13440912774

Committed 20 Feb 2025 05:14PM UTC coverage: 57.697% (-1.1%) from 58.802%

Build # 13440912774

Build Type

Pull #9535

github

Committed by

guggero

Commit Message

GitHub: remove duplicate caching

Turns out that actions/setup-go starting with @v4 also adds caching.
With that, our cache size on disk has almost doubled, leading to the
GitHub runner running out of space in certain situation.
We fix that by disabling the automated caching since we already have our
own, custom-tailored version.

Pull Request Pull Request #9535: GitHub: remove duplicate caching

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73.88

/routing/router.go

package routing

import (
        "context"
        "fmt"
        "math"
        "math/big"
        "sort"
        "sync"
        "sync/atomic"
        "time"

        "github.com/btcsuite/btcd/btcec/v2"
        "github.com/btcsuite/btcd/btcutil"
        "github.com/davecgh/go-spew/spew"
        "github.com/go-errors/errors"
        "github.com/lightningnetwork/lnd/amp"
        "github.com/lightningnetwork/lnd/channeldb"
        "github.com/lightningnetwork/lnd/clock"
        "github.com/lightningnetwork/lnd/fn/v2"
        "github.com/lightningnetwork/lnd/graph/db/models"
        "github.com/lightningnetwork/lnd/htlcswitch"
        "github.com/lightningnetwork/lnd/lntypes"
        "github.com/lightningnetwork/lnd/lnutils"
        "github.com/lightningnetwork/lnd/lnwallet"
        "github.com/lightningnetwork/lnd/lnwire"
        "github.com/lightningnetwork/lnd/record"
        "github.com/lightningnetwork/lnd/routing/route"
        "github.com/lightningnetwork/lnd/routing/shards"
        "github.com/lightningnetwork/lnd/tlv"
        "github.com/lightningnetwork/lnd/zpay32"
)

const (
        // DefaultPayAttemptTimeout is the default payment attempt timeout. The
        // payment attempt timeout defines the duration after which we stop
        // trying more routes for a payment.
        DefaultPayAttemptTimeout = time.Second * 60

        // MinCLTVDelta is the minimum CLTV value accepted by LND for all
        // timelock deltas. This includes both forwarding CLTV deltas set on
        // channel updates, as well as final CLTV deltas used to create BOLT 11
        // payment requests.
        //
        // NOTE: For payment requests, BOLT 11 stipulates that a final CLTV
        // delta of 9 should be used when no value is decoded. This however
        // leads to inflexibility in upgrading this default parameter, since it
        // can create inconsistencies around the assumed value between sender
        // and receiver. Specifically, if the receiver assumes a higher value
        // than the sender, the receiver will always see the received HTLCs as
        // invalid due to their timelock not meeting the required delta.
        //
        // We skirt this by always setting an explicit CLTV delta when creating
        // invoices. This allows LND nodes to freely update the minimum without
        // creating incompatibilities during the upgrade process. For some time
        // LND has used an explicit default final CLTV delta of 40 blocks for
        // bitcoin, though we now clamp the lower end of this
        // range for user-chosen deltas to 18 blocks to be conservative.
        MinCLTVDelta = 18

        // MaxCLTVDelta is the maximum CLTV value accepted by LND for all
        // timelock deltas.
        MaxCLTVDelta = math.MaxUint16
)

var (
        // ErrRouterShuttingDown is returned if the router is in the process of
        // shutting down.
        ErrRouterShuttingDown = fmt.Errorf("router shutting down")

        // ErrSelfIntro is a failure returned when the source node of a
        // route request is also the introduction node. This is not yet
        // supported because LND does not support blinded forwardingg.
        ErrSelfIntro = errors.New("introduction point as own node not " +
                "supported")

        // ErrHintsAndBlinded is returned if a route request has both
        // bolt 11 route hints and a blinded path set.
        ErrHintsAndBlinded = errors.New("bolt 11 route hints and blinded " +
                "paths are mutually exclusive")

        // ErrExpiryAndBlinded is returned if a final cltv and a blinded path
        // are provided, as the cltv should be provided within the blinded
        // path.
        ErrExpiryAndBlinded = errors.New("final cltv delta and blinded " +
                "paths are mutually exclusive")

        // ErrTargetAndBlinded is returned is a target destination and a
        // blinded path are both set (as the target is inferred from the
        // blinded path).
        ErrTargetAndBlinded = errors.New("target node and blinded paths " +
                "are mutually exclusive")

        // ErrNoTarget is returned when the target node for a route is not
        // provided by either a blinded route or a cleartext pubkey.
        ErrNoTarget = errors.New("destination not set in target or blinded " +
                "path")

        // ErrSkipTempErr is returned when a non-MPP is made yet the
        // skipTempErr flag is set.
        ErrSkipTempErr = errors.New("cannot skip temp error for non-MPP")
)

// PaymentAttemptDispatcher is used by the router to send payment attempts onto
// the network, and receive their results.
type PaymentAttemptDispatcher interface {
        // SendHTLC is a function that directs a link-layer switch to
        // forward a fully encoded payment to the first hop in the route
        // denoted by its public key. A non-nil error is to be returned if the
        // payment was unsuccessful.
        SendHTLC(firstHop lnwire.ShortChannelID,
                attemptID uint64,
                htlcAdd *lnwire.UpdateAddHTLC) error

        // 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.
        GetAttemptResult(attemptID uint64, paymentHash lntypes.Hash,
                deobfuscator htlcswitch.ErrorDecrypter) (
                <-chan *htlcswitch.PaymentResult, error)

        // 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 periodically to let the switch clean up payment results
        // that we have handled.
        // NOTE: New payment attempts MUST NOT be made after the keepPids map
        // has been created and this method has returned.
        CleanStore(keepPids map[uint64]struct{}) error

        // HasAttemptResult reads the network result store to fetch the
        // specified attempt. Returns true if the attempt result exists.
        //
        // NOTE: This method is used and should only be used by the router to
        // resume payments during startup. It can be viewed as a subset of
        // `GetAttemptResult` in terms of its functionality, and can be removed
        // once we move the construction of `UpdateAddHTLC` and
        // `ErrorDecrypter` into `htlcswitch`.
        HasAttemptResult(attemptID uint64) (bool, error)
}

// PaymentSessionSource is an interface that defines a source for the router to
// retrieve new payment sessions.
type PaymentSessionSource interface {
        // NewPaymentSession creates a new payment session that will produce
        // routes to the given target. An optional set of routing hints can be
        // provided in order to populate additional edges to explore when
        // finding a path to the payment's destination.
        NewPaymentSession(p *LightningPayment,
                firstHopBlob fn.Option[tlv.Blob],
                ts fn.Option[htlcswitch.AuxTrafficShaper]) (PaymentSession,
                error)

        // NewPaymentSessionEmpty creates a new paymentSession instance that is
        // empty, and will be exhausted immediately. Used for failure reporting
        // to missioncontrol for resumed payment we don't want to make more
        // attempts for.
        NewPaymentSessionEmpty() PaymentSession
}

// MissionControlQuerier is an interface that exposes failure reporting and
// probability estimation.
type MissionControlQuerier interface {
        // ReportPaymentFail reports a failed payment to mission control as
        // input for future probability estimates. It returns a bool indicating
        // whether this error is a final error and no further payment attempts
        // need to be made.
        ReportPaymentFail(attemptID uint64, rt *route.Route,
                failureSourceIdx *int, failure lnwire.FailureMessage) (
                *channeldb.FailureReason, error)

        // ReportPaymentSuccess reports a successful payment to mission control
        // as input for future probability estimates.
        ReportPaymentSuccess(attemptID uint64, rt *route.Route) error

        // GetProbability is expected to return the success probability of a
        // payment from fromNode along edge.
        GetProbability(fromNode, toNode route.Vertex,
                amt lnwire.MilliSatoshi, capacity btcutil.Amount) float64
}

// FeeSchema is the set fee configuration for a Lightning Node on the network.
// Using the coefficients described within the schema, the required fee to
// forward outgoing payments can be derived.
type FeeSchema struct {
        // BaseFee is the base amount of milli-satoshis that will be chained
        // for ANY payment forwarded.
        BaseFee lnwire.MilliSatoshi

        // FeeRate is the rate that will be charged for forwarding payments.
        // This value should be interpreted as the numerator for a fraction
        // (fixed point arithmetic) whose denominator is 1 million. As a result
        // the effective fee rate charged per mSAT will be: (amount *
        // FeeRate/1,000,000).
        FeeRate uint32

        // InboundFee is the inbound fee schedule that applies to forwards
        // coming in through a channel to which this FeeSchema pertains.
        InboundFee fn.Option[models.InboundFee]
}

// ChannelPolicy holds the parameters that determine the policy we enforce
// when forwarding payments on a channel. These parameters are communicated
// to the rest of the network in ChannelUpdate messages.
type ChannelPolicy struct {
        // FeeSchema holds the fee configuration for a channel.
        FeeSchema

