struct{}),
        }

        s.aliasToReal = make(map[lnwire.ShortChannelID]lnwire.ShortChannelID)
        s.baseIndex = make(map[lnwire.ShortChannelID]lnwire.ShortChannelID)

        s.mailOrchestrator = newMailOrchestrator(&mailOrchConfig{
                forwardPackets:    s.ForwardPackets,
                clock:             s.cfg.Clock,
                expiry:            s.cfg.MailboxDeliveryTimeout,
                failMailboxUpdate: s.failMailboxUpdate,
        })

        return s, nil
}

// resolutionMsg is a struct that wraps an existing ResolutionMsg with a done
// channel. We'll use this channel to synchronize delivery of the message with
// the caller.
type resolutionMsg struct {
        contractcourt.ResolutionMsg

        errChan chan error
}

// ProcessContractResolution is called by active contract resolvers once a
// contract they are watching over has been fully resolved. The message carries
// an external signal that *would* have been sent if the outgoing channel
// didn't need to go to the chain in order to fulfill a contract. We'll process
// this message just as if it came from an active outgoing channel.
func (s *Switch) ProcessContractResolution(msg contractcourt.ResolutionMsg) error {
        errChan := make(chan error, 1)

        select {
        case s.resolutionMsgs <- &resolutionMsg{
                ResolutionMsg: msg,
                errChan:       errChan,
        }:
        case <-s.quit:
                return ErrSwitchExiting
        }

        select {
        case err := <-errChan:
                return err
        case <-s.quit:
                return ErrSwitchExiting
        }
}

// GetAttemptResult returns the result of the payment attempt with the given
// attemptID. The paymentHash should be set to the payment's overall hash, or
// in case of AMP payments the payment's unique identifier.
//
// The method returns a channel where the payment result will be sent when
// available, or an error is encountered during forwarding. When a result is
// received on the channel, the HTLC is guaranteed to no longer be in flight.
// The switch shutting down is signaled by closing the channel. If the
// attemptID is unknown, ErrPaymentIDNotFound will be returned.
func (s *Switch) GetAttemptResult(attemptID uint64, paymentHash lntypes.Hash,
        deobfuscator ErrorDecrypter) (<-chan *PaymentResult, error) {

        var (
                nChan <-chan *networkResult
                err   error
                inKey = CircuitKey{
                        ChanID: hop.Source,
                        HtlcID: attemptID,
                }
        )

        // If the payment is not found in the circuit map, check whether a
        // result is already available.
        // Assumption: no one will add this payment ID other than the caller.
        if s.circuits.LookupCircuit(inKey) == nil {
                res, err := s.networkResults.getResult(attemptID)
                if err != nil {
                        return nil, err
                }
                c := make(chan *networkResult, 1)
                c <- res
                nChan = c
        } else {
                // The payment was committed to the circuits, subscribe for a
                // result.
                nChan, err = s.networkResults.subscribeResult(attemptID)
                if err != nil {
                        return nil, err
                }
        }

        resultChan := make(chan *PaymentResult, 1)

        // Since the payment was known, we can start a goroutine that can
        // extract the result when it is available, and pass it on to the
        // caller.
        s.wg.Add(1)
        go func() {
                defer s.wg.Done()

                var n *networkResult
                select {
                case n = <-nChan:
                case <-s.quit:
                        // We close the result channel to signal a shutdown. We
                        // don't send any result in this case since the HTLC is
                        // still in flight.
                        close(resultChan)
                        return
                }

                log.Debugf("Received network result %T for attemptID=%v", n.msg,
                        attemptID)

                // Extract the result and pass it to the result channel.
                result, err := s.extractResult(
                        deobfuscator, n, attemptID, paymentHash,
                )
                if err != nil {
                        e := fmt.Errorf("unable to extract result: %w", err)
                        log.Error(e)
                        resultChan <- &PaymentResult{
                                Error: e,
                        }
                        return
                }
                resultChan <- result
        }()

        return resultChan, nil
}

// CleanStore calls the underlying result store, telling it is safe to delete
// all entries except the ones in the keepPids map. This should be called
// preiodically to let the switch clean up payment results that we have
// handled.
func (s *Switch) CleanStore(keepPids map[uint64]struct{}) error {
        return s.networkResults.cleanStore(keepPids)
}

// SendHTLC is used by other subsystems which aren't belong to htlc switch
// package in order to send the htlc update. The attemptID used MUST be unique
// for this HTLC, and MUST be used only once, otherwise the switch might reject
// it.
func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID, attemptID uint64,
        htlc *lnwire.UpdateAddHTLC) error {

        // Generate and send new update packet, if error will be received on
        // this stage it means that packet haven't left boundaries of our
        // system and something wrong happened.
        packet := &htlcPacket{
                incomingChanID: hop.Source,
                incomingHTLCID: attemptID,
                outgoingChanID: firstHop,
                htlc:           htlc,
                amount:         htlc.Amount,
        }

        // Attempt to fetch the target link before creating a circuit so that
        // we don't leave dangling circuits. The getLocalLink method does not
        // require the circuit variable to be set on the *htlcPacket.
        link, linkErr := s.getLocalLink(packet, htlc)
        if linkErr != nil {
                // Notify the htlc notifier of a link failure on our outgoing
                // link. Incoming timelock/amount values are not set because
                // they are not present for local sends.
                s.cfg.HtlcNotifier.NotifyLinkFailEvent(
                        newHtlcKey(packet),
                        HtlcInfo{
                                OutgoingTimeLock: htlc.Expiry,
                                OutgoingAmt:      htlc.Amount,
                        },
                        HtlcEventTypeSend,
                        linkErr,
                        false,
                )

                return linkErr
        }

        // Evaluate whether this HTLC would bypass our fee exposure. If it
        // does, don't send it out and instead return an error.
        if s.dustExceedsFeeThreshold(link, htlc.Amount, false) {
                // Notify the htlc notifier of a link failure on our outgoing
                // link. We use the FailTemporaryChannelFailure in place of a
                // more descriptive error message.
                linkErr := NewLinkError(
                        &lnwire.FailTemporaryChannelFailure{},
                )
                s.cfg.HtlcNotifier.NotifyLinkFailEvent(
                        newHtlcKey(packet),
                        HtlcInfo{
                                OutgoingTimeLock: htlc.Expiry,
                                OutgoingAmt:      htlc.Amount,
                        },
                        HtlcEventTypeSend,
                        linkErr,
                        false,
                )

                return errFeeExposureExceeded
        }

        circuit := newPaymentCircuit(&htlc.PaymentHash, packet)
        actions, err := s.circuits.CommitCircuits(circuit)
        if err != nil {
                log.Errorf("unable to commit circuit in switch: %v", err)
                return err
        }

        // Drop duplicate packet if it has already been seen.
        switch {
        case len(actions.Drops) == 1:
                return ErrDuplicateAdd

        case len(actions.Fails) == 1:
                return ErrLocalAddFailed
        }

        // Give the packet to the link's mailbox so that HTLC's are properly
        // canceled back if the mailbox timeout elapses.
        packet.circuit = circuit

        return link.handleSwitchPacket(packet)
}

// UpdateForwardingPolicies sends a message to the switch to update the
// forwarding policies for the set of target channels, keyed in chanPolicies.
//
// NOTE: This function is synchronous and will block until either the
// forwarding policies for all links have been updated, or the switch shuts
// down.
func (s *Switch) UpdateForwardingPolicies(
        chanPolicies map[wire.OutPoint]models.ForwardingPolicy) {

        log.Tracef("Updating link policies: %v", lnutils.SpewLogClosure(
                chanPolicies))

        s.indexMtx.RLock()

        // Update each link in chanPolicies.
        for targetLink, policy := range chanPolicies {
                cid := lnwire.NewChanIDFromOutPoint(targetLink)

                link, ok := s.linkIndex[cid]
                if !ok {
                        log.Debugf("Unable to find ChannelPoint(%v) to update "+
                                "link policy", targetLink)
                        continue
                }

                link.UpdateForwardingPolicy(policy)
        }

        s.indexMtx.RUnlock()
}

// IsForwardedHTLC checks for a given channel and htlc index if it is related
// to an opened circuit that represents a forwarded payment.
func (s *Switch) IsForwardedHTLC(chanID lnwire.ShortChannelID,
        htlcIndex uint64) bool {

        circuit := s.circuits.LookupOpenCircuit(models.CircuitKey{
                ChanID: chanID,
                HtlcID: htlcIndex,
        })
        return circuit != nil && circuit.Incoming.ChanID != hop.Source
}

// ForwardPackets adds a list of packets to the switch for processing. Fails
// and settles are added on a first past, simultaneously constructing circuits
// for any adds. After persisting the circuits, another pass of the adds is
// given to forward them through the router. The sending link's quit channel is
// used to prevent deadlocks when the switch stops a link in the midst of
// forwarding.
func (s *Switch) ForwardPackets(linkQuit chan struct{},
        packets ...*htlcPacket) error {

        var (
                // fwdChan is a buffered channel used to receive err msgs from
                // the htlcPlex when forwarding this batch.
                fwdChan = make(chan error, len(packets))

                // numSent keeps a running count of how many packets are
                // forwarded to the switch, which determines how many responses
                // we will wait for on the fwdChan..
                numSent int
        )

        // No packets, nothing to do.
        if len(packets) == 0 {
                return nil
        }

        // Setup a barrier to prevent the background tasks from processing
        // responses until this function returns to the user.
        var wg sync.WaitGroup
        wg.Add(1)
        defer wg.Done()

        // Before spawning the following goroutine to proxy our error responses,
        // check to see if we have already been issued a shutdown request. If
        // so, we exit early to avoid incrementing the switch's waitgroup while
        // it is already in the process of shutting down.
        select {
        case <-linkQuit:
                return nil
        case <-s.quit:
                return nil
        default:
                // Spawn a goroutine to log the errors returned from failed packets.
                s.wg.Add(1)
                go s.logFwdErrs(&numSent, &wg, fwdChan)
        }

        // Make a first pass over the packets, forwarding any settles or fails.
        // As adds are found, we create a circuit and append it to our set of
        // circuits to be written to disk.
        var circuits []*PaymentCircuit
        var addBatch []*htlcPacket
        for _, packet := range packets {
                switch htlc := packet.htlc.(type) {
                case *lnwire.UpdateAddHTLC:
                        circuit := newPaymentCircuit(&htlc.PaymentHash, packet)
                        packet.circuit = circuit
                        circuits = append(circuits, circuit)
                        addBatch = append(addBatch, packet)
                default:
                        err := s.routeAsync(packet, fwdChan, linkQuit)
                        if err != nil {
                                return fmt.Errorf("failed to forward packet %w",
                                        err)
                        }
                        numSent++
                }
        }

        // If this batch did not contain any circuits to commit, we can return
        // early.
        if len(circuits) == 0 {
                return nil
        }

        // Write any circuits that we found to disk.
        actions, err := s.circuits.CommitCircuits(circuits...)
        if err != nil {
                log.Errorf("unable to commit circuits in switch: %v", err)
        }

        // Split the htlc packets by comparing an in-order seek to the head of
        // the added, dropped, or failed circuits.
        //
        // NOTE: This assumes each list is guaranteed to be a subsequence of the
        // circuits, and that the union of the sets results in the original set
        // of circuits.
        var addedPackets, failedPackets []*htlcPacket
        for _, packet := range addBatch {
                switch {
                case len(actions.Adds) > 0 && packet.circuit == actions.Adds[0]:
                        addedPackets = append(addedPackets, packet)
                        actions.Adds = actions.Adds[1:]

                case len(actions.Drops) > 0 && packet.circuit == actions.Drops[0]:
                        actions.Drops = actions.Drops[1:]

                case len(actions.Fails) > 0 && packet.circuit == actions.Fails[0]:
                        failedPackets = append(failedPackets, packet)
                        actions.Fails = actions.Fails[1:]
                }
        }

        // Now, forward any packets for circuits that were successfully added to
        // the switch's circuit map.
        for _, packet := range addedPackets {
                err := s.routeAsync(packet, fwdChan, linkQuit)
                if err != nil {
                        return fmt.Errorf("failed to forward packet %w", err)
                }
                numSent++
        }

        // Lastly, for any packets that failed, this implies that they were
        // left in a half added state, which can happen when recovering from
        // failures.
        if len(failedPackets) > 0 {
                var failure lnwire.FailureMessage
                incomingID := failedPackets[0].incomingChanID

                // If the incoming channel is an option_scid_alias channel,
                // then we'll need to replace the SCID in the ChannelUpdate.
                update := s.failAliasUpdate(incomingID, true)
                if update == nil {
                        // Fallback to the original non-option behavior.
                        update, err := s.cfg.FetchLastChannelUpdate(
                                incomingID,
                        )
                        if err != nil {
                                failure = &lnwire.FailTemporaryNodeFailure{}
                        } else {
                                failure = lnwire.NewTemporaryChannelFailure(
                                        update,
                                )
                        }
                } else {
                        // This is an option_scid_alias channel.
                        failure = lnwire.NewTemporaryChannelFailure(update)
                }

                linkError := NewDetailedLinkError(
                        failure, OutgoingFailureIncompleteForward,
                )

                for _, packet := range failedPackets {
                        // We don't handle the error here since this method
                        // always returns an error.
                        _ = s.failAddPacket(packet, linkError)
                }
        }

        return nil
}

// logFwdErrs logs any errors received on `fwdChan`.
func (s *Switch) logFwdErrs(num *int, wg *sync.WaitGroup, fwdChan chan error) {
        defer s.wg.Done()

        // Wait here until the outer function has finished persisting
        // and routing the packets. This guarantees we don't read from num until
        // the value is accurate.
        wg.Wait()

        numSent := *num
        for i := 0; i < numSent; i++ {
                select {
                case err := <-fwdChan:
                        if err != nil {
                                log.Errorf("Unhandled error while reforwarding htlc "+
                                        "settle/fail over htlcswitch: %v", err)
                        }
                case <-s.quit:
                        log.Errorf("unable to forward htlc packet " +
                                "htlc switch was stopped")
                        return
                }
        }
}

// routeAsync sends a packet through the htlc switch, using the provided err
// chan to propagate errors back to the caller. The link's quit channel is
// provided so that the send can be canceled if either the link or the switch
// receive a shutdown requuest. This method does not wait for a response from
// the htlcForwarder before returning.
func (s *Switch) routeAsync(packet *htlcPacket, errChan chan error,
        linkQuit chan struct{}) error {

        command := &plexPacket{
                pkt: packet,
                err: errChan,
        }

        select {
        case s.htlcPlex <- command:
                return nil
        case <-linkQuit:
                return ErrLinkShuttingDown
        case <-s.quit:
                return errors.New("htlc switch was stopped")
        }
}

// getLocalLink handles the addition of a htlc for a send that originates from
// our node. It returns the link that the htlc should be forwarded outwards on,
// and a link error if the htlc cannot be forwarded.
func (s *Switch) getLocalLink(pkt *htlcPacket, htlc *lnwire.UpdateAddHTLC) (
        ChannelLink, *LinkError) {

        // Try to find links by node destination.
        s.indexMtx.RLock()
        link, err := s.getLinkByShortID(pkt.outgoingChanID)
        defer s.indexMtx.RUnlock()
        if err != nil {
                // If the link was not found for the outgoingChanID, an outside
                // subsystem may be using the confirmed SCID of a zero-conf
                // channel. In this case, we'll consult the Switch maps to see
                // if an alias exists and use the alias to lookup the link.
                // This extra step is a consequence of not updating the Switch
                // forwardingIndex when a zero-conf channel is confirmed. We
                // don't need to change the outgoingChanID since the link will
                // do that upon receiving the packet.
                baseScid, ok := s.baseIndex[pkt.outgoingChanID]
                if !ok {
                        log.Errorf("Link %v not found", pkt.outgoingChanID)
                        return nil, NewLinkError(&lnwire.FailUnknownNextPeer{})
                }

                // The base SCID was found, so we'll use that to fetch the
                // link.
                link, err = s.getLinkByShortID(baseScid)
                if err != nil {
                        log.Errorf("Link %v not found", baseScid)
                        return nil, NewLinkError(&lnwire.FailUnknownNextPeer{})
                }
        }

        if !link.EligibleToForward() {
                log.Errorf("Link %v is not available to forward",
                        pkt.outgoingChanID)

                // The update does not need to be populated as the error
                // will be returned back to the router.
                return nil, NewDetailedLinkError(
                        lnwire.NewTemporaryChannelFailure(nil),
                        OutgoingFailureLinkNotEligible,
                )
        }

        // Ensure that the htlc satisfies the outgoing channel policy.
        currentHeight := atomic.LoadUint32(&s.bestHeight)
        htlcErr := link.CheckHtlcTransit(
                htlc.PaymentHash, htlc.Amount, htlc.Expiry, currentHeight,
        )
        if htlcErr != nil {
                log.Errorf("Link %v policy for local forward not "+
                        "satisfied", pkt.outgoingChanID)
                return nil, htlcErr
        }
        return link, nil
}

// handleLocalResponse processes a Settle or Fail responding to a
// locally-initiated payment. This is handled asynchronously to avoid blocking
// the main event loop within the switch, as these operations can require
// multiple db transactions. The guarantees of the circuit map are stringent
// enough such that we are able to tolerate reordering of these operations
// without side effects. The primary operations handled are:
//  1. Save the payment result to the pending payment store.
//  2. Notify subscribers about the payment result.
//  3. Ack settle/fail references, to avoid resending this response internally
//  4. Teardown the closing circuit in the circuit map
//
// NOTE: This method MUST be spawned as a goroutine.
func (s *Switch) handleLocalResponse(pkt *htlcPacket) {
        defer s.wg.Done()

        attemptID := pkt.incomingHTLCID

        // The error reason will be unencypted in case this a local
        // failure or a converted error.
        unencrypted := pkt.localFailure || pkt.convertedError
        n := &networkResult{
                msg:          pkt.htlc,
                unencrypted:  unencrypted,
                isResolution: pkt.isResolution,
        }

        // Store the result to the db. This will also notify subscribers about
        // the result.
        if err := s.networkResults.storeResult(attemptID, n); err != nil {
                log.Errorf("Unable to complete payment for pid=%v: %v",
                        attemptID, err)
                return
        }

        // First, we'll clean up any fwdpkg references, circuit entries, and
        // mark in our db that the payment for this payment hash has either
        // succeeded or failed.
        //
        // If this response is contained in a forwarding package, we'll start by
        // acking the settle/fail so that we don't continue to retransmit the
        // HTLC internally.
        if pkt.destRef != nil {
                if err := s.ackSettleFail(*pkt.destRef); err != nil {
                        log.Warnf("Unable to ack settle/fail reference: %s: %v",
                                *pkt.destRef, err)
                        return
                }
        }

        // Next, we'll remove the circuit since we are about to complete an
        // fulfill/fail of this HTLC. Since we've already removed the
        // settle/fail fwdpkg reference, the response from the peer cannot be
        // replayed internally if this step fails. If this happens, this logic
        // will be executed when a provided resolution message comes through.
        // This can only happen if the circuit is still open, which is why this
        // ordering is chosen.
        if err := s.teardownCircuit(pkt); err != nil {
                log.Warnf("Unable to teardown circuit %s: %v",
                        pkt.inKey(), err)
                return
        }

        // Finally, notify on the htlc failure or success that has been handled.
        key := newHtlcKey(pkt)
        eventType := getEventType(pkt)

        switch htlc := pkt.htlc.(type) {
        case *lnwire.UpdateFulfillHTLC:
                s.cfg.HtlcNotifier.NotifySettleEvent(key, htlc.PaymentPreimage,
                        eventType)

        case *lnwire.UpdateFailHTLC:
                s.cfg.HtlcNotifier.NotifyForwardingFailEvent(key, eventType)
        }
}

// extractResult uses the given deobfuscator to extract the payment result from
// the given network message.
func (s *Switch) extractResult(deobfuscator ErrorDecrypter, n *networkResult,
        attemptID uint64, paymentHash lntypes.Hash) (*PaymentResult, error) {

        switch htlc := n.msg.(type) {

        // We've received a settle update which means we can finalize the user
        // payment and return successful response.
        case *lnwire.UpdateFulfillHTLC:
                return &PaymentResult{
                        Preimage: htlc.PaymentPreimage,
                }, nil

        // We've received a fail update which means we can finalize the
        // user payment and return fail response.
        case *lnwire.UpdateFailHTLC:
                // TODO(yy): construct deobfuscator here to avoid creating it
                // in paymentLifecycle even for settled HTLCs.
                paymentErr := s.parseFailedPayment(
                        deobfuscator, attemptID, paymentHash, n.unencrypted,
                        n.isResolution, htlc,
                )

                return &PaymentResult{
                        Error: paymentErr,
                }, nil

        default:
                return nil, fmt.Errorf("received unknown response type: %T",
                        htlc)
        }
}

// parseFailedPayment determines the appropriate failure message to return to
// a user initiated payment. The three cases handled are:
//  1. An unencrypted failure, which should already plaintext.
//  2. A resolution from the chain arbitrator, which possibly has no failure
//     reason attached.
//  3. A failure from the remote party, which will need to be decrypted using
//     the payment deobfuscator.
func (s *Switch) parseFailedPayment(deobfuscator ErrorDecrypter,
        attemptID uint64, paymentHash lntypes.Hash, unencrypted,
        isResolution bool, htlc *lnwire.UpdateFailHTLC) error {

        switch {

        // The payment never cleared the link, so we don't need to
        // decrypt the error, simply decode it them report back to the
        // user.
        case unencrypted:
                r := bytes.NewReader(htlc.Reason)
                failureMsg, err := lnwire.DecodeFailure(r, 0)
                if err != nil {
                        // If we could not decode the failure reason, return a link
                        // error indicating that we failed to decode the onion.
                        linkError := NewDetailedLinkError(
                                // As this didn't even clear the link, we don't
                                // need to apply an update here since it goes
                                // directly to the router.
                                lnwire.NewTemporaryChannelFailure(nil),
                                OutgoingFailureDecodeError,
                        )

                        log.Errorf("%v: (hash=%v, pid=%d): %v",
                                linkError.FailureDetail.FailureString(),
                                paymentHash, attemptID, err)

                        return linkError
                }

                // If we successfully decoded the failure reason, return it.
                return NewLinkError(failureMsg)