        // TimeLockDelta is the required HTLC timelock delta to be used
        // when forwarding payments.
        TimeLockDelta uint32

        // MaxHTLC is the maximum HTLC size including fees we are allowed to
        // forward over this channel.
        MaxHTLC lnwire.MilliSatoshi

        // MinHTLC is the minimum HTLC size including fees we are allowed to
        // forward over this channel.
        MinHTLC *lnwire.MilliSatoshi
}

// Config defines the configuration for the ChannelRouter. ALL elements within
// the configuration MUST be non-nil for the ChannelRouter to carry out its
// duties.
type Config struct {
        // SelfNode is the public key of the node that this channel router
        // belongs to.
        SelfNode route.Vertex

        // RoutingGraph is a graph source that will be used for pathfinding.
        RoutingGraph Graph

        // Chain is the router's source to the most up-to-date blockchain data.
        // All incoming advertised channels will be checked against the chain
        // to ensure that the channels advertised are still open.
        Chain lnwallet.BlockChainIO

        // Payer is an instance of a PaymentAttemptDispatcher and is used by
        // the router to send payment attempts onto the network, and receive
        // their results.
        Payer PaymentAttemptDispatcher

        // Control keeps track of the status of ongoing payments, ensuring we
        // can properly resume them across restarts.
        Control ControlTower

        // MissionControl is a shared memory of sorts that executions of
        // payment path finding use in order to remember which vertexes/edges
        // were pruned from prior attempts. During SendPayment execution,
        // errors sent by nodes are mapped into a vertex or edge to be pruned.
        // Each run will then take into account this set of pruned
        // vertexes/edges to reduce route failure and pass on graph information
        // gained to the next execution.
        MissionControl MissionControlQuerier

        // SessionSource defines a source for the router to retrieve new payment
        // sessions.
        SessionSource PaymentSessionSource

        // GetLink is a method that allows the router to query the lower link
        // layer to determine the up-to-date available bandwidth at a
        // prospective link to be traversed. If the link isn't available, then
        // a value of zero should be returned. Otherwise, the current up-to-
        // date knowledge of the available bandwidth of the link should be
        // returned.
        GetLink getLinkQuery

        // NextPaymentID is a method that guarantees to return a new, unique ID
        // each time it is called. This is used by the router to generate a
        // unique payment ID for each payment it attempts to send, such that
        // the switch can properly handle the HTLC.
        NextPaymentID func() (uint64, error)

        // PathFindingConfig defines global path finding parameters.
        PathFindingConfig PathFindingConfig

        // Clock is mockable time provider.
        Clock clock.Clock

        // ApplyChannelUpdate can be called to apply a new channel update to the
        // graph that we received from a payment failure.
        ApplyChannelUpdate func(msg *lnwire.ChannelUpdate1) bool

        // ClosedSCIDs is used by the router to fetch closed channels.
        //
        // TODO(yy): remove it once the root cause of stuck payments is found.
        ClosedSCIDs map[lnwire.ShortChannelID]struct{}

        // TrafficShaper is an optional traffic shaper that can be used to
        // control the outgoing channel of a payment.
        TrafficShaper fn.Option[htlcswitch.AuxTrafficShaper]
}

// EdgeLocator is a struct used to identify a specific edge.
type EdgeLocator struct {
        // ChannelID is the channel of this edge.
        ChannelID uint64

        // Direction takes the value of 0 or 1 and is identical in definition to
        // the channel direction flag. A value of 0 means the direction from the
        // lower node pubkey to the higher.
        Direction uint8
}

// String returns a human-readable version of the edgeLocator values.
func (e *EdgeLocator) String() string {
        return fmt.Sprintf("%v:%v", e.ChannelID, e.Direction)
}

// ChannelRouter is the layer 3 router within the Lightning stack. Below the
// ChannelRouter is the HtlcSwitch, and below that is the Bitcoin blockchain
// itself. The primary role of the ChannelRouter is to respond to queries for
// potential routes that can support a payment amount, and also general graph
// reachability questions. The router will prune the channel graph
// automatically as new blocks are discovered which spend certain known funding
// outpoints, thereby closing their respective channels.
type ChannelRouter struct {
        started uint32 // To be used atomically.
        stopped uint32 // To be used atomically.

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

        quit chan struct{}
        wg   sync.WaitGroup
}

// New creates a new instance of the ChannelRouter with the specified
// configuration parameters. As part of initialization, if the router detects
// that the channel graph isn't fully in sync with the latest UTXO (since the
// channel graph is a subset of the UTXO set) set, then the router will proceed
// to fully sync to the latest state of the UTXO set.
func New(cfg Config) (*ChannelRouter, error) {
        return &ChannelRouter{
                cfg:  &cfg,
                quit: make(chan struct{}),
        }, nil
}

// Start launches all the goroutines the ChannelRouter requires to carry out
// its duties. If the router has already been started, then this method is a
// noop.
func (r *ChannelRouter) Start() error {
        if !atomic.CompareAndSwapUint32(&r.started, 0, 1) {
                return nil
        }

        log.Info("Channel Router starting")

        // If any payments are still in flight, we resume, to make sure their
        // results are properly handled.
        if err := r.resumePayments(); err != nil {
                log.Error("Failed to resume payments during startup")
        }

        return nil
}

// Stop signals the ChannelRouter to gracefully halt all routines. This method
// will *block* until all goroutines have excited. If the channel router has
// already stopped then this method will return immediately.
func (r *ChannelRouter) Stop() error {
        if !atomic.CompareAndSwapUint32(&r.stopped, 0, 1) {
                return nil
        }

        log.Info("Channel Router shutting down...")
        defer log.Debug("Channel Router shutdown complete")

        close(r.quit)
        r.wg.Wait()

        return nil
}

// RouteRequest contains the parameters for a pathfinding request. It may
// describe a request to make a regular payment or one to a blinded path
// (incdicated by a non-nil BlindedPayment field).
type RouteRequest struct {
        // Source is the node that the path originates from.
        Source route.Vertex

        // Target is the node that the path terminates at. If the route
        // includes a blinded path, target will be the blinded node id of the
        // final hop in the blinded route.
        Target route.Vertex

        // Amount is the Amount in millisatoshis to be delivered to the target
        // node.
        Amount lnwire.MilliSatoshi

        // TimePreference expresses the caller's time preference for
        // pathfinding.
        TimePreference float64

        // Restrictions provides a set of additional Restrictions that the
        // route must adhere to.
        Restrictions *RestrictParams

        // CustomRecords is a set of custom tlv records to include for the
        // final hop.
        CustomRecords record.CustomSet

        // RouteHints contains an additional set of edges to include in our
        // view of the graph. This may either be a set of hints for private
        // channels or a "virtual" hop hint that represents a blinded route.
        RouteHints RouteHints

        // FinalExpiry is the cltv delta for the final hop. If paying to a
        // blinded path, this value is a duplicate of the delta provided
        // in blinded payment.
        FinalExpiry uint16

        // BlindedPathSet contains a set of optional blinded paths and
        // parameters used to reach a target node blinded paths. This field is
        // mutually exclusive with the Target field.
        BlindedPathSet *BlindedPaymentPathSet
}

// RouteHints is an alias type for a set of route hints, with the source node
// as the map's key and the details of the hint(s) in the edge policy.
type RouteHints map[route.Vertex][]AdditionalEdge

// NewRouteRequest produces a new route request for a regular payment or one
// to a blinded route, validating that the target, routeHints and finalExpiry
// parameters are mutually exclusive with the blindedPayment parameter (which
// contains these values for blinded payments).
func NewRouteRequest(source route.Vertex, target *route.Vertex,
        amount lnwire.MilliSatoshi, timePref float64,
        restrictions *RestrictParams, customRecords record.CustomSet,
        routeHints RouteHints, blindedPathSet *BlindedPaymentPathSet,
        finalExpiry uint16) (*RouteRequest, error) {

        var (
                // Assume that we're starting off with a regular payment.
                requestHints  = routeHints
                requestExpiry = finalExpiry
                err           error
        )

        if blindedPathSet != nil {
                if blindedPathSet.IsIntroNode(source) {
                        return nil, ErrSelfIntro
                }