        // A payment had to be timed out on chain before it got past
        // the first hop. In this case, we'll report a permanent
        // channel failure as this means us, or the remote party had to
        // go on chain.
        case isResolution && htlc.Reason == nil:
                linkError := NewDetailedLinkError(
                        &lnwire.FailPermanentChannelFailure{},
                        OutgoingFailureOnChainTimeout,
                )

                log.Infof("%v: hash=%v, pid=%d",
                        linkError.FailureDetail.FailureString(),
                        paymentHash, attemptID)

                return linkError

        // A regular multi-hop payment error that we'll need to
        // decrypt.
        default:
                // We'll attempt to fully decrypt the onion encrypted
                // error. If we're unable to then we'll bail early.
                failure, err := deobfuscator.DecryptError(htlc.Reason)
                if err != nil {
                        log.Errorf("unable to de-obfuscate onion failure "+
                                "(hash=%v, pid=%d): %v",
                                paymentHash, attemptID, err)

                        return ErrUnreadableFailureMessage
                }

                return failure
        }
}

// handlePacketForward is used in cases when we need forward the htlc update
// from one channel link to another and be able to propagate the settle/fail
// updates back. This behaviour is achieved by creation of payment circuits.
func (s *Switch) handlePacketForward(packet *htlcPacket) error {
        switch htlc := packet.htlc.(type) {

        // Channel link forwarded us a new htlc, therefore we initiate the
        // payment circuit within our internal state so we can properly forward
        // the ultimate settle message back latter.
        case *lnwire.UpdateAddHTLC:
                // Check if the node is set to reject all onward HTLCs and also make
                // sure that HTLC is not from the source node.
                if s.cfg.RejectHTLC {
                        failure := NewDetailedLinkError(
                                &lnwire.FailChannelDisabled{},
                                OutgoingFailureForwardsDisabled,
                        )

                        return s.failAddPacket(packet, failure)
                }

                // Before we attempt to find a non-strict forwarding path for
                // this htlc, check whether the htlc is being routed over the
                // same incoming and outgoing channel. If our node does not
                // allow forwards of this nature, we fail the htlc early. This
                // check is in place to disallow inefficiently routed htlcs from
                // locking up our balance. With channels where the
                // option-scid-alias feature was negotiated, we also have to be
                // sure that the IDs aren't the same since one or both could be
                // an alias.
                linkErr := s.checkCircularForward(
                        packet.incomingChanID, packet.outgoingChanID,
                        s.cfg.AllowCircularRoute, htlc.PaymentHash,
                )
                if linkErr != nil {
                        return s.failAddPacket(packet, linkErr)
                }

                s.indexMtx.RLock()
                targetLink, err := s.getLinkByMapping(packet)
                if err != nil {
                        s.indexMtx.RUnlock()

                        log.Debugf("unable to find link with "+
                                "destination %v", packet.outgoingChanID)

                        // If packet was forwarded from another channel link
                        // than we should notify this link that some error
                        // occurred.
                        linkError := NewLinkError(
                                &lnwire.FailUnknownNextPeer{},
                        )

                        return s.failAddPacket(packet, linkError)
                }
                targetPeerKey := targetLink.PeerPubKey()
                interfaceLinks, _ := s.getLinks(targetPeerKey)
                s.indexMtx.RUnlock()

                // We'll keep track of any HTLC failures during the link
                // selection process. This way we can return the error for
                // precise link that the sender selected, while optimistically
                // trying all links to utilize our available bandwidth.
                linkErrs := make(map[lnwire.ShortChannelID]*LinkError)

                // Find all destination channel links with appropriate
                // bandwidth.
                var destinations []ChannelLink
                for _, link := range interfaceLinks {
                        var failure *LinkError

                        // We'll skip any links that aren't yet eligible for
                        // forwarding.
                        if !link.EligibleToForward() {
                                failure = NewDetailedLinkError(
                                        &lnwire.FailUnknownNextPeer{},
                                        OutgoingFailureLinkNotEligible,
                                )
                        } else {
                                // We'll ensure that the HTLC satisfies the
                                // current forwarding conditions of this target
                                // link.
                                currentHeight := atomic.LoadUint32(&s.bestHeight)
                                failure = link.CheckHtlcForward(
                                        htlc.PaymentHash, packet.incomingAmount,
                                        packet.amount, packet.incomingTimeout,
                                        packet.outgoingTimeout,
                                        packet.inboundFee,
                                        currentHeight,
                                        packet.originalOutgoingChanID,
                                )
                        }

                        // If this link can forward the htlc, add it to the set
                        // of destinations.
                        if failure == nil {
                                destinations = append(destinations, link)
                                continue
                        }

                        linkErrs[link.ShortChanID()] = failure
                }

                // If we had a forwarding failure due to the HTLC not
                // satisfying the current policy, then we'll send back an
                // error, but ensure we send back the error sourced at the
                // *target* link.
                if len(destinations) == 0 {
                        // At this point, some or all of the links rejected the
                        // HTLC so we couldn't forward it. So we'll try to look
                        // up the error that came from the source.
                        linkErr, ok := linkErrs[packet.outgoingChanID]
                        if !ok {
                                // If we can't find the error of the source,
                                // then we'll return an unknown next peer,
                                // though this should never happen.
                                linkErr = NewLinkError(
                                        &lnwire.FailUnknownNextPeer{},
                                )
                                log.Warnf("unable to find err source for "+
                                        "outgoing_link=%v, errors=%v",
                                        packet.outgoingChanID,
                                        lnutils.SpewLogClosure(linkErrs))
                        }

                        log.Tracef("incoming HTLC(%x) violated "+
                                "target outgoing link (id=%v) policy: %v",
                                htlc.PaymentHash[:], packet.outgoingChanID,
                                linkErr)

                        return s.failAddPacket(packet, linkErr)
                }

                // Choose a random link out of the set of links that can forward
                // this htlc. The reason for randomization is to evenly
                // distribute the htlc load without making assumptions about
                // what the best channel is.
                destination := destinations[rand.Intn(len(destinations))] // nolint:gosec

                // Retrieve the incoming link by its ShortChannelID. Note that
                // the incomingChanID is never set to hop.Source here.
                s.indexMtx.RLock()
                incomingLink, err := s.getLinkByShortID(packet.incomingChanID)
                s.indexMtx.RUnlock()
                if err != nil {
                        // If we couldn't find the incoming link, we can't
                        // evaluate the incoming's exposure to dust, so we just
                        // fail the HTLC back.
                        linkErr := NewLinkError(
                                &lnwire.FailTemporaryChannelFailure{},
                        )

                        return s.failAddPacket(packet, linkErr)
                }

                // Evaluate whether this HTLC would increase our fee exposure
                // over the threshold on the incoming link. If it does, fail it
                // backwards.
                if s.dustExceedsFeeThreshold(
                        incomingLink, packet.incomingAmount, true,
                ) {
                        // The incoming dust exceeds the threshold, so we fail
                        // the add back.
                        linkErr := NewLinkError(
                                &lnwire.FailTemporaryChannelFailure{},
                        )

                        return s.failAddPacket(packet, linkErr)
                }

                // Also evaluate whether this HTLC would increase our fee
                // exposure over the threshold on the destination link. If it
                // does, fail it back.
                if s.dustExceedsFeeThreshold(
                        destination, packet.amount, false,
                ) {
                        // The outgoing dust exceeds the threshold, so we fail
                        // the add back.
                        linkErr := NewLinkError(
                                &lnwire.FailTemporaryChannelFailure{},
                        )

                        return s.failAddPacket(packet, linkErr)
                }

                // Send the packet to the destination channel link which
                // manages the channel.
                packet.outgoingChanID = destination.ShortChanID()
                return destination.handleSwitchPacket(packet)

        case *lnwire.UpdateFailHTLC, *lnwire.UpdateFulfillHTLC:
                // If the source of this packet has not been set, use the
                // circuit map to lookup the origin.
                circuit, err := s.closeCircuit(packet)
                if err != nil {
                        return err
                }

                // closeCircuit returns a nil circuit when a settle packet returns an
                // ErrUnknownCircuit error upon the inner call to CloseCircuit.
                if circuit == nil {
                        return nil
                }

                fail, isFail := htlc.(*lnwire.UpdateFailHTLC)
                if isFail && !packet.hasSource {
                        // HTLC resolutions and messages restored from disk
                        // don't have the obfuscator set from the original htlc
                        // add packet - set it here for use in blinded errors.
                        packet.obfuscator = circuit.ErrorEncrypter

                        switch {
                        // No message to encrypt, locally sourced payment.
                        case circuit.ErrorEncrypter == nil:

                        // If this is a resolution message, then we'll need to
                        // encrypt it as it's actually internally sourced.
                        case packet.isResolution:
                                var err error
                                // TODO(roasbeef): don't need to pass actually?
                                failure := &lnwire.FailPermanentChannelFailure{}
                                fail.Reason, err = circuit.ErrorEncrypter.EncryptFirstHop(
                                        failure,
                                )
                                if err != nil {
                                        err = fmt.Errorf("unable to obfuscate "+
                                                "error: %v", err)
                                        log.Error(err)
                                }

                        // Alternatively, if the remote party send us an
                        // UpdateFailMalformedHTLC, then we'll need to convert
                        // this into a proper well formatted onion error as
                        // there's no HMAC currently.
                        case packet.convertedError:
                                log.Infof("Converting malformed HTLC error "+
                                        "for circuit for Circuit(%x: "+
                                        "(%s, %d) <-> (%s, %d))", packet.circuit.PaymentHash,
                                        packet.incomingChanID, packet.incomingHTLCID,
                                        packet.outgoingChanID, packet.outgoingHTLCID)

                                fail.Reason = circuit.ErrorEncrypter.EncryptMalformedError(
                                        fail.Reason,
                                )

                        default:
                                // Otherwise, it's a forwarded error, so we'll perform a
                                // wrapper encryption as normal.
                                fail.Reason = circuit.ErrorEncrypter.IntermediateEncrypt(
                                        fail.Reason,
                                )
                        }
                } else if !isFail && circuit.Outgoing != nil {
                        // If this is an HTLC settle, and it wasn't from a
                        // locally initiated HTLC, then we'll log a forwarding
                        // event so we can flush it to disk later.
                        //
                        // TODO(roasbeef): only do this once link actually
                        // fully settles?
                        localHTLC := packet.incomingChanID == hop.Source
                        if !localHTLC {
                                log.Infof("Forwarded HTLC(%x) of %v (fee: %v) "+
                                        "from IncomingChanID(%v) to OutgoingChanID(%v)",
                                        circuit.PaymentHash[:], circuit.OutgoingAmount,
                                        circuit.IncomingAmount-circuit.OutgoingAmount,
                                        circuit.Incoming.ChanID, circuit.Outgoing.ChanID)
                                s.fwdEventMtx.Lock()
                                s.pendingFwdingEvents = append(
                                        s.pendingFwdingEvents,
                                        channeldb.ForwardingEvent{
                                                Timestamp:      time.Now(),
                                                IncomingChanID: circuit.Incoming.ChanID,
                                                OutgoingChanID: circuit.Outgoing.ChanID,
                                                AmtIn:          circuit.IncomingAmount,
                                                AmtOut:         circuit.OutgoingAmount,
                                        },
                                )
                                s.fwdEventMtx.Unlock()
                        }
                }