                // Check that the values for a clear path have not been set,
                // as this is an ambiguous signal from the caller.
                if routeHints != nil {
                        return nil, ErrHintsAndBlinded
                }

                if finalExpiry != 0 {
                        return nil, ErrExpiryAndBlinded
                }

                requestExpiry = blindedPathSet.FinalCLTVDelta()

                requestHints, err = blindedPathSet.ToRouteHints()
                if err != nil {
                        return nil, err
                }
        }

        requestTarget, err := getTargetNode(target, blindedPathSet)
        if err != nil {
                return nil, err
        }

        return &RouteRequest{
                Source:         source,
                Target:         requestTarget,
                Amount:         amount,
                TimePreference: timePref,
                Restrictions:   restrictions,
                CustomRecords:  customRecords,
                RouteHints:     requestHints,
                FinalExpiry:    requestExpiry,
                BlindedPathSet: blindedPathSet,
        }, nil
}

func getTargetNode(target *route.Vertex,
        blindedPathSet *BlindedPaymentPathSet) (route.Vertex, error) {

        var (
                blinded   = blindedPathSet != nil
                targetSet = target != nil
        )

        switch {
        case blinded && targetSet:
                return route.Vertex{}, ErrTargetAndBlinded

        case blinded:
                return route.NewVertex(blindedPathSet.TargetPubKey()), nil

        case targetSet:
                return *target, nil

        default:
                return route.Vertex{}, ErrNoTarget
        }
}

// FindRoute attempts to query the ChannelRouter for the optimum path to a
// particular target destination to which it is able to send `amt` after
// factoring in channel capacities and cumulative fees along the route.
func (r *ChannelRouter) FindRoute(req *RouteRequest) (*route.Route, float64,
        error) {

        log.Debugf("Searching for path to %v, sending %v", req.Target,
                req.Amount)

        // We'll attempt to obtain a set of bandwidth hints that can help us
        // eliminate certain routes early on in the path finding process.
        bandwidthHints, err := newBandwidthManager(
                r.cfg.RoutingGraph, r.cfg.SelfNode, r.cfg.GetLink,
                fn.None[tlv.Blob](), r.cfg.TrafficShaper,
        )
        if err != nil {
                return nil, 0, err
        }

        // We'll fetch the current block height, so we can properly calculate
        // the required HTLC time locks within the route.
        _, currentHeight, err := r.cfg.Chain.GetBestBlock()
        if err != nil {
                return nil, 0, err
        }

        // Now that we know the destination is reachable within the graph, we'll
        // execute our path finding algorithm.
        finalHtlcExpiry := currentHeight + int32(req.FinalExpiry)

        // Validate time preference.
        timePref := req.TimePreference
        if timePref < -1 || timePref > 1 {
                return nil, 0, errors.New("time preference out of range")
        }

        path, probability, err := findPath(
                &graphParams{
                        additionalEdges: req.RouteHints,
                        bandwidthHints:  bandwidthHints,
                        graph:           r.cfg.RoutingGraph,
                },
                req.Restrictions, &r.cfg.PathFindingConfig,
                r.cfg.SelfNode, req.Source, req.Target, req.Amount,
                req.TimePreference, finalHtlcExpiry,
        )
        if err != nil {
                return nil, 0, err
        }

        // Create the route with absolute time lock values.
        route, err := newRoute(
                req.Source, path, uint32(currentHeight),
                finalHopParams{
                        amt:       req.Amount,
                        totalAmt:  req.Amount,
                        cltvDelta: req.FinalExpiry,
                        records:   req.CustomRecords,
                }, req.BlindedPathSet,
        )
        if err != nil {
                return nil, 0, err
        }

        go log.Tracef("Obtained path to send %v to %x: %v",
                req.Amount, req.Target, lnutils.SpewLogClosure(route))

        return route, probability, nil
}

// probabilitySource defines the signature of a function that can be used to
// query the success probability of sending a given amount between the two
// given vertices.
type probabilitySource func(route.Vertex, route.Vertex, lnwire.MilliSatoshi,
        btcutil.Amount) float64

// BlindedPathRestrictions are a set of constraints to adhere to when
// choosing a set of blinded paths to this node.
type BlindedPathRestrictions struct {
        // MinDistanceFromIntroNode is the minimum number of _real_ (non-dummy)
        // hops to include in a blinded path. Since we post-fix dummy hops, this
        // is the minimum distance between our node and the introduction node
        // of the path. This doesn't include our node, so if the minimum is 1,
        // then the path will contain at minimum our node along with an
        // introduction node hop.
        MinDistanceFromIntroNode uint8

        // NumHops is the number of hops that each blinded path should consist
        // of. If paths are found with a number of hops less that NumHops, then
        // dummy hops will be padded on to the route. This value doesn't
        // include our node, so if the maximum is 1, then the path will contain
        // our node along with an introduction node hop.
        NumHops uint8

        // MaxNumPaths is the maximum number of blinded paths to select.
        MaxNumPaths uint8

        // NodeOmissionSet is a set of nodes that should not be used within any
        // of the blinded paths that we generate.
        NodeOmissionSet fn.Set[route.Vertex]
}

// FindBlindedPaths finds a selection of paths to the destination node that can
// be used in blinded payment paths.
func (r *ChannelRouter) FindBlindedPaths(destination route.Vertex,
        amt lnwire.MilliSatoshi, probabilitySrc probabilitySource,
        restrictions *BlindedPathRestrictions) ([]*route.Route, error) {

        // First, find a set of candidate paths given the destination node and
        // path length restrictions.
        paths, err := findBlindedPaths(
                r.cfg.RoutingGraph, destination, &blindedPathRestrictions{
                        minNumHops:      restrictions.MinDistanceFromIntroNode,
                        maxNumHops:      restrictions.NumHops,
                        nodeOmissionSet: restrictions.NodeOmissionSet,
                },
        )
        if err != nil {
                return nil, err
        }

        // routeWithProbability groups a route with the probability of a
        // payment of the given amount succeeding on that path.
        type routeWithProbability struct {
                route       *route.Route
                probability float64
        }

        // Iterate over all the candidate paths and determine the success
        // probability of each path given the data we have about forwards
        // between any two nodes on a path.
        routes := make([]*routeWithProbability, 0, len(paths))
        for _, path := range paths {
                if len(path) < 1 {
                        return nil, fmt.Errorf("a blinded path must have at " +
                                "least one hop")
                }

                var (
                        introNode = path[0].vertex
                        prevNode  = introNode
                        hops      = make(
                                []*route.Hop, 0, len(path)-1,
                        )
                        totalRouteProbability = float64(1)
                )

                // For each set of hops on the path, get the success probability
                // of a forward between those two vertices and use that to
                // update the overall route probability.
                for j := 1; j < len(path); j++ {
                        probability := probabilitySrc(
                                prevNode, path[j].vertex, amt,
                                path[j-1].edgeCapacity,
                        )

                        totalRouteProbability *= probability

                        hops = append(hops, &route.Hop{
                                PubKeyBytes: path[j].vertex,
                                ChannelID:   path[j-1].channelID,
                        })

                        prevNode = path[j].vertex
                }

                // Don't bother adding a route if its success probability less
                // minimum that can be assigned to any single pair.
                if totalRouteProbability <= DefaultMinRouteProbability {
                        continue
                }

                routes = append(routes, &routeWithProbability{
                        route: &route.Route{
                                SourcePubKey: introNode,
                                Hops:         hops,
                        },
                        probability: totalRouteProbability,
                })
        }

        // Sort the routes based on probability.
        sort.Slice(routes, func(i, j int) bool {
                return routes[i].probability > routes[j].probability
        })

        // Now just choose the best paths up until the maximum number of allowed
        // paths.
        bestRoutes := make([]*route.Route, 0, restrictions.MaxNumPaths)
        for _, route := range routes {
                if len(bestRoutes) >= int(restrictions.MaxNumPaths) {
                        break
                }

                bestRoutes = append(bestRoutes, route.route)
        }

        return bestRoutes, nil
}

// generateNewSessionKey generates a new ephemeral private key to be used for a
// payment attempt.
func generateNewSessionKey() (*btcec.PrivateKey, error) {
        // Generate a new random session key to ensure that we don't trigger
        // any replay.
        //
        // TODO(roasbeef): add more sources of randomness?
        return btcec.NewPrivateKey()
}

// LightningPayment describes a payment to be sent through the network to the
// final destination.
type LightningPayment struct {
        // Target is the node in which the payment should be routed towards.
        Target route.Vertex

        // Amount is the value of the payment to send through the network in
        // milli-satoshis.
        Amount lnwire.MilliSatoshi

        // FeeLimit is the maximum fee in millisatoshis that the payment should
        // accept when sending it through the network. The payment will fail
        // if there isn't a route with lower fees than this limit.
        FeeLimit lnwire.MilliSatoshi

        // CltvLimit is the maximum time lock that is allowed for attempts to
        // complete this payment.
        CltvLimit uint32

        // paymentHash is the r-hash value to use within the HTLC extended to
        // the first hop. This won't be set for AMP payments.
        paymentHash *lntypes.Hash

        // amp is an optional field that is set if and only if this is am AMP
        // payment.
        amp *AMPOptions

        // FinalCLTVDelta is the CTLV expiry delta to use for the _final_ hop
        // in the route. This means that the final hop will have a CLTV delta
        // of at least: currentHeight + FinalCLTVDelta.
        FinalCLTVDelta uint16