                // A blank IncomingChanID in a circuit indicates that it is a pending
                // user-initiated payment.
                if packet.incomingChanID == hop.Source {
                        s.wg.Add(1)
                        go s.handleLocalResponse(packet)
                        return nil
                }

                // Check to see that the source link is online before removing
                // the circuit.
                return s.mailOrchestrator.Deliver(packet.incomingChanID, packet)

        default:
                return errors.New("wrong update type")
        }
}

// checkCircularForward checks whether a forward is circular (arrives and
// departs on the same link) and returns a link error if the switch is
// configured to disallow this behaviour.
func (s *Switch) checkCircularForward(incoming, outgoing lnwire.ShortChannelID,
        allowCircular bool, paymentHash lntypes.Hash) *LinkError {

        // If they are equal, we can skip the alias mapping checks.
        if incoming == outgoing {
                // The switch may be configured to allow circular routes, so
                // just log and return nil.
                if allowCircular {
                        log.Debugf("allowing circular route over link: %v "+
                                "(payment hash: %x)", incoming, paymentHash)
                        return nil
                }

                // Otherwise, we'll return a temporary channel failure.
                return NewDetailedLinkError(
                        lnwire.NewTemporaryChannelFailure(nil),
                        OutgoingFailureCircularRoute,
                )
        }

        // We'll fetch the "base" SCID from the baseIndex for the incoming and
        // outgoing SCIDs. If either one does not have a base SCID, then the
        // two channels are not equal since one will be a channel that does not
        // need a mapping and SCID equality was checked above. If the "base"
        // SCIDs are equal, then this is a circular route. Otherwise, it isn't.
        s.indexMtx.RLock()
        incomingBaseScid, ok := s.baseIndex[incoming]
        if !ok {
                // This channel does not use baseIndex, bail out.
                s.indexMtx.RUnlock()
                return nil
        }

        outgoingBaseScid, ok := s.baseIndex[outgoing]
        if !ok {
                // This channel does not use baseIndex, bail out.
                s.indexMtx.RUnlock()
                return nil
        }
        s.indexMtx.RUnlock()

        // Check base SCID equality.
        if incomingBaseScid != outgoingBaseScid {
                // The base SCIDs are not equal so these are not the same
                // channel.
                return nil
        }

        // If the incoming and outgoing link are equal, the htlc is part of a
        // circular route which may be used to lock up our liquidity. If the
        // switch is configured to allow circular routes, log that we are
        // allowing the route then return nil.
        if allowCircular {
                log.Debugf("allowing circular route over link: %v "+
                        "(payment hash: %x)", incoming, paymentHash)
                return nil
        }

        // If our node disallows circular routes, return a temporary channel
        // failure. There is nothing wrong with the policy used by the remote
        // node, so we do not include a channel update.
        return NewDetailedLinkError(
                lnwire.NewTemporaryChannelFailure(nil),
                OutgoingFailureCircularRoute,
        )
}

// failAddPacket encrypts a fail packet back to an add packet's source.
// The ciphertext will be derived from the failure message proivded by context.
// This method returns the failErr if all other steps complete successfully.
func (s *Switch) failAddPacket(packet *htlcPacket, failure *LinkError) error {
        // Encrypt the failure so that the sender will be able to read the error
        // message. Since we failed this packet, we use EncryptFirstHop to
        // obfuscate the failure for their eyes only.
        reason, err := packet.obfuscator.EncryptFirstHop(failure.WireMessage())
        if err != nil {
                err := fmt.Errorf("unable to obfuscate "+
                        "error: %v", err)
                log.Error(err)
                return err
        }

        log.Error(failure.Error())

        // Create a failure packet for this htlc. The full set of
        // information about the htlc failure is included so that they can
        // be included in link failure notifications.
        failPkt := &htlcPacket{
                sourceRef:       packet.sourceRef,
                incomingChanID:  packet.incomingChanID,
                incomingHTLCID:  packet.incomingHTLCID,
                outgoingChanID:  packet.outgoingChanID,
                outgoingHTLCID:  packet.outgoingHTLCID,
                incomingAmount:  packet.incomingAmount,
                amount:          packet.amount,
                incomingTimeout: packet.incomingTimeout,
                outgoingTimeout: packet.outgoingTimeout,
                circuit:         packet.circuit,
                obfuscator:      packet.obfuscator,
                linkFailure:     failure,
                htlc: &lnwire.UpdateFailHTLC{
                        Reason: reason,
                },
        }

        // Route a fail packet back to the source link.
        err = s.mailOrchestrator.Deliver(failPkt.incomingChanID, failPkt)
        if err != nil {
                err = fmt.Errorf("source chanid=%v unable to "+
                        "handle switch packet: %v",
                        packet.incomingChanID, err)
                log.Error(err)
                return err
        }

        return failure
}

// closeCircuit accepts a settle or fail htlc and the associated htlc packet and
// attempts to determine the source that forwarded this htlc. This method will
// set the incoming chan and htlc ID of the given packet if the source was
// found, and will properly [re]encrypt any failure messages.
func (s *Switch) closeCircuit(pkt *htlcPacket) (*PaymentCircuit, error) {
        // If the packet has its source, that means it was failed locally by
        // the outgoing link. We fail it here to make sure only one response
        // makes it through the switch.
        if pkt.hasSource {
                circuit, err := s.circuits.FailCircuit(pkt.inKey())
                switch err {

                // Circuit successfully closed.
                case nil:
                        return circuit, nil

                // Circuit was previously closed, but has not been deleted.
                // We'll just drop this response until the circuit has been
                // fully removed.
                case ErrCircuitClosing:
                        return nil, err

                // Failed to close circuit because it does not exist. This is
                // likely because the circuit was already successfully closed.
                // Since this packet failed locally, there is no forwarding
                // package entry to acknowledge.
                case ErrUnknownCircuit:
                        return nil, err

                // Unexpected error.
                default:
                        return nil, err
                }
        }

        // Otherwise, this is packet was received from the remote party.  Use
        // circuit map to find the incoming link to receive the settle/fail.
        circuit, err := s.circuits.CloseCircuit(pkt.outKey())
        switch err {

        // Open circuit successfully closed.
        case nil:
                pkt.incomingChanID = circuit.Incoming.ChanID
                pkt.incomingHTLCID = circuit.Incoming.HtlcID
                pkt.circuit = circuit
                pkt.sourceRef = &circuit.AddRef

                pktType := "SETTLE"
                if _, ok := pkt.htlc.(*lnwire.UpdateFailHTLC); ok {
                        pktType = "FAIL"
                }

                log.Debugf("Closed completed %s circuit for %x: "+
                        "(%s, %d) <-> (%s, %d)", pktType, pkt.circuit.PaymentHash,
                        pkt.incomingChanID, pkt.incomingHTLCID,
                        pkt.outgoingChanID, pkt.outgoingHTLCID)

                return circuit, nil

        // Circuit was previously closed, but has not been deleted. We'll just
        // drop this response until the circuit has been removed.
        case ErrCircuitClosing:
                return nil, err

        // Failed to close circuit because it does not exist. This is likely
        // because the circuit was already successfully closed.
        case ErrUnknownCircuit:
                if pkt.destRef != nil {
                        // Add this SettleFailRef to the set of pending settle/fail entries
                        // awaiting acknowledgement.
                        s.pendingSettleFails = append(s.pendingSettleFails, *pkt.destRef)
                }

                // If this is a settle, we will not log an error message as settles
                // are expected to hit the ErrUnknownCircuit case. The only way fails
                // can hit this case if the link restarts after having just sent a fail
                // to the switch.
                _, isSettle := pkt.htlc.(*lnwire.UpdateFulfillHTLC)
                if !isSettle {
                        err := fmt.Errorf("unable to find target channel "+
                                "for HTLC fail: channel ID = %s, "+
                                "HTLC ID = %d", pkt.outgoingChanID,
                                pkt.outgoingHTLCID)
                        log.Error(err)

                        return nil, err
                }

                return nil, nil

        // Unexpected error.
        default:
                return nil, err
        }
}

// ackSettleFail is used by the switch to ACK any settle/fail entries in the
// forwarding package of the outgoing link for a payment circuit. We do this if
// we're the originator of the payment, so the link stops attempting to
// re-broadcast.
func (s *Switch) ackSettleFail(settleFailRefs ...channeldb.SettleFailRef) error {
        return kvdb.Batch(s.cfg.DB, func(tx kvdb.RwTx) error {
                return s.cfg.SwitchPackager.AckSettleFails(tx, settleFailRefs...)
        })
}

// teardownCircuit removes a pending or open circuit from the switch's circuit
// map and prints useful logging statements regarding the outcome.
func (s *Switch) teardownCircuit(pkt *htlcPacket) error {
        var pktType string
        switch htlc := pkt.htlc.(type) {
        case *lnwire.UpdateFulfillHTLC:
                pktType = "SETTLE"
        case *lnwire.UpdateFailHTLC:
                pktType = "FAIL"
        default:
                err := fmt.Errorf("cannot tear down packet of type: %T", htlc)
                log.Errorf(err.Error())
                return err
        }

        switch {
        case pkt.circuit.HasKeystone():
                log.Debugf("Tearing down open circuit with %s pkt, removing circuit=%v "+
                        "with keystone=%v", pktType, pkt.inKey(), pkt.outKey())

                err := s.circuits.DeleteCircuits(pkt.inKey())
                if err != nil {
                        log.Warnf("Failed to tear down open circuit (%s, %d) <-> (%s, %d) "+
                                "with payment_hash-%v using %s pkt",
                                pkt.incomingChanID, pkt.incomingHTLCID,
                                pkt.outgoingChanID, pkt.outgoingHTLCID,
                                pkt.circuit.PaymentHash, pktType)
                        return err
                }

                log.Debugf("Closed completed %s circuit for %x: "+
                        "(%s, %d) <-> (%s, %d)", pktType, pkt.circuit.PaymentHash,
                        pkt.incomingChanID, pkt.incomingHTLCID,
                        pkt.outgoingChanID, pkt.outgoingHTLCID)

        default:
                log.Debugf("Tearing down incomplete circuit with %s for inkey=%v",
                        pktType, pkt.inKey())

                err := s.circuits.DeleteCircuits(pkt.inKey())
                if err != nil {
                        log.Warnf("Failed to tear down pending %s circuit for %x: "+
                                "(%s, %d)", pktType, pkt.circuit.PaymentHash,
                                pkt.incomingChanID, pkt.incomingHTLCID)
                        return err
                }

                log.Debugf("Removed pending onion circuit for %x: "+
                        "(%s, %d)", pkt.circuit.PaymentHash,
                        pkt.incomingChanID, pkt.incomingHTLCID)
        }

        return nil
}

// CloseLink creates and sends the close channel command to the target link
// directing the specified closure type. If the closure type is CloseRegular,
// targetFeePerKw parameter should be the ideal fee-per-kw that will be used as
// a starting point for close negotiation. The deliveryScript parameter is an
// optional parameter which sets a user specified script to close out to.
func (s *Switch) CloseLink(chanPoint *wire.OutPoint,
        closeType contractcourt.ChannelCloseType,
        targetFeePerKw, maxFee chainfee.SatPerKWeight,
        deliveryScript lnwire.DeliveryAddress) (chan interface{}, chan error) {