        // PayAttemptTimeout is a timeout value that we'll use to determine
        // when we should should abandon the payment attempt after consecutive
        // payment failure. This prevents us from attempting to send a payment
        // indefinitely. A zero value means the payment will never time out.
        //
        // TODO(halseth): make wallclock time to allow resume after startup.
        PayAttemptTimeout time.Duration

        // RouteHints represents the different routing hints that can be used to
        // assist a payment in reaching its destination successfully. These
        // hints will act as intermediate hops along the route.
        //
        // NOTE: This is optional unless required by the payment. When providing
        // multiple routes, ensure the hop hints within each route are chained
        // together and sorted in forward order in order to reach the
        // destination successfully. This is mutually exclusive to the
        // BlindedPayment field.
        RouteHints [][]zpay32.HopHint

        // BlindedPathSet holds the information about a set of blinded paths to
        // the payment recipient. This is mutually exclusive to the RouteHints
        // field.
        BlindedPathSet *BlindedPaymentPathSet

        // OutgoingChannelIDs is the list of channels that are allowed for the
        // first hop. If nil, any channel may be used.
        OutgoingChannelIDs []uint64

        // LastHop is the pubkey of the last node before the final destination
        // is reached. If nil, any node may be used.
        LastHop *route.Vertex

        // DestFeatures specifies the set of features we assume the final node
        // has for pathfinding. Typically, these will be taken directly from an
        // invoice, but they can also be manually supplied or assumed by the
        // sender. If a nil feature vector is provided, the router will try to
        // fall back to the graph in order to load a feature vector for a node
        // in the public graph.
        DestFeatures *lnwire.FeatureVector

        // PaymentAddr is the payment address specified by the receiver. This
        // field should be a random 32-byte nonce presented in the receiver's
        // invoice to prevent probing of the destination.
        PaymentAddr fn.Option[[32]byte]

        // PaymentRequest is an optional payment request that this payment is
        // attempting to complete.
        PaymentRequest []byte

        // DestCustomRecords are TLV records that are to be sent to the final
        // hop in the new onion payload format. If the destination does not
        // understand this new onion payload format, then the payment will
        // fail.
        DestCustomRecords record.CustomSet

        // FirstHopCustomRecords are the TLV records that are to be sent to the
        // first hop of this payment. These records will be transmitted via the
        // wire message and therefore do not affect the onion payload size.
        FirstHopCustomRecords lnwire.CustomRecords

        // MaxParts is the maximum number of partial payments that may be used
        // to complete the full amount.
        MaxParts uint32

        // MaxShardAmt is the largest shard that we'll attempt to split using.
        // If this field is set, and we need to split, rather than attempting
        // half of the original payment amount, we'll use this value if half
        // the payment amount is greater than it.
        //
        // NOTE: This field is _optional_.
        MaxShardAmt *lnwire.MilliSatoshi

        // TimePref is the time preference for this payment. Set to -1 to
        // optimize for fees only, to 1 to optimize for reliability only or a
        // value in between for a mix.
        TimePref float64

        // Metadata is additional data that is sent along with the payment to
        // the payee.
        Metadata []byte
}

// AMPOptions houses information that must be known in order to send an AMP
// payment.
type AMPOptions struct {
        SetID     [32]byte
        RootShare [32]byte
}

// SetPaymentHash sets the given hash as the payment's overall hash. This
// should only be used for non-AMP payments.
func (l *LightningPayment) SetPaymentHash(hash lntypes.Hash) error {
        if l.amp != nil {
                return fmt.Errorf("cannot set payment hash for AMP payment")
        }

        l.paymentHash = &hash
        return nil
}

// SetAMP sets the given AMP options for the payment.
func (l *LightningPayment) SetAMP(amp *AMPOptions) error {
        if l.paymentHash != nil {
                return fmt.Errorf("cannot set amp options for payment " +
                        "with payment hash")
        }

        l.amp = amp
        return nil
}

// Identifier returns a 32-byte slice that uniquely identifies this single
// payment. For non-AMP payments this will be the payment hash, for AMP
// payments this will be the used SetID.
func (l *LightningPayment) Identifier() [32]byte {
        if l.amp != nil {
                return l.amp.SetID
        }

        return *l.paymentHash
}

// SendPayment attempts to send a payment as described within the passed
// LightningPayment. This function is blocking and will return either: when the
// payment is successful, or all candidates routes have been attempted and
// resulted in a failed payment. If the payment succeeds, then a non-nil Route
// will be returned which describes the path the successful payment traversed
// within the network to reach the destination. Additionally, the payment
// preimage will also be returned.
func (r *ChannelRouter) SendPayment(payment *LightningPayment) ([32]byte,
        *route.Route, error) {

        paySession, shardTracker, err := r.PreparePayment(payment)
        if err != nil {
                return [32]byte{}, nil, err
        }

        log.Tracef("Dispatching SendPayment for lightning payment: %v",
                spewPayment(payment))

        return r.sendPayment(
                context.Background(), payment.FeeLimit, payment.Identifier(),
                payment.PayAttemptTimeout, paySession, shardTracker,
                payment.FirstHopCustomRecords,
        )
}

// SendPaymentAsync is the non-blocking version of SendPayment. The payment
// result needs to be retrieved via the control tower.
func (r *ChannelRouter) SendPaymentAsync(ctx context.Context,
        payment *LightningPayment, ps PaymentSession, st shards.ShardTracker) {

        // Since this is the first time this payment is being made, we pass nil
        // for the existing attempt.
        r.wg.Add(1)
        go func() {
                defer r.wg.Done()

                log.Tracef("Dispatching SendPayment for lightning payment: %v",
                        spewPayment(payment))

                _, _, err := r.sendPayment(
                        ctx, payment.FeeLimit, payment.Identifier(),
                        payment.PayAttemptTimeout, ps, st,
                        payment.FirstHopCustomRecords,
                )
                if err != nil {
                        log.Errorf("Payment %x failed: %v",
                                payment.Identifier(), err)
                }
        }()
}

// spewPayment returns a log closures that provides a spewed string
// representation of the passed payment.
func spewPayment(payment *LightningPayment) lnutils.LogClosure {
        return lnutils.NewLogClosure(func() string {
                // Make a copy of the payment with a nilled Curve
                // before spewing.
                var routeHints [][]zpay32.HopHint
                for _, routeHint := range payment.RouteHints {
                        var hopHints []zpay32.HopHint
                        for _, hopHint := range routeHint {
                                h := hopHint.Copy()
                                hopHints = append(hopHints, h)
                        }
                        routeHints = append(routeHints, hopHints)
                }
                p := *payment
                p.RouteHints = routeHints
                return spew.Sdump(p)
        })
}

// PreparePayment creates the payment session and registers the payment with the
// control tower.
func (r *ChannelRouter) PreparePayment(payment *LightningPayment) (
        PaymentSession, shards.ShardTracker, error) {

        // Assemble any custom data we want to send to the first hop only.
        var firstHopData fn.Option[tlv.Blob]
        if len(payment.FirstHopCustomRecords) > 0 {
                if err := payment.FirstHopCustomRecords.Validate(); err != nil {
                        return nil, nil, fmt.Errorf("invalid first hop custom "+
                                "records: %w", err)
                }

                firstHopBlob, err := payment.FirstHopCustomRecords.Serialize()
                if err != nil {
                        return nil, nil, fmt.Errorf("unable to serialize "+
                                "first hop custom records: %w", err)
                }

                firstHopData = fn.Some(firstHopBlob)
        }

        // Before starting the HTLC routing attempt, we'll create a fresh
        // payment session which will report our errors back to mission
        // control.
        paySession, err := r.cfg.SessionSource.NewPaymentSession(
                payment, firstHopData, r.cfg.TrafficShaper,
        )
        if err != nil {
                return nil, nil, err
        }

        // Record this payment hash with the ControlTower, ensuring it is not
        // already in-flight.
        //
        // TODO(roasbeef): store records as part of creation info?
        info := &channeldb.PaymentCreationInfo{
                PaymentIdentifier:     payment.Identifier(),
                Value:                 payment.Amount,
                CreationTime:          r.cfg.Clock.Now(),
                PaymentRequest:        payment.PaymentRequest,
                FirstHopCustomRecords: payment.FirstHopCustomRecords,
        }

        // Create a new ShardTracker that we'll use during the life cycle of
        // this payment.
        var shardTracker shards.ShardTracker
        switch {
        // If this is an AMP payment, we'll use the AMP shard tracker.
        case payment.amp != nil:
                addr := payment.PaymentAddr.UnwrapOr([32]byte{})
                shardTracker = amp.NewShardTracker(
                        payment.amp.RootShare, payment.amp.SetID, addr,
                        payment.Amount,
                )