        // TODO(roasbeef) abstract out the close updates.
        updateChan := make(chan interface{}, 2)
        errChan := make(chan error, 1)

        command := &ChanClose{
                CloseType:      closeType,
                ChanPoint:      chanPoint,
                Updates:        updateChan,
                TargetFeePerKw: targetFeePerKw,
                MaxFee:         maxFee,
                DeliveryScript: deliveryScript,
                Err:            errChan,
        }

        select {
        case s.chanCloseRequests <- command:
                return updateChan, errChan

        case <-s.quit:
                errChan <- ErrSwitchExiting
                close(updateChan)
                return updateChan, errChan
        }
}

// htlcForwarder is responsible for optimally forwarding (and possibly
// fragmenting) incoming/outgoing HTLCs amongst all active interfaces and their
// links. The duties of the forwarder are similar to that of a network switch,
// in that it facilitates multi-hop payments by acting as a central messaging
// bus. The switch communicates will active links to create, manage, and tear
// down active onion routed payments. Each active channel is modeled as
// networked device with metadata such as the available payment bandwidth, and
// total link capacity.
//
// NOTE: This MUST be run as a goroutine.
func (s *Switch) htlcForwarder() {
        defer s.wg.Done()

        defer func() {
                s.blockEpochStream.Cancel()

                // Remove all links once we've been signalled for shutdown.
                var linksToStop []ChannelLink
                s.indexMtx.Lock()
                for _, link := range s.linkIndex {
                        activeLink := s.removeLink(link.ChanID())
                        if activeLink == nil {
                                log.Errorf("unable to remove ChannelLink(%v) "+
                                        "on stop", link.ChanID())
                                continue
                        }
                        linksToStop = append(linksToStop, activeLink)
                }
                for _, link := range s.pendingLinkIndex {
                        pendingLink := s.removeLink(link.ChanID())
                        if pendingLink == nil {
                                log.Errorf("unable to remove ChannelLink(%v) "+
                                        "on stop", link.ChanID())
                                continue
                        }
                        linksToStop = append(linksToStop, pendingLink)
                }
                s.indexMtx.Unlock()

                // Now that all pending and live links have been removed from
                // the forwarding indexes, stop each one before shutting down.
                // We'll shut them down in parallel to make exiting as fast as
                // possible.
                var wg sync.WaitGroup
                for _, link := range linksToStop {
                        wg.Add(1)
                        go func(l ChannelLink) {
                                defer wg.Done()

                                l.Stop()
                        }(link)
                }
                wg.Wait()

                // Before we exit fully, we'll attempt to flush out any
                // forwarding events that may still be lingering since the last
                // batch flush.
                if err := s.FlushForwardingEvents(); err != nil {
                        log.Errorf("unable to flush forwarding events: %v", err)
                }
        }()

        // TODO(roasbeef): cleared vs settled distinction
        var (
                totalNumUpdates uint64
                totalSatSent    btcutil.Amount
                totalSatRecv    btcutil.Amount
        )
        s.cfg.LogEventTicker.Resume()
        defer s.cfg.LogEventTicker.Stop()

        // Every 15 seconds, we'll flush out the forwarding events that
        // occurred during that period.
        s.cfg.FwdEventTicker.Resume()
        defer s.cfg.FwdEventTicker.Stop()

        defer s.cfg.AckEventTicker.Stop()

out:
        for {

                // If the set of pending settle/fail entries is non-zero,
                // reinstate the ack ticker so we can batch ack them.
                if len(s.pendingSettleFails) > 0 {
                        s.cfg.AckEventTicker.Resume()
                }

                select {
                case blockEpoch, ok := <-s.blockEpochStream.Epochs:
                        if !ok {
                                break out
                        }

                        atomic.StoreUint32(&s.bestHeight, uint32(blockEpoch.Height))

                // A local close request has arrived, we'll forward this to the
                // relevant link (if it exists) so the channel can be
                // cooperatively closed (if possible).
                case req := <-s.chanCloseRequests:
                        chanID := lnwire.NewChanIDFromOutPoint(*req.ChanPoint)

                        s.indexMtx.RLock()
                        link, ok := s.linkIndex[chanID]
                        if !ok {
                                s.indexMtx.RUnlock()

                                req.Err <- fmt.Errorf("no peer for channel with "+
                                        "chan_id=%x", chanID[:])
                                continue
                        }
                        s.indexMtx.RUnlock()

                        peerPub := link.PeerPubKey()
                        log.Debugf("Requesting local channel close: peer=%v, "+
                                "chan_id=%x", link.PeerPubKey(), chanID[:])

                        go s.cfg.LocalChannelClose(peerPub[:], req)

                case resolutionMsg := <-s.resolutionMsgs:
                        // We'll persist the resolution message to the Switch's
                        // resolution store.
                        resMsg := resolutionMsg.ResolutionMsg
                        err := s.resMsgStore.addResolutionMsg(&resMsg)
                        if err != nil {
                                // This will only fail if there is a database
                                // error or a serialization error. Sending the
                                // error prevents the contractcourt from being
                                // in a state where it believes the send was
                                // successful, when it wasn't.
                                log.Errorf("unable to add resolution msg: %v",
                                        err)
                                resolutionMsg.errChan <- err
                                continue
                        }

                        // At this point, the resolution message has been
                        // persisted. It is safe to signal success by sending
                        // a nil error since the Switch will re-deliver the
                        // resolution message on restart.
                        resolutionMsg.errChan <- nil

                        // Create a htlc packet for this resolution. We do
                        // not have some of the information that we'll need
                        // for blinded error handling here , so we'll rely on
                        // our forwarding logic to fill it in later.
                        pkt := &htlcPacket{
                                outgoingChanID: resolutionMsg.SourceChan,
                                outgoingHTLCID: resolutionMsg.HtlcIndex,
                                isResolution:   true,
                        }

                        // Resolution messages will either be cancelling
                        // backwards an existing HTLC, or settling a previously
                        // outgoing HTLC. Based on this, we'll map the message
                        // to the proper htlcPacket.
                        if resolutionMsg.Failure != nil {
                                pkt.htlc = &lnwire.UpdateFailHTLC{}
                        } else {
                                pkt.htlc = &lnwire.UpdateFulfillHTLC{
                                        PaymentPreimage: *resolutionMsg.PreImage,
                                }
                        }

                        log.Infof("Received outside contract resolution, "+
                                "mapping to: %v", spew.Sdump(pkt))

                        // We don't check the error, as the only failure we can
                        // encounter is due to the circuit already being
                        // closed. This is fine, as processing this message is
                        // meant to be idempotent.
                        err = s.handlePacketForward(pkt)
                        if err != nil {
                                log.Errorf("Unable to forward resolution msg: %v", err)
                        }

                // A new packet has arrived for forwarding, we'll interpret the
                // packet concretely, then either forward it along, or
                // interpret a return packet to a locally initialized one.
                case cmd := <-s.htlcPlex:
                        cmd.err <- s.handlePacketForward(cmd.pkt)

                // When this time ticks, then it indicates that we should
                // collect all the forwarding events since the last internal,
                // and write them out to our log.
                case <-s.cfg.FwdEventTicker.Ticks():
                        s.wg.Add(1)
                        go func() {
                                defer s.wg.Done()

                                if err := s.FlushForwardingEvents(); err != nil {
                                        log.Errorf("unable to flush "+
                                                "forwarding events: %v", err)
                                }
                        }()

                // The log ticker has fired, so we'll calculate some forwarding
                // stats for the last 10 seconds to display within the logs to
                // users.
                case <-s.cfg.LogEventTicker.Ticks():
                        // First, we'll collate the current running tally of
                        // our forwarding stats.
                        prevSatSent := totalSatSent
                        prevSatRecv := totalSatRecv
                        prevNumUpdates := totalNumUpdates

                        var (
                                newNumUpdates uint64
                                newSatSent    btcutil.Amount
                                newSatRecv    btcutil.Amount
                        )

                        // Next, we'll run through all the registered links and
                        // compute their up-to-date forwarding stats.
                        s.indexMtx.RLock()
                        for _, link := range s.linkIndex {
                                // TODO(roasbeef): when links first registered
                                // stats printed.
                                updates, sent, recv := link.Stats()
                                newNumUpdates += updates
                                newSatSent += sent.ToSatoshis()
                                newSatRecv += recv.ToSatoshis()
                        }
                        s.indexMtx.RUnlock()

                        var (
                                diffNumUpdates uint64
                                diffSatSent    btcutil.Amount
                                diffSatRecv    btcutil.Amount
                        )

                        // If this is the first time we're computing these
                        // stats, then the diff is just the new value. We do
                        // this in order to avoid integer underflow issues.
                        if prevNumUpdates == 0 {
                                diffNumUpdates = newNumUpdates
                                diffSatSent = newSatSent
                                diffSatRecv = newSatRecv
                        } else {
                                diffNumUpdates = newNumUpdates - prevNumUpdates
                                diffSatSent = newSatSent - prevSatSent
                                diffSatRecv = newSatRecv - prevSatRecv
                        }

                        // If the diff of num updates is zero, then we haven't
                        // forwarded anything in the last 10 seconds, so we can
                        // skip this update.
                        if diffNumUpdates == 0 {
                                continue
                        }

                        // If the diff of num updates is negative, then some
                        // links may have been unregistered from the switch, so
                        // we'll update our stats to only include our registered
                        // links.
                        if int64(diffNumUpdates) < 0 {
                                totalNumUpdates = newNumUpdates
                                totalSatSent = newSatSent
                                totalSatRecv = newSatRecv
                                continue
                        }

                        // Otherwise, we'll log this diff, then accumulate the
                        // new stats into the running total.
                        log.Debugf("Sent %d satoshis and received %d satoshis "+
                                "in the last 10 seconds (%f tx/sec)",
                                diffSatSent, diffSatRecv,
                                float64(diffNumUpdates)/10)

                        totalNumUpdates += diffNumUpdates
                        totalSatSent += diffSatSent
                        totalSatRecv += diffSatRecv

                // The ack ticker has fired so if we have any settle/fail entries
                // for a forwarding package to ack, we will do so here in a batch
                // db call.
                case <-s.cfg.AckEventTicker.Ticks():
                        // If the current set is empty, pause the ticker.
                        if len(s.pendingSettleFails) == 0 {
                                s.cfg.AckEventTicker.Pause()
                                continue
                        }

                        // Batch ack the settle/fail entries.
                        if err := s.ackSettleFail(s.pendingSettleFails...); err != nil {
                                log.Errorf("Unable to ack batch of settle/fails: %v", err)
                                continue
                        }

                        log.Tracef("Acked %d settle fails: %v",
                                len(s.pendingSettleFails),
                                lnutils.SpewLogClosure(s.pendingSettleFails))

                        // Reset the pendingSettleFails buffer while keeping acquired
                        // memory.
                        s.pendingSettleFails = s.pendingSettleFails[:0]

                case <-s.quit:
                        return
                }
        }
}

// Start starts all helper goroutines required for the operation of the switch.
func (s *Switch) Start() error {
        if !atomic.CompareAndSwapInt32(&s.started, 0, 1) {
                log.Warn("Htlc Switch already started")
                return errors.New("htlc switch already started")
        }

        log.Infof("HTLC Switch starting")

        blockEpochStream, err := s.cfg.Notifier.RegisterBlockEpochNtfn(nil)
        if err != nil {
                return err
        }
        s.blockEpochStream = blockEpochStream

        s.wg.Add(1)
        go s.htlcForwarder()

        if err := s.reforwardResponses(); err != nil {
                s.Stop()
                log.Errorf("unable to reforward responses: %v", err)
                return err
        }

        if err := s.reforwardResolutions(); err != nil {
                // We are already stopping so we can ignore the error.
                _ = s.Stop()
                log.Errorf("unable to reforward resolutions: %v", err)
                return err
        }

        return nil
}

// reforwardResolutions fetches the set of resolution messages stored on-disk
// and reforwards them if their circuits are still open. If the circuits have
// been deleted, then we will delete the resolution message from the database.
func (s *Switch) reforwardResolutions() error {
        // Fetch all stored resolution messages, deleting the ones that are
        // resolved.
        resMsgs, err := s.resMsgStore.fetchAllResolutionMsg()
        if err != nil {
                return err
        }

        switchPackets := make([]*htlcPacket, 0, len(resMsgs))
        for _, resMsg := range resMsgs {
                // If the open circuit no longer exists, then we can remove the
                // message from the store.
                outKey := CircuitKey{
                        ChanID: resMsg.SourceChan,
                        HtlcID: resMsg.HtlcIndex,
                }

                if s.circuits.LookupOpenCircuit(outKey) == nil {
                        // The open circuit doesn't exist.
                        err := s.resMsgStore.deleteResolutionMsg(&outKey)
                        if err != nil {
                                return err
                        }

                        continue
                }

                // The circuit is still open, so we can assume that the link or
                // switch (if we are the source) hasn't cleaned it up yet.
                // We rely on our forwarding logic to fill in details that
                // are not currently available to us.
                resPkt := &htlcPacket{
                        outgoingChanID: resMsg.SourceChan,
                        outgoingHTLCID: resMsg.HtlcIndex,
                        isResolution:   true,
                }

                if resMsg.Failure != nil {
                        resPkt.htlc = &lnwire.UpdateFailHTLC{}
                } else {
                        resPkt.htlc = &lnwire.UpdateFulfillHTLC{
                                PaymentPreimage: *resMsg.PreImage,
                        }
                }

                switchPackets = append(switchPackets, resPkt)
        }

        // We'll now dispatch the set of resolution messages to the proper
        // destination. An error is only encountered here if the switch is
        // shutting down.
        if err := s.ForwardPackets(nil, switchPackets...); err != nil {
                return err
        }

        return nil
}

// reforwardResponses for every known, non-pending channel, loads all associated
// forwarding packages and reforwards any Settle or Fail HTLCs found. This is
// used to resurrect the switch's mailboxes after a restart. This also runs for
// waiting close channels since there may be settles or fails that need to be
// reforwarded before they completely close.
func (s *Switch) reforwardResponses() error {
        openChannels, err := s.cfg.FetchAllChannels()
        if err != nil {
                return err
        }

        for _, openChannel := range openChannels {
                shortChanID := openChannel.ShortChanID()

                // Locally-initiated payments never need reforwarding.
                if shortChanID == hop.Source {
                        continue
                }

                // If the channel is pending, it should have no forwarding
                // packages, and nothing to reforward.
                if openChannel.IsPending {
                        continue
                }

                // Channels in open or waiting-close may still have responses in
                // their forwarding packages. We will continue to reattempt
                // forwarding on startup until the channel is fully-closed.
                //
                // Load this channel's forwarding packages, and deliver them to
                // the switch.
                fwdPkgs, err := s.loadChannelFwdPkgs(shortChanID)
                if err != nil {
                        log.Errorf("unable to load forwarding "+
                                "packages for %v: %v", shortChanID, err)
                        return err
                }

                s.reforwardSettleFails(fwdPkgs)
        }

        return nil
}

// loadChannelFwdPkgs loads all forwarding packages owned by the `source` short
// channel identifier.
func (s *Switch) loadChannelFwdPkgs(source lnwire.ShortChannelID) ([]*channeldb.FwdPkg, error) {

        var fwdPkgs []*channeldb.FwdPkg
        if err := kvdb.View(s.cfg.DB, func(tx kvdb.RTx) error {
                var err error
                fwdPkgs, err = s.cfg.SwitchPackager.LoadChannelFwdPkgs(
                        tx, source,
                )
                return err
        }, func() {
                fwdPkgs = nil
        }); err != nil {
                return nil, err
        }

        return fwdPkgs, nil
}

// reforwardSettleFails parses the Settle and Fail HTLCs from the list of
// forwarding packages, and reforwards those that have not been acknowledged.
// This is intended to occur on startup, in order to recover the switch's
// mailboxes, and to ensure that responses can be propagated in case the
// outgoing link never comes back online.
//
// NOTE: This should mimic the behavior processRemoteSettleFails.
func (s *Switch) reforwardSettleFails(fwdPkgs []*channeldb.FwdPkg) {
        for _, fwdPkg := range fwdPkgs {
                settleFails, err := lnwallet.PayDescsFromRemoteLogUpdates(
                        fwdPkg.Source, fwdPkg.Height, fwdPkg.SettleFails,
                )
                if err != nil {
                        log.Errorf("Unable to process remote log updates: %v",
                                err)
                        continue
                }

                switchPackets := make([]*htlcPacket, 0, len(settleFails))
                for i, pd := range settleFails {

                        // Skip any settles or fails that have already been
                        // acknowledged by the incoming link that originated the
                        // forwarded Add.
                        if fwdPkg.SettleFailFilter.Contains(uint16(i)) {
                                continue
                        }

                        switch pd.EntryType {

                        // A settle for an HTLC we previously forwarded HTLC has
                        // been received. So we'll forward the HTLC to the
                        // switch which will handle propagating the settle to
                        // the prior hop.
                        case lnwallet.Settle:
                                settlePacket := &htlcPacket{
                                        outgoingChanID: fwdPkg.Source,
                                        outgoingHTLCID: pd.ParentIndex,
                                        destRef:        pd.DestRef,
                                        htlc: &lnwire.UpdateFulfillHTLC{
                                                PaymentPreimage: pd.RPreimage,
                                        },
                                }

                                // Add the packet to the batch to be forwarded, and
                                // notify the overflow queue that a spare spot has been
                                // freed up within the commitment state.
                                switchPackets = append(switchPackets, settlePacket)

                        // A failureCode message for a previously forwarded HTLC has been
                        // received. As a result a new slot will be freed up in our
                        // commitment state, so we'll forward this to the switch so the
                        // backwards undo can continue.
                        case lnwallet.Fail:
                                // Fetch the reason the HTLC was canceled so
                                // we can continue to propagate it. This
                                // failure originated from another node, so
                                // the linkFailure field is not set on this
                                // packet. We rely on the link to fill in
                                // additional circuit information for us.
                                failPacket := &htlcPacket{
                                        outgoingChanID: fwdPkg.Source,
                                        outgoingHTLCID: pd.ParentIndex,
                                        destRef:        pd.DestRef,
                                        htlc: &lnwire.UpdateFailHTLC{
                                                Reason: lnwire.OpaqueReason(pd.FailReason),
                                        },
                                }

                                // Add the packet to the batch to be forwarded, and
                                // notify the overflow queue that a spare spot has been
                                // freed up within the commitment state.
                                switchPackets = append(switchPackets, failPacket)
                        }
                }

                // Since this send isn't tied to a specific link, we pass a nil
                // link quit channel, meaning the send will fail only if the
                // switch receives a shutdown request.
                if err := s.ForwardPackets(nil, switchPackets...); err != nil {
                        log.Errorf("Unhandled error while reforwarding packets "+
                                "settle/fail over htlcswitch: %v", err)
                }
        }
}

// Stop gracefully stops all active helper goroutines, then waits until they've
// exited.
func (s *Switch) Stop() error {
        if !atomic.CompareAndSwapInt32(&s.shutdown, 0, 1) {
                log.Warn("Htlc Switch already stopped")
                return errors.New("htlc switch already shutdown")
        }

        log.Info("HTLC Switch shutting down...")
        defer log.Debug("HTLC Switch shutdown complete")

        close(s.quit)

        s.wg.Wait()

        // Wait until all active goroutines have finished exiting before
        // stopping the mailboxes, otherwise the mailbox map could still be
        // accessed and modified.
        s.mailOrchestrator.Stop()

        return nil
}

// CreateAndAddLink will create a link and then add it to the internal maps
// when given a ChannelLinkConfig and LightningChannel.
func (s *Switch) CreateAndAddLink(linkCfg ChannelLinkConfig,
        lnChan *lnwallet.LightningChannel) error {

        link := NewChannelLink(linkCfg, lnChan)
        return s.AddLink(link)
}

// AddLink is used to initiate the handling of the add link command. The
// request will be propagated and handled in the main goroutine.
func (s *Switch) AddLink(link ChannelLink) error {
        s.indexMtx.Lock()
        defer s.indexMtx.Unlock()

        chanID := link.ChanID()

        // First, ensure that this link is not already active in the switch.
        _, err := s.getLink(chanID)
        if err == nil {
                return fmt.Errorf("unable to add ChannelLink(%v), already "+
                        "active", chanID)
        }

        // Get and attach the mailbox for this link, which buffers packets in
        // case there packets that we tried to deliver while this link was
        // offline.
        shortChanID := link.ShortChanID()
        mailbox := s.mailOrchestrator.GetOrCreateMailBox(chanID, shortChanID)
        link.AttachMailBox(mailbox)

        // Attach the Switch's failAliasUpdate function to the link.
        link.attachFailAliasUpdate(s.failAliasUpdate)

        if err := link.Start(); err != nil {
                log.Errorf("AddLink failed to start link with chanID=%v: %v",
                        chanID, err)
                s.removeLink(chanID)
                return err
        }

        if shortChanID == hop.Source {
                log.Infof("Adding pending link chan_id=%v, short_chan_id=%v",
                        chanID, shortChanID)

                s.pendingLinkIndex[chanID] = link
        } else {
                log.Infof("Adding live link chan_id=%v, short_chan_id=%v",
                        chanID, shortChanID)

                s.addLiveLink(link)
                s.mailOrchestrator.BindLiveShortChanID(
                        mailbox, chanID, shortChanID,
                )
        }

        return nil
}

// addLiveLink adds a link to all associated forwarding index, this makes it a
// candidate for forwarding HTLCs.
func (s *Switch) addLiveLink(link ChannelLink) {
        linkScid := link.ShortChanID()

        // We'll add the link to the linkIndex which lets us quickly
        // look up a channel when we need to close or register it, and
        // the forwarding index which'll be used when forwarding HTLC's
        // in the multi-hop setting.
        s.linkIndex[link.ChanID()] = link
        s.forwardingIndex[linkScid] = link