        // Otherwise we'll use the simple tracker that will map each attempt to
        // the same payment hash.
        default:
                shardTracker = shards.NewSimpleShardTracker(
                        payment.Identifier(), nil,
                )
        }

        err = r.cfg.Control.InitPayment(payment.Identifier(), info)
        if err != nil {
                return nil, nil, err
        }

        return paySession, shardTracker, nil
}

// SendToRoute sends a payment using the provided route and fails the payment
// when an error is returned from the attempt.
func (r *ChannelRouter) SendToRoute(htlcHash lntypes.Hash, rt *route.Route,
        firstHopCustomRecords lnwire.CustomRecords) (*channeldb.HTLCAttempt,
        error) {

        return r.sendToRoute(htlcHash, rt, false, firstHopCustomRecords)
}

// SendToRouteSkipTempErr sends a payment using the provided route and fails
// the payment ONLY when a terminal error is returned from the attempt.
func (r *ChannelRouter) SendToRouteSkipTempErr(htlcHash lntypes.Hash,
        rt *route.Route,
        firstHopCustomRecords lnwire.CustomRecords) (*channeldb.HTLCAttempt,
        error) {

        return r.sendToRoute(htlcHash, rt, true, firstHopCustomRecords)
}

// sendToRoute attempts to send a payment with the given hash through the
// provided route. This function is blocking and will return the attempt
// information as it is stored in the database. For a successful htlc, this
// information will contain the preimage. If an error occurs after the attempt
// was initiated, both return values will be non-nil. If skipTempErr is true,
// the payment won't be failed unless a terminal error has occurred.
func (r *ChannelRouter) sendToRoute(htlcHash lntypes.Hash, rt *route.Route,
        skipTempErr bool,
        firstHopCustomRecords lnwire.CustomRecords) (*channeldb.HTLCAttempt,
        error) {

        log.Debugf("SendToRoute for payment %v with skipTempErr=%v",
                htlcHash, skipTempErr)

        // Calculate amount paid to receiver.
        amt := rt.ReceiverAmt()

        // If this is meant as an MP payment shard, we set the amount for the
        // creating info to the total amount of the payment.
        finalHop := rt.Hops[len(rt.Hops)-1]
        mpp := finalHop.MPP
        if mpp != nil {
                amt = mpp.TotalMsat()
        }

        // For non-MPP, there's no such thing as temp error as there's only one
        // HTLC attempt being made. When this HTLC is failed, the payment is
        // failed hence cannot be retried.
        if skipTempErr && mpp == nil {
                return nil, ErrSkipTempErr
        }

        // For non-AMP payments the overall payment identifier will be the same
        // hash as used for this HTLC.
        paymentIdentifier := htlcHash

        // For AMP-payments, we'll use the setID as the unique ID for the
        // overall payment.
        amp := finalHop.AMP
        if amp != nil {
                paymentIdentifier = amp.SetID()
        }

        // Record this payment hash with the ControlTower, ensuring it is not
        // already in-flight.
        info := &channeldb.PaymentCreationInfo{
                PaymentIdentifier:     paymentIdentifier,
                Value:                 amt,
                CreationTime:          r.cfg.Clock.Now(),
                PaymentRequest:        nil,
                FirstHopCustomRecords: firstHopCustomRecords,
        }

        err := r.cfg.Control.InitPayment(paymentIdentifier, info)
        switch {
        // If this is an MPP attempt and the hash is already registered with
        // the database, we can go on to launch the shard.
        case mpp != nil && errors.Is(err, channeldb.ErrPaymentInFlight):
        case mpp != nil && errors.Is(err, channeldb.ErrPaymentExists):

        // Any other error is not tolerated.
        case err != nil:
                return nil, err
        }

        log.Tracef("Dispatching SendToRoute for HTLC hash %v: %v", htlcHash,
                lnutils.SpewLogClosure(rt))

        // Since the HTLC hashes and preimages are specified manually over the
        // RPC for SendToRoute requests, we don't have to worry about creating
        // a ShardTracker that can generate hashes for AMP payments. Instead, we
        // create a simple tracker that can just return the hash for the single
        // shard we'll now launch.
        shardTracker := shards.NewSimpleShardTracker(htlcHash, nil)

        // Create a payment lifecycle using the given route with,
        // - zero fee limit as we are not requesting routes.
        // - nil payment session (since we already have a route).
        // - no payment timeout.
        // - no current block height.
        p := newPaymentLifecycle(
                r, 0, paymentIdentifier, nil, shardTracker, 0,
                firstHopCustomRecords,
        )

        // Allow the traffic shaper to add custom records to the outgoing HTLC
        // and also adjust the amount if needed.
        err = p.amendFirstHopData(rt)
        if err != nil {
                return nil, err
        }

        // We found a route to try, create a new HTLC attempt to try.
        //
        // NOTE: we use zero `remainingAmt` here to simulate the same effect of
        // setting the lastShard to be false, which is used by previous
        // implementation.
        attempt, err := p.registerAttempt(rt, 0)
        if err != nil {
                return nil, err
        }

        // Once the attempt is created, send it to the htlcswitch. Notice that
        // the `err` returned here has already been processed by
        // `handleSwitchErr`, which means if there's a terminal failure, the
        // payment has been failed.
        result, err := p.sendAttempt(attempt)
        if err != nil {
                return nil, err
        }

        // We now look up the payment to see if it's already failed.
        payment, err := p.router.cfg.Control.FetchPayment(p.identifier)
        if err != nil {
                return result.attempt, err
        }

        // Exit if the above error has caused the payment to be failed, we also
        // return the error from sending attempt to mimic the old behavior of
        // this method.
        _, failedReason := payment.TerminalInfo()
        if failedReason != nil {
                return result.attempt, result.err
        }

        // Since for SendToRoute we won't retry in case the shard fails, we'll
        // mark the payment failed with the control tower immediately if the
        // skipTempErr is false.
        reason := channeldb.FailureReasonError

        // If we failed to send the HTLC, we need to further decide if we want
        // to fail the payment.
        if result.err != nil {
                // If skipTempErr, we'll return the attempt and the temp error.
                if skipTempErr {
                        return result.attempt, result.err
                }

                // Otherwise we need to fail the payment.
                err := r.cfg.Control.FailPayment(paymentIdentifier, reason)
                if err != nil {
                        return nil, err
                }

                return result.attempt, result.err
        }

        // The attempt was successfully sent, wait for the result to be
        // available.
        result, err = p.collectAndHandleResult(attempt)
        if err != nil {
                return nil, err
        }

        // We got a successful result.
        if result.err == nil {
                return result.attempt, nil
        }

        // An error returned from collecting the result, we'll mark the payment
        // as failed if we don't skip temp error.
        if !skipTempErr {
                err := r.cfg.Control.FailPayment(paymentIdentifier, reason)
                if err != nil {
                        return nil, err
                }
        }

        return result.attempt, result.err
}

// sendPayment attempts to send a payment to the passed payment hash. This
// function is blocking and will return either: when the payment is successful,
// or all candidates routes have been attempted and resulted in a failed
// payment. If the payment succeeds, then a non-nil Route will be returned
// which describes the path the successful payment traversed within the network
// to reach the destination. Additionally, the payment preimage will also be
// returned.
//
// This method relies on the ControlTower's internal payment state machine to
// carry out its execution. After restarts, it is safe, and assumed, that the
// router will call this method for every payment still in-flight according to
// the ControlTower.
func (r *ChannelRouter) sendPayment(ctx context.Context,
        feeLimit lnwire.MilliSatoshi, identifier lntypes.Hash,
        paymentAttemptTimeout time.Duration, paySession PaymentSession,
        shardTracker shards.ShardTracker,
        firstHopCustomRecords lnwire.CustomRecords) ([32]byte, *route.Route,
        error) {

        // If the user provides a timeout, we will additionally wrap the context
        // in a deadline.
        cancel := func() {}
        if paymentAttemptTimeout > 0 {
                ctx, cancel = context.WithTimeout(ctx, paymentAttemptTimeout)
        }

        // Since resumePayment is a blocking call, we'll cancel this
        // context if the payment completes before the optional
        // deadline.
        defer cancel()

        // We'll also fetch the current block height, so we can properly
        // calculate the required HTLC time locks within the route.
        _, currentHeight, err := r.cfg.Chain.GetBestBlock()
        if err != nil {
                return [32]byte{}, nil, err
        }

        // Validate the custom records before we attempt to send the payment.
        if err := firstHopCustomRecords.Validate(); err != nil {
                return [32]byte{}, nil, err
        }

        // Now set up a paymentLifecycle struct with these params, such that we
        // can resume the payment from the current state.
        p := newPaymentLifecycle(
                r, feeLimit, identifier, paySession, shardTracker,
                currentHeight, firstHopCustomRecords,
        )

        return p.resumePayment(ctx)
}

// extractChannelUpdate examines the error and extracts the channel update.
func (r *ChannelRouter) extractChannelUpdate(
        failure lnwire.FailureMessage) *lnwire.ChannelUpdate1 {

        var update *lnwire.ChannelUpdate1
        switch onionErr := failure.(type) {
        case *lnwire.FailExpiryTooSoon:
                update = &onionErr.Update
        case *lnwire.FailAmountBelowMinimum:
                update = &onionErr.Update
        case *lnwire.FailFeeInsufficient:
                update = &onionErr.Update
        case *lnwire.FailIncorrectCltvExpiry:
                update = &onionErr.Update
        case *lnwire.FailChannelDisabled:
                update = &onionErr.Update
        case *lnwire.FailTemporaryChannelFailure:
                update = onionErr.Update
        }

        return update
}

// ErrNoChannel is returned when a route cannot be built because there are no
// channels that satisfy all requirements.
type ErrNoChannel struct {
        position int
}

// Error returns a human-readable string describing the error.
func (e ErrNoChannel) Error() string {
        return fmt.Sprintf("no matching outgoing channel available for "+
                "node index %v", e.position)
}

// BuildRoute returns a fully specified route based on a list of pubkeys. If
// amount is nil, the minimum routable amount is used. To force a specific
// outgoing channel, use the outgoingChan parameter.
func (r *ChannelRouter) BuildRoute(amt fn.Option[lnwire.MilliSatoshi],
        hops []route.Vertex, outgoingChan *uint64, finalCltvDelta int32,
        payAddr fn.Option[[32]byte], firstHopBlob fn.Option[[]byte]) (
        *route.Route, error) {

        log.Tracef("BuildRoute called: hopsCount=%v, amt=%v", len(hops), amt)

        var outgoingChans map[uint64]struct{}
        if outgoingChan != nil {
                outgoingChans = map[uint64]struct{}{
                        *outgoingChan: {},
                }
        }

        // We'll attempt to obtain a set of bandwidth hints that helps us select
        // the best outgoing channel to use in case no outgoing channel is set.
        bandwidthHints, err := newBandwidthManager(
                r.cfg.RoutingGraph, r.cfg.SelfNode, r.cfg.GetLink, firstHopBlob,
                r.cfg.TrafficShaper,
        )
        if err != nil {
                return nil, err
        }

        sourceNode := r.cfg.SelfNode

        // We check that each node in the route has a connection to others that
        // can forward in principle.
        unifiers, err := getEdgeUnifiers(
                r.cfg.SelfNode, hops, outgoingChans, r.cfg.RoutingGraph,
        )
        if err != nil {
                return nil, err
        }

        var (
                receiverAmt lnwire.MilliSatoshi
                senderAmt   lnwire.MilliSatoshi
                pathEdges   []*unifiedEdge
        )

        // We determine the edges compatible with the requested amount, as well
        // as the amount to send, which can be used to determine the final
        // receiver amount, if a minimal amount was requested.
        pathEdges, senderAmt, err = senderAmtBackwardPass(
                unifiers, amt, bandwidthHints,
        )
        if err != nil {
                return nil, err
        }

        // For the minimal amount search, we need to do a forward pass to find a
        // larger receiver amount due to possible min HTLC bumps, otherwise we
        // just use the requested amount.
        receiverAmt, err = fn.ElimOption(
                amt,
                func() fn.Result[lnwire.MilliSatoshi] {
                        return fn.NewResult(
                                receiverAmtForwardPass(senderAmt, pathEdges),
                        )
                },
                fn.Ok[lnwire.MilliSatoshi],
        ).Unpack()
        if err != nil {
                return nil, err
        }

        // Fetch the current block height outside the routing transaction, to
        // prevent the rpc call blocking the database.
        _, height, err := r.cfg.Chain.GetBestBlock()
        if err != nil {
                return nil, err
        }

        // Build and return the final route.
        return newRoute(
                sourceNode, pathEdges, uint32(height),
                finalHopParams{
                        amt:         receiverAmt,
                        totalAmt:    receiverAmt,
                        cltvDelta:   uint16(finalCltvDelta),
                        records:     nil,
                        paymentAddr: payAddr,
                }, nil,
        )
}

// resumePayments fetches inflight payments and resumes their payment
// lifecycles.
func (r *ChannelRouter) resumePayments() error {
        // Get all payments that are inflight.
        payments, err := r.cfg.Control.FetchInFlightPayments()
        if err != nil {
                return err
        }

        // Before we restart existing payments and start accepting more
        // payments to be made, we clean the network result store of the
        // Switch. We do this here at startup to ensure no more payments can be
        // made concurrently, so we know the toKeep map will be up-to-date
        // until the cleaning has finished.
        toKeep := make(map[uint64]struct{})
        for _, p := range payments {
                for _, a := range p.HTLCs {
                        toKeep[a.AttemptID] = struct{}{}

                        // Try to fail the attempt if the route contains a dead
                        // channel.
                        r.failStaleAttempt(a, p.Info.PaymentIdentifier)
                }
        }

        log.Debugf("Cleaning network result store.")
        if err := r.cfg.Payer.CleanStore(toKeep); err != nil {
                return err
        }

        // launchPayment is a helper closure that handles resuming the payment.
        launchPayment := func(payment *channeldb.MPPayment) {
                defer r.wg.Done()

                // Get the hashes used for the outstanding HTLCs.
                htlcs := make(map[uint64]lntypes.Hash)
                for _, a := range payment.HTLCs {
                        a := a

                        // We check whether the individual attempts have their
                        // HTLC hash set, if not we'll fall back to the overall
                        // payment hash.
                        hash := payment.Info.PaymentIdentifier
                        if a.Hash != nil {
                                hash = *a.Hash
                        }

                        htlcs[a.AttemptID] = hash
                }

                payHash := payment.Info.PaymentIdentifier

                // Since we are not supporting creating more shards after a
                // restart (only receiving the result of the shards already
                // outstanding), we create a simple shard tracker that will map
                // the attempt IDs to hashes used for the HTLCs. This will be
                // enough also for AMP payments, since we only need the hashes
                // for the individual HTLCs to regenerate the circuits, and we
                // don't currently persist the root share necessary to
                // re-derive them.
                shardTracker := shards.NewSimpleShardTracker(payHash, htlcs)

                // We create a dummy, empty payment session such that we won't
                // make another payment attempt when the result for the
                // in-flight attempt is received.
                paySession := r.cfg.SessionSource.NewPaymentSessionEmpty()

                // We pass in a non-timeout context, to indicate we don't need
                // it to timeout. It will stop immediately after the existing
                // attempt has finished anyway. We also set a zero fee limit,
                // as no more routes should be tried.
                noTimeout := time.Duration(0)
                _, _, err := r.sendPayment(
                        context.Background(), 0, payHash, noTimeout, paySession,
                        shardTracker, payment.Info.FirstHopCustomRecords,
                )
                if err != nil {
                        log.Errorf("Resuming payment %v failed: %v", payHash,
                                err)

                        return
                }

                log.Infof("Resumed payment %v completed", payHash)
        }

        for _, payment := range payments {
                log.Infof("Resuming payment %v", payment.Info.PaymentIdentifier)

                r.wg.Add(1)
                go launchPayment(payment)
        }

        return nil
}

// failStaleAttempt will fail an HTLC attempt if it's using an unknown channel
// in its route. It first consults the switch to see if there's already a
// network result stored for this attempt. If not, it will check whether the
// first hop of this attempt is using an active channel known to us. If
// inactive, this attempt will be failed.
//
// NOTE: there's an unknown bug that caused the network result for a particular
// attempt to NOT be saved, resulting a payment being stuck forever. More info:
// - https://github.com/lightningnetwork/lnd/issues/8146
// - https://github.com/lightningnetwork/lnd/pull/8174
func (r *ChannelRouter) failStaleAttempt(a channeldb.HTLCAttempt,
        payHash lntypes.Hash) {

        // We can only fail inflight HTLCs so we skip the settled/failed ones.
        if a.Failure != nil || a.Settle != nil {
                return
        }

        // First, check if we've already had a network result for this attempt.
        // If no result is found, we'll check whether the reference link is
        // still known to us.
        ok, err := r.cfg.Payer.HasAttemptResult(a.AttemptID)
        if err != nil {
                log.Errorf("Failed to fetch network result for attempt=%v",
                        a.AttemptID)
                return
        }

        // There's already a network result, no need to fail it here as the
        // payment lifecycle will take care of it, so we can exit early.
        if ok {
                log.Debugf("Already have network result for attempt=%v",
                        a.AttemptID)
                return
        }

        // We now need to decide whether this attempt should be failed here.
        // For very old payments, it's possible that the network results were
        // never saved, causing the payments to be stuck inflight. We now check
        // whether the first hop is referencing an active channel ID and, if
        // not, we will fail the attempt as it has no way to be retried again.
        var shouldFail bool

        // Validate that the attempt has hop info. If this attempt has no hop
        // info it indicates an error in our db.
        if len(a.Route.Hops) == 0 {
                log.Errorf("Found empty hop for attempt=%v", a.AttemptID)

                shouldFail = true
        } else {
                // Get the short channel ID.
                chanID := a.Route.Hops[0].ChannelID
                scid := lnwire.NewShortChanIDFromInt(chanID)