        // Next we'll add the link to the interface index so we can
        // quickly look up all the channels for a particular node.
        peerPub := link.PeerPubKey()
        if _, ok := s.interfaceIndex[peerPub]; !ok {
                s.interfaceIndex[peerPub] = make(map[lnwire.ChannelID]ChannelLink)
        }
        s.interfaceIndex[peerPub][link.ChanID()] = link

        aliases := link.getAliases()
        if link.isZeroConf() {
                if link.zeroConfConfirmed() {
                        // Since the zero-conf channel has confirmed, we can
                        // populate the aliasToReal mapping.
                        confirmedScid := link.confirmedScid()

                        for _, alias := range aliases {
                                s.aliasToReal[alias] = confirmedScid
                        }

                        // Add the confirmed SCID as a key in the baseIndex.
                        s.baseIndex[confirmedScid] = linkScid
                }

                // Now we populate the baseIndex which will be used to fetch
                // the link given any of the channel's alias SCIDs or the real
                // SCID. The link's SCID is an alias, so we don't need to
                // special-case it like the option-scid-alias feature-bit case
                // further down.
                for _, alias := range aliases {
                        s.baseIndex[alias] = linkScid
                }
        } else if link.negotiatedAliasFeature() {
                // The link's SCID is the confirmed SCID for non-zero-conf
                // option-scid-alias feature bit channels.
                for _, alias := range aliases {
                        s.aliasToReal[alias] = linkScid
                        s.baseIndex[alias] = linkScid
                }

                // Since the link's SCID is confirmed, it was not included in
                // the baseIndex above as a key. Add it now.
                s.baseIndex[linkScid] = linkScid
        }
}

// GetLink is used to initiate the handling of the get link command. The
// request will be propagated/handled to/in the main goroutine.
func (s *Switch) GetLink(chanID lnwire.ChannelID) (ChannelUpdateHandler,
        error) {

        s.indexMtx.RLock()
        defer s.indexMtx.RUnlock()

        return s.getLink(chanID)
}

// getLink returns the link stored in either the pending index or the live
// lindex.
func (s *Switch) getLink(chanID lnwire.ChannelID) (ChannelLink, error) {
        link, ok := s.linkIndex[chanID]
        if !ok {
                link, ok = s.pendingLinkIndex[chanID]
                if !ok {
                        return nil, ErrChannelLinkNotFound
                }
        }

        return link, nil
}

// GetLinkByShortID attempts to return the link which possesses the target short
// channel ID.
func (s *Switch) GetLinkByShortID(chanID lnwire.ShortChannelID) (ChannelLink,
        error) {

        s.indexMtx.RLock()
        defer s.indexMtx.RUnlock()

        link, err := s.getLinkByShortID(chanID)
        if err != nil {
                // If we failed to find the link under the passed-in SCID, we
                // consult the Switch's baseIndex map to see if the confirmed
                // SCID was used for a zero-conf channel.
                aliasID, ok := s.baseIndex[chanID]
                if !ok {
                        return nil, err
                }

                // An alias was found, use it to lookup if a link exists.
                return s.getLinkByShortID(aliasID)
        }

        return link, nil
}

// getLinkByShortID attempts to return the link which possesses the target
// short channel ID.
//
// NOTE: This MUST be called with the indexMtx held.
func (s *Switch) getLinkByShortID(chanID lnwire.ShortChannelID) (ChannelLink, error) {
        link, ok := s.forwardingIndex[chanID]
        if !ok {
                return nil, ErrChannelLinkNotFound
        }

        return link, nil
}

// getLinkByMapping attempts to fetch the link via the htlcPacket's
// outgoingChanID, possibly using a mapping. If it finds the link via mapping,
// the outgoingChanID will be changed so that an error can be properly
// attributed when looping over linkErrs in handlePacketForward.
//
// * If the outgoingChanID is an alias, we'll fetch the link regardless if it's
// public or not.
//
// * If the outgoingChanID is a confirmed SCID, we'll need to do more checks.
//   - If there is no entry found in baseIndex, fetch the link. This channel
//     did not have the option-scid-alias feature negotiated (which includes
//     zero-conf and option-scid-alias channel-types).
//   - If there is an entry found, fetch the link from forwardingIndex and
//     fail if this is a private link.
//
// NOTE: This MUST be called with the indexMtx read lock held.
func (s *Switch) getLinkByMapping(pkt *htlcPacket) (ChannelLink, error) {
        // Determine if this ShortChannelID is an alias or a confirmed SCID.
        chanID := pkt.outgoingChanID
        aliasID := s.cfg.IsAlias(chanID)

        // Set the originalOutgoingChanID so the proper channel_update can be
        // sent back if the option-scid-alias feature bit was negotiated.
        pkt.originalOutgoingChanID = chanID

        if aliasID {
                // Since outgoingChanID is an alias, we'll fetch the link via
                // baseIndex.
                baseScid, ok := s.baseIndex[chanID]
                if !ok {
                        // No mapping exists, bail.
                        return nil, ErrChannelLinkNotFound
                }

                // A mapping exists, so use baseScid to find the link in the
                // forwardingIndex.
                link, ok := s.forwardingIndex[baseScid]
                if !ok {
                        // Link not found, bail.
                        return nil, ErrChannelLinkNotFound
                }

                // Change the packet's outgoingChanID field so that errors are
                // properly attributed.
                pkt.outgoingChanID = baseScid

                // Return the link without checking if it's private or not.
                return link, nil
        }

        // The outgoingChanID is a confirmed SCID. Attempt to fetch the base
        // SCID from baseIndex.
        baseScid, ok := s.baseIndex[chanID]
        if !ok {
                // outgoingChanID is not a key in base index meaning this
                // channel did not have the option-scid-alias feature bit
                // negotiated. We'll fetch the link and return it.
                link, ok := s.forwardingIndex[chanID]
                if !ok {
                        // The link wasn't found, bail out.
                        return nil, ErrChannelLinkNotFound
                }

                return link, nil
        }

        // Fetch the link whose internal SCID is baseScid.
        link, ok := s.forwardingIndex[baseScid]
        if !ok {
                // Link wasn't found, bail out.
                return nil, ErrChannelLinkNotFound
        }

        // If the link is unadvertised, we fail since the real SCID was used to
        // forward over it and this is a channel where the option-scid-alias
        // feature bit was negotiated.
        if link.IsUnadvertised() {
                return nil, ErrChannelLinkNotFound
        }

        // The link is public so the confirmed SCID can be used to forward over
        // it. We'll also replace pkt's outgoingChanID field so errors can
        // properly be attributed in the calling function.
        pkt.outgoingChanID = baseScid
        return link, nil
}

// HasActiveLink returns true if the given channel ID has a link in the link
// index AND the link is eligible to forward.
func (s *Switch) HasActiveLink(chanID lnwire.ChannelID) bool {
        s.indexMtx.RLock()
        defer s.indexMtx.RUnlock()

        if link, ok := s.linkIndex[chanID]; ok {
                return link.EligibleToForward()
        }

        return false
}

// RemoveLink purges the switch of any link associated with chanID. If a pending
// or active link is not found, this method does nothing. Otherwise, the method
// returns after the link has been completely shutdown.
func (s *Switch) RemoveLink(chanID lnwire.ChannelID) {
        s.indexMtx.Lock()
        link, err := s.getLink(chanID)
        if err != nil {
                // If err is non-nil, this means that link is also nil. The
                // link variable cannot be nil without err being non-nil.
                s.indexMtx.Unlock()
                log.Tracef("Unable to remove link for ChannelID(%v): %v",
                        chanID, err)
                return
        }

        // Check if the link is already stopping and grab the stop chan if it
        // is.
        stopChan, ok := s.linkStopIndex[chanID]
        if !ok {
                // If the link is non-nil, it is not currently stopping, so
                // we'll add a stop chan to the linkStopIndex.
                stopChan = make(chan struct{})
                s.linkStopIndex[chanID] = stopChan
        }
        s.indexMtx.Unlock()

        if ok {
                // If the stop chan exists, we will wait for it to be closed.
                // Once it is closed, we will exit.
                select {
                case <-stopChan:
                        return
                case <-s.quit:
                        return
                }
        }

        // Stop the link before removing it from the maps.
        link.Stop()

        s.indexMtx.Lock()
        _ = s.removeLink(chanID)

        // Close stopChan and remove this link from the linkStopIndex.
        // Deleting from the index and removing from the link must be done
        // in the same block while the mutex is held.
        close(stopChan)
        delete(s.linkStopIndex, chanID)
        s.indexMtx.Unlock()
}

// removeLink is used to remove and stop the channel link.
//
// NOTE: This MUST be called with the indexMtx held.
func (s *Switch) removeLink(chanID lnwire.ChannelID) ChannelLink {
        log.Infof("Removing channel link with ChannelID(%v)", chanID)

        link, err := s.getLink(chanID)
        if err != nil {
                return nil
        }

        // Remove the channel from live link indexes.
        delete(s.pendingLinkIndex, link.ChanID())
        delete(s.linkIndex, link.ChanID())
        delete(s.forwardingIndex, link.ShortChanID())

        // If the link has been added to the peer index, then we'll move to
        // delete the entry within the index.
        peerPub := link.PeerPubKey()
        if peerIndex, ok := s.interfaceIndex[peerPub]; ok {
                delete(peerIndex, link.ChanID())

                // If after deletion, there are no longer any links, then we'll
                // remove the interface map all together.
                if len(peerIndex) == 0 {
                        delete(s.interfaceIndex, peerPub)
                }
        }

        return link
}

// UpdateShortChanID locates the link with the passed-in chanID and updates the
// underlying channel state. This is only used in zero-conf channels to allow
// the confirmed SCID to be updated.
func (s *Switch) UpdateShortChanID(chanID lnwire.ChannelID) error {
        s.indexMtx.Lock()
        defer s.indexMtx.Unlock()

        // Locate the target link in the link index. If no such link exists,
        // then we will ignore the request.
        link, ok := s.linkIndex[chanID]
        if !ok {
                return fmt.Errorf("link %v not found", chanID)
        }

        // Try to update the link's underlying channel state, returning early
        // if this update failed.
        _, err := link.UpdateShortChanID()
        if err != nil {
                return err
        }

        // Since the zero-conf channel is confirmed, we should populate the
        // aliasToReal map and update the baseIndex.
        aliases := link.getAliases()

        confirmedScid := link.confirmedScid()

        for _, alias := range aliases {
                s.aliasToReal[alias] = confirmedScid
        }

        s.baseIndex[confirmedScid] = link.ShortChanID()

        return nil
}

// GetLinksByInterface fetches all the links connected to a particular node
// identified by the serialized compressed form of its public key.
func (s *Switch) GetLinksByInterface(hop [33]byte) ([]ChannelUpdateHandler,
        error) {

        s.indexMtx.RLock()
        defer s.indexMtx.RUnlock()

        var handlers []ChannelUpdateHandler

        links, err := s.getLinks(hop)
        if err != nil {
                return nil, err
        }

        // Range over the returned []ChannelLink to convert them into
        // []ChannelUpdateHandler.
        for _, link := range links {
                handlers = append(handlers, link)
        }

        return handlers, nil
}

// getLinks is function which returns the channel links of the peer by hop
// destination id.
//
// NOTE: This MUST be called with the indexMtx held.
func (s *Switch) getLinks(destination [33]byte) ([]ChannelLink, error) {
        links, ok := s.interfaceIndex[destination]
        if !ok {
                return nil, ErrNoLinksFound
        }

        channelLinks := make([]ChannelLink, 0, len(links))
        for _, link := range links {
                channelLinks = append(channelLinks, link)
        }

        return channelLinks, nil
}

// CircuitModifier returns a reference to subset of the interfaces provided by
// the circuit map, to allow links to open and close circuits.
func (s *Switch) CircuitModifier() CircuitModifier {
        return s.circuits
}