                // Check whether this link is active. If so, we won't fail the
                // attempt but keep waiting for its result.
                _, err := r.cfg.GetLink(scid)
                if err == nil {
                        return
                }

                // We should get the link not found err. If not, we will log an
                // error and skip failing this attempt since an unknown error
                // occurred.
                if !errors.Is(err, htlcswitch.ErrChannelLinkNotFound) {
                        log.Errorf("Failed to get link for attempt=%v for "+
                                "payment=%v: %v", a.AttemptID, payHash, err)

                        return
                }

                // The channel link is not active, we now check whether this
                // channel is already closed. If so, we fail the HTLC attempt
                // as there's no need to wait for its network result because
                // there's no link. If the channel is still pending, we'll keep
                // waiting for the result as we may get a contract resolution
                // for this HTLC.
                if _, ok := r.cfg.ClosedSCIDs[scid]; ok {
                        shouldFail = true
                }
        }

        // Exit if there's no need to fail.
        if !shouldFail {
                return
        }

        log.Errorf("Failing stale attempt=%v for payment=%v", a.AttemptID,
                payHash)

        // Fail the attempt in db. If there's an error, there's nothing we can
        // do here but logging it.
        failInfo := &channeldb.HTLCFailInfo{
                Reason:   channeldb.HTLCFailUnknown,
                FailTime: r.cfg.Clock.Now(),
        }
        _, err = r.cfg.Control.FailAttempt(payHash, a.AttemptID, failInfo)
        if err != nil {
                log.Errorf("Fail attempt=%v got error: %v", a.AttemptID, err)
        }
}

// getEdgeUnifiers returns a list of edge unifiers for the given route.
func getEdgeUnifiers(source route.Vertex, hops []route.Vertex,
        outgoingChans map[uint64]struct{},
        graph Graph) ([]*edgeUnifier, error) {

        // Allocate a list that will contain the edge unifiers for this route.
        unifiers := make([]*edgeUnifier, len(hops))

        // Traverse hops backwards to accumulate fees in the running amounts.
        for i := len(hops) - 1; i >= 0; i-- {
                toNode := hops[i]

                var fromNode route.Vertex
                if i == 0 {
                        fromNode = source
                } else {
                        fromNode = hops[i-1]
                }

                // Build unified policies for this hop based on the channels
                // known in the graph. Inbound fees are only active if the edge
                // is not the last hop.
                isExitHop := i == len(hops)-1
                u := newNodeEdgeUnifier(
                        source, toNode, !isExitHop, outgoingChans,
                )

                err := u.addGraphPolicies(graph)
                if err != nil {
                        return nil, err
                }

                // Exit if there are no channels.
                edgeUnifier, ok := u.edgeUnifiers[fromNode]
                if !ok {
                        log.Errorf("Cannot find policy for node %v", fromNode)
                        return nil, ErrNoChannel{position: i}
                }

                unifiers[i] = edgeUnifier
        }

        return unifiers, nil
}

// senderAmtBackwardPass returns a list of unified edges for the given route and
// determines the amount that should be sent to fulfill min HTLC requirements.
// The minimal sender amount can be searched for by using amt=None.
func senderAmtBackwardPass(unifiers []*edgeUnifier,
        amt fn.Option[lnwire.MilliSatoshi],
        bandwidthHints bandwidthHints) ([]*unifiedEdge, lnwire.MilliSatoshi,
        error) {

        if len(unifiers) == 0 {
                return nil, 0, fmt.Errorf("no unifiers provided")
        }

        var unifiedEdges = make([]*unifiedEdge, len(unifiers))

        // We traverse the route backwards and handle the last hop separately.
        edgeUnifier := unifiers[len(unifiers)-1]

        // incomingAmt tracks the amount that is forwarded on the edges of a
        // route. The last hop only forwards the amount that the receiver should
        // receive, as there are no fees paid to the last node.
        // For minimum amount routes, aim to deliver at least 1 msat to
        // the destination. There are nodes in the wild that have a
        // min_htlc channel policy of zero, which could lead to a zero
        // amount payment being made.
        incomingAmt := amt.UnwrapOr(1)

        // If using min amt, increase the amount if needed to fulfill min HTLC
        // requirements.
        if amt.IsNone() {
                min := edgeUnifier.minAmt()
                if min > incomingAmt {
                        incomingAmt = min
                }
        }

        // Get an edge for the specific amount that we want to forward.
        edge := edgeUnifier.getEdge(incomingAmt, bandwidthHints, 0)
        if edge == nil {
                log.Errorf("Cannot find policy with amt=%v "+
                        "for hop %v", incomingAmt, len(unifiers)-1)

                return nil, 0, ErrNoChannel{position: len(unifiers) - 1}
        }

        unifiedEdges[len(unifiers)-1] = edge

        // Handle the rest of the route except the last hop.
        for i := len(unifiers) - 2; i >= 0; i-- {
                edgeUnifier = unifiers[i]

                // If using min amt, increase the amount if needed to fulfill
                // min HTLC requirements.
                if amt.IsNone() {
                        min := edgeUnifier.minAmt()
                        if min > incomingAmt {
                                incomingAmt = min
                        }
                }

                // A --current hop-- B --next hop: incomingAmt-- C
                // The outbound fee paid to the current end node B is based on
                // the amount that the next hop forwards and B's policy for that
                // hop.
                outboundFee := unifiedEdges[i+1].policy.ComputeFee(
                        incomingAmt,
                )

                netAmount := incomingAmt + outboundFee

                // We need to select an edge that can forward the requested
                // amount.
                edge = edgeUnifier.getEdge(
                        netAmount, bandwidthHints, outboundFee,
                )
                if edge == nil {
                        return nil, 0, ErrNoChannel{position: i}
                }

                // The fee paid to B depends on the current hop's inbound fee
                // policy and on the outbound fee for the next hop as any
                // inbound fee discount is capped by the outbound fee such that
                // the total fee for B can't become negative.
                inboundFee := calcCappedInboundFee(edge, netAmount, outboundFee)

                fee := lnwire.MilliSatoshi(int64(outboundFee) + inboundFee)

                log.Tracef("Select channel %v at position %v",
                        edge.policy.ChannelID, i)

                // Finally, we update the amount that needs to flow into node B
                // from A, which is the next hop's forwarding amount plus the
                // fee for B: A --current hop: incomingAmt-- B --next hop-- C
                incomingAmt += fee

                unifiedEdges[i] = edge
        }

        return unifiedEdges, incomingAmt, nil
}

// receiverAmtForwardPass returns the amount that a receiver will receive after
// deducting all fees from the sender amount.
func receiverAmtForwardPass(runningAmt lnwire.MilliSatoshi,
        unifiedEdges []*unifiedEdge) (lnwire.MilliSatoshi, error) {

        if len(unifiedEdges) == 0 {
                return 0, fmt.Errorf("no edges to forward through")
        }

        inEdge := unifiedEdges[0]
        if !inEdge.amtInRange(runningAmt) {
                log.Errorf("Amount %v not in range for hop index %v",
                        runningAmt, 0)

                return 0, ErrNoChannel{position: 0}
        }

        // Now that we arrived at the start of the route and found out the route
        // total amount, we make a forward pass. Because the amount may have
        // been increased in the backward pass, fees need to be recalculated and
        // amount ranges re-checked.
        for i := 1; i < len(unifiedEdges); i++ {
                inEdge := unifiedEdges[i-1]
                outEdge := unifiedEdges[i]

                // Decrease the amount to send while going forward.
                runningAmt = outgoingFromIncoming(runningAmt, inEdge, outEdge)

                if !outEdge.amtInRange(runningAmt) {
                        log.Errorf("Amount %v not in range for hop index %v",
                                runningAmt, i)

                        return 0, ErrNoChannel{position: i}
                }
        }

        return runningAmt, nil
}

// incomingFromOutgoing computes the incoming amount based on the outgoing
// amount by adding fees to the outgoing amount, replicating the path finding
// and routing process, see also CheckHtlcForward.
func incomingFromOutgoing(outgoingAmt lnwire.MilliSatoshi,
        incoming, outgoing *unifiedEdge) lnwire.MilliSatoshi {

        outgoingFee := outgoing.policy.ComputeFee(outgoingAmt)

        // Net amount is the amount the inbound fees are calculated with.
        netAmount := outgoingAmt + outgoingFee

        inboundFee := incoming.inboundFees.CalcFee(netAmount)

        // The inbound fee is not allowed to reduce the incoming amount below
        // the outgoing amount.
        if int64(outgoingFee)+inboundFee < 0 {
                return outgoingAmt
        }

        return netAmount + lnwire.MilliSatoshi(inboundFee)
}

// outgoingFromIncoming computes the outgoing amount based on the incoming
// amount by subtracting fees from the incoming amount. Note that this is not
// exactly the inverse of incomingFromOutgoing, because of some rounding.
func outgoingFromIncoming(incomingAmt lnwire.MilliSatoshi,
        incoming, outgoing *unifiedEdge) lnwire.MilliSatoshi {

        // Convert all quantities to big.Int to be able to hande negative
        // values. The formulas to compute the outgoing amount involve terms
        // with PPM*PPM*A, which can easily overflow an int64.
        A := big.NewInt(int64(incomingAmt))
        Ro := big.NewInt(int64(outgoing.policy.FeeProportionalMillionths))
        Bo := big.NewInt(int64(outgoing.policy.FeeBaseMSat))
        Ri := big.NewInt(int64(incoming.inboundFees.Rate))
        Bi := big.NewInt(int64(incoming.inboundFees.Base))
        PPM := big.NewInt(1_000_000)

        // The following discussion was contributed by user feelancer21, see
        //nolint:ll
        // https://github.com/feelancer21/lnd/commit/f6f05fa930985aac0d27c3f6681aada1b599162a.

        // The incoming amount Ai based on the outgoing amount Ao is computed by
        // Ai = max(Ai(Ao), Ao), which caps the incoming amount such that the
        // total node fee (Ai - Ao) is non-negative. This is commonly enforced
        // by routing nodes.

        // The function Ai(Ao) is given by:
        // Ai(Ao) = (Ao + Bo + Ro/PPM) + (Bi + (Ao + Ro/PPM + Bo)*Ri/PPM), where
        // the first term is the net amount (the outgoing amount plus the
        // outbound fee), and the second is the inbound fee computed based on
        // the net amount.

        // Ai(Ao) can potentially become more negative in absolute value than
        // Ao, which is why the above mentioned capping is needed. We can
        // abbreviate Ai(Ao) with Ai(Ao) = m*Ao + n, where m and n are:
        // m := (1 + Ro/PPM) * (1 + Ri/PPM)
        // n := Bi + Bo*(1 + Ri/PPM)

        // If we know that m > 0, this is equivalent of Ri/PPM > -1, because Ri
        // is the only factor that can become negative. A value or Ri/PPM = -1,
        // means that the routing node is willing to give up on 100% of the
        // net amount (based on the fee rate), which is likely to not happen in
        // practice. This condition will be important for a later trick.

        // If we want to compute the incoming amount based on the outgoing
        // amount, which is the reverse problem, we need to solve Ai =
        // max(Ai(Ao), Ao) for Ao(Ai). Given an incoming amount A,
        // we look for an Ao such that A = max(Ai(Ao), Ao).

        // The max function separates this into two cases. The case to take is
        // not clear yet, because we don't know Ao, but later we see a trick
        // how to determine which case is the one to take.

        // first case: Ai(Ao) <= Ao:
        // Therefore, A = max(Ai(Ao), Ao) = Ao, we find Ao = A.
        // This also leads to Ai(A) <= A by substitution into the condition.

        // second case: Ai(Ao) > Ao:
        // Therefore, A = max(Ai(Ao), Ao) = Ai(Ao) = m*Ao + n. Solving for Ao
        // gives Ao = (A - n)/m.
        //
        // We know
        // Ai(Ao) > Ao  <=>  A = Ai(Ao) > Ao = (A - n)/m,
        // so A > (A - n)/m.
        //
        // **Assuming m > 0**, by multiplying with m, we can transform this to
        // A * m + n > A.
        //
        // We know Ai(A) = A*m + n, therefore Ai(A) > A.
        //
        // This means that if we apply the incoming amount calculation to the
        // **incoming** amount, and this condition holds, then we know that we
        // deal with the second case, being able to compute the outgoing amount
        // based off the formula Ao = (A - n)/m, otherwise we will just return
        // the incoming amount.

        // In case the inbound fee rate is less than -1 (-100%), we fail to
        // compute the outbound amount and return the incoming amount. This also
        // protects against zero division later.

        // We compute m in terms of big.Int to be safe from overflows and to be
        // consistent with later calculations.
        // m := (PPM*PPM + Ri*PPM + Ro*PPM + Ro*Ri)/(PPM*PPM)

        // Compute terms in (PPM*PPM + Ri*PPM + Ro*PPM + Ro*Ri).
        m1 := new(big.Int).Mul(PPM, PPM)
        m2 := new(big.Int).Mul(Ri, PPM)
        m3 := new(big.Int).Mul(Ro, PPM)
        m4 := new(big.Int).Mul(Ro, Ri)

        // Add up terms m1..m4.
        m := big.NewInt(0)
        m.Add(m, m1)
        m.Add(m, m2)
        m.Add(m, m3)
        m.Add(m, m4)

        // Since we compare to 0, we can multiply by PPM*PPM to avoid the
        // division.
        if m.Int64() <= 0 {
                return incomingAmt
        }

        // In order to decide if the total fee is negative, we apply the fee
        // to the *incoming* amount as mentioned before.

        // We compute the test amount in terms of big.Int to be safe from
        // overflows and to be consistent later calculations.
        // testAmtF := A*m + n =
        // = A + Bo + Bi + (PPM*(A*Ri + A*Ro + Ro*Ri) + A*Ri*Ro)/(PPM*PPM)

        // Compute terms in (A*Ri + A*Ro + Ro*Ri).
        t1 := new(big.Int).Mul(A, Ri)
        t2 := new(big.Int).Mul(A, Ro)
        t3 := new(big.Int).Mul(Ro, Ri)

        // Sum up terms t1-t3.
        t4 := big.NewInt(0)
        t4.Add(t4, t1)
        t4.Add(t4, t2)
        t4.Add(t4, t3)

        // Compute PPM*(A*Ri + A*Ro + Ro*Ri).
        t6 := new(big.Int).Mul(PPM, t4)

        // Compute A*Ri*Ro.
        t7 := new(big.Int).Mul(A, Ri)
        t7.Mul(t7, Ro)

        // Compute (PPM*(A*Ri + A*Ro + Ro*Ri) + A*Ri*Ro)/(PPM*PPM).
        num := new(big.Int).Add(t6, t7)
        denom := new(big.Int).Mul(PPM, PPM)
        fraction := new(big.Int).Div(num, denom)

        // Sum up all terms.
        testAmt := big.NewInt(0)
        testAmt.Add(testAmt, A)
        testAmt.Add(testAmt, Bo)
        testAmt.Add(testAmt, Bi)
        testAmt.Add(testAmt, fraction)

        // Protect against negative values for the integer cast to Msat.
        if testAmt.Int64() < 0 {
                return incomingAmt
        }

        // If the second case holds, we have to compute the outgoing amount.
        if lnwire.MilliSatoshi(testAmt.Int64()) > incomingAmt {
                // Compute the outgoing amount by integer ceiling division. This
                // precision is needed because PPM*PPM*A and other terms can
                // easily overflow with int64, which happens with about
                // A = 10_000 sat.

                // out := (A - n) / m = numerator / denominator
                // numerator := PPM*(PPM*(A - Bo - Bi) - Bo*Ri)
                // denominator := PPM*(PPM + Ri + Ro) + Ri*Ro

                var numerator big.Int

                // Compute (A - Bo - Bi).
                temp1 := new(big.Int).Sub(A, Bo)
                temp2 := new(big.Int).Sub(temp1, Bi)

                // Compute terms in (PPM*(A - Bo - Bi) - Bo*Ri).
                temp3 := new(big.Int).Mul(PPM, temp2)
                temp4 := new(big.Int).Mul(Bo, Ri)

                // Compute PPM*(PPM*(A - Bo - Bi) - Bo*Ri)
                temp5 := new(big.Int).Sub(temp3, temp4)
                numerator.Mul(PPM, temp5)

                var denominator big.Int

                // Compute (PPM + Ri + Ro).
                temp1 = new(big.Int).Add(PPM, Ri)
                temp2 = new(big.Int).Add(temp1, Ro)

                // Compute PPM*(PPM + Ri + Ro) + Ri*Ro.
                temp3 = new(big.Int).Mul(PPM, temp2)
                temp4 = new(big.Int).Mul(Ri, Ro)
                denominator.Add(temp3, temp4)

                // We overestimate the outgoing amount by taking the ceiling of
                // the division. This means that we may round slightly up by a
                // MilliSatoshi, but this helps to ensure that we don't hit min
                // HTLC constrains in the context of finding the minimum amount
                // of a route.
                // ceil = floor((numerator + denominator - 1) / denominator)
                ceil := new(big.Int).Add(&numerator, &denominator)
                ceil.Sub(ceil, big.NewInt(1))
                ceil.Div(ceil, &denominator)

                return lnwire.MilliSatoshi(ceil.Int64())
        }

        // Otherwise the inbound fee made up for the outbound fee, which is why
        // we just return the incoming amount.
        return incomingAmt
}

lightningnetwork / lnd / 13440912774

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