// CircuitLookup returns a reference to subset of the interfaces provided by the
// circuit map, to allow looking up circuits.
func (s *Switch) CircuitLookup() CircuitLookup {
        return s.circuits
}

// commitCircuits persistently adds a circuit to the switch's circuit map.
func (s *Switch) commitCircuits(circuits ...*PaymentCircuit) (
        *CircuitFwdActions, error) {

        return s.circuits.CommitCircuits(circuits...)
}

// FlushForwardingEvents flushes out the set of pending forwarding events to
// the persistent log. This will be used by the switch to periodically flush
// out the set of forwarding events to disk. External callers can also use this
// method to ensure all data is flushed to dis before querying the log.
func (s *Switch) FlushForwardingEvents() error {
        // First, we'll obtain a copy of the current set of pending forwarding
        // events.
        s.fwdEventMtx.Lock()

        // If we won't have any forwarding events, then we can exit early.
        if len(s.pendingFwdingEvents) == 0 {
                s.fwdEventMtx.Unlock()
                return nil
        }

        events := make([]channeldb.ForwardingEvent, len(s.pendingFwdingEvents))
        copy(events[:], s.pendingFwdingEvents[:])

        // With the copy obtained, we can now clear out the header pointer of
        // the current slice. This way, we can re-use the underlying storage
        // allocated for the slice.
        s.pendingFwdingEvents = s.pendingFwdingEvents[:0]
        s.fwdEventMtx.Unlock()

        // Finally, we'll write out the copied events to the persistent
        // forwarding log.
        return s.cfg.FwdingLog.AddForwardingEvents(events)
}

// BestHeight returns the best height known to the switch.
func (s *Switch) BestHeight() uint32 {
        return atomic.LoadUint32(&s.bestHeight)
}

// dustExceedsFeeThreshold takes in a ChannelLink, HTLC amount, and a boolean
// to determine whether the default fee threshold has been exceeded. This
// heuristic takes into account the trimmed-to-dust mechanism. The sum of the
// commitment's dust with the mailbox's dust with the amount is checked against
// the fee exposure threshold. If incoming is true, then the amount is not
// included in the sum as it was already included in the commitment's dust. A
// boolean is returned telling the caller whether the HTLC should be failed
// back.
func (s *Switch) dustExceedsFeeThreshold(link ChannelLink,
        amount lnwire.MilliSatoshi, incoming bool) bool {

        // Retrieve the link's current commitment feerate and dustClosure.
        feeRate := link.getFeeRate()
        isDust := link.getDustClosure()

        // Evaluate if the HTLC is dust on either sides' commitment.
        isLocalDust := isDust(
                feeRate, incoming, lntypes.Local, amount.ToSatoshis(),
        )
        isRemoteDust := isDust(
                feeRate, incoming, lntypes.Remote, amount.ToSatoshis(),
        )

        if !(isLocalDust || isRemoteDust) {
                // If the HTLC is not dust on either commitment, it's fine to
                // forward.
                return false
        }

        // Fetch the dust sums currently in the mailbox for this link.
        cid := link.ChanID()
        sid := link.ShortChanID()
        mailbox := s.mailOrchestrator.GetOrCreateMailBox(cid, sid)
        localMailDust, remoteMailDust := mailbox.DustPackets()

        // If the htlc is dust on the local commitment, we'll obtain the dust
        // sum for it.
        if isLocalDust {
                localSum := link.getDustSum(
                        lntypes.Local, fn.None[chainfee.SatPerKWeight](),
                )
                localSum += localMailDust

                // Optionally include the HTLC amount only for outgoing
                // HTLCs.
                if !incoming {
                        localSum += amount
                }

                // Finally check against the defined fee threshold.
                if localSum > s.cfg.MaxFeeExposure {
                        return true
                }
        }

        // Also check if the htlc is dust on the remote commitment, if we've
        // reached this point.
        if isRemoteDust {
                remoteSum := link.getDustSum(
                        lntypes.Remote, fn.None[chainfee.SatPerKWeight](),
                )
                remoteSum += remoteMailDust

                // Optionally include the HTLC amount only for outgoing
                // HTLCs.
                if !incoming {
                        remoteSum += amount
                }

                // Finally check against the defined fee threshold.
                if remoteSum > s.cfg.MaxFeeExposure {
                        return true
                }
        }

        // If we reached this point, this HTLC is fine to forward.
        return false
}

// failMailboxUpdate is passed to the mailbox orchestrator which in turn passes
// it to individual mailboxes. It allows the mailboxes to construct a
// FailureMessage when failing back HTLC's due to expiry and may include an
// alias in the ShortChannelID field. The outgoingScid is the SCID originally
// used in the onion. The mailboxScid is the SCID that the mailbox and link
// use. The mailboxScid is only used in the non-alias case, so it is always
// the confirmed SCID.
func (s *Switch) failMailboxUpdate(outgoingScid,
        mailboxScid lnwire.ShortChannelID) lnwire.FailureMessage {

        // Try to use the failAliasUpdate function in case this is a channel
        // that uses aliases. If it returns nil, we'll fallback to the original
        // pre-alias behavior.
        update := s.failAliasUpdate(outgoingScid, false)
        if update == nil {
                // Execute the fallback behavior.
                var err error
                update, err = s.cfg.FetchLastChannelUpdate(mailboxScid)
                if err != nil {
                        return &lnwire.FailTemporaryNodeFailure{}
                }
        }

        return lnwire.NewTemporaryChannelFailure(update)
}

// failAliasUpdate prepares a ChannelUpdate for a failed incoming or outgoing
// HTLC on a channel where the option-scid-alias feature bit was negotiated. If
// the associated channel is not one of these, this function will return nil
// and the caller is expected to handle this properly. In this case, a return
// to the original non-alias behavior is expected.
func (s *Switch) failAliasUpdate(scid lnwire.ShortChannelID,
        incoming bool) *lnwire.ChannelUpdate {

        // This function does not defer the unlocking because of the database
        // lookups for ChannelUpdate.
        s.indexMtx.RLock()

        if s.cfg.IsAlias(scid) {
                // The alias SCID was used. In the incoming case this means
                // the channel is zero-conf as the link sets the scid. In the
                // outgoing case, the sender set the scid to use and may be
                // either the alias or the confirmed one, if it exists.
                realScid, ok := s.aliasToReal[scid]
                if !ok {
                        // The real, confirmed SCID does not exist yet. Find
                        // the "base" SCID that the link uses via the
                        // baseIndex. If we can't find it, return nil. This
                        // means the channel is zero-conf.
                        baseScid, ok := s.baseIndex[scid]
                        s.indexMtx.RUnlock()
                        if !ok {
                                return nil
                        }

                        update, err := s.cfg.FetchLastChannelUpdate(baseScid)
                        if err != nil {
                                return nil
                        }

                        // Replace the baseScid with the passed-in alias.
                        update.ShortChannelID = scid
                        sig, err := s.cfg.SignAliasUpdate(update)
                        if err != nil {
                                return nil
                        }

                        update.Signature, err = lnwire.NewSigFromSignature(sig)
                        if err != nil {
                                return nil
                        }

                        return update
                }

                s.indexMtx.RUnlock()

                // Fetch the SCID via the confirmed SCID and replace it with
                // the alias.
                update, err := s.cfg.FetchLastChannelUpdate(realScid)
                if err != nil {
                        return nil
                }

                // In the incoming case, we want to ensure that we don't leak
                // the UTXO in case the channel is private. In the outgoing
                // case, since the alias was used, we do the same thing.
                update.ShortChannelID = scid
                sig, err := s.cfg.SignAliasUpdate(update)
                if err != nil {
                        return nil
                }

                update.Signature, err = lnwire.NewSigFromSignature(sig)
                if err != nil {
                        return nil
                }

                return update
        }

        // If the confirmed SCID is not in baseIndex, this is not an
        // option-scid-alias or zero-conf channel.
        baseScid, ok := s.baseIndex[scid]
        if !ok {
                s.indexMtx.RUnlock()
                return nil
        }

        // Fetch the link so we can get an alias to use in the ShortChannelID
        // of the ChannelUpdate.
        link, ok := s.forwardingIndex[baseScid]
        s.indexMtx.RUnlock()
        if !ok {
                // This should never happen, but if it does for some reason,
                // fallback to the old behavior.
                return nil
        }

        aliases := link.getAliases()
        if len(aliases) == 0 {
                // This should never happen, but if it does, fallback.
                return nil
        }

        // Fetch the ChannelUpdate via the real, confirmed SCID.
        update, err := s.cfg.FetchLastChannelUpdate(scid)
        if err != nil {
                return nil
        }

        // The incoming case will replace the ShortChannelID in the retrieved
        // ChannelUpdate with the alias to ensure no privacy leak occurs. This
        // would happen if a private non-zero-conf option-scid-alias
        // feature-bit channel leaked its UTXO here rather than supplying an
        // alias. In the outgoing case, the confirmed SCID was actually used
        // for forwarding in the onion, so no replacement is necessary as the
        // sender knows the scid.
        if incoming {
                // We will replace and sign the update with the first alias.
                // Since this happens on the incoming side, it's not actually
                // possible to know what the sender used in the onion.
                update.ShortChannelID = aliases[0]
                sig, err := s.cfg.SignAliasUpdate(update)
                if err != nil {
                        return nil
                }

                update.Signature, err = lnwire.NewSigFromSignature(sig)
                if err != nil {
                        return nil
                }
        }

        return update
}

// AddAliasForLink instructs the Switch to update its in-memory maps to reflect
// that a link has a new alias.
func (s *Switch) AddAliasForLink(chanID lnwire.ChannelID,
        alias lnwire.ShortChannelID) error {

        // Fetch the link so that we can update the underlying channel's set of
        // aliases.
        s.indexMtx.RLock()
        link, err := s.getLink(chanID)
        s.indexMtx.RUnlock()
        if err != nil {
                return err
        }

        // If the link is a channel where the option-scid-alias feature bit was
        // not negotiated, we'll return an error.
        if !link.negotiatedAliasFeature() {
                return fmt.Errorf("attempted to update non-alias channel")
        }

        linkScid := link.ShortChanID()

        // We'll update the maps so the Switch includes this alias in its
        // forwarding decisions.
        if link.isZeroConf() {
                if link.zeroConfConfirmed() {
                        // If the channel has confirmed on-chain, we'll
                        // add this alias to the aliasToReal map.
                        confirmedScid := link.confirmedScid()

                        s.aliasToReal[alias] = confirmedScid
                }

                // Add this alias to the baseIndex mapping.
                s.baseIndex[alias] = linkScid
        } else if link.negotiatedAliasFeature() {
                // The channel is confirmed, so we'll populate the aliasToReal
                // and baseIndex maps.
                s.aliasToReal[alias] = linkScid
                s.baseIndex[alias] = linkScid
        }

        return nil
}

lightningnetwork / lnd / 10189747109

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