19087480965

Committed 05 Nov 2025 12:44AM UTC coverage: 56.868%. First build

Build # 19087480965

Build Type

Pull #10346

github

Committed by

web-flow

Commit Message

Merge 1387ac1e1 into dc5d57f28

Pull Request Pull Request #10346: protofsm: extend EmittedEvents with new Outbox field

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56.13

/protofsm/state_machine.go

package protofsm

import (
        "context"
        "fmt"
        "sync"
        "sync/atomic"
        "time"

        "github.com/btcsuite/btcd/btcec/v2"
        "github.com/btcsuite/btcd/chaincfg/chainhash"
        "github.com/btcsuite/btcd/wire"
        "github.com/btcsuite/btclog/v2"
        "github.com/lightningnetwork/lnd/actor"
        "github.com/lightningnetwork/lnd/chainntnfs"
        "github.com/lightningnetwork/lnd/fn/v2"
        "github.com/lightningnetwork/lnd/lnutils"
        "github.com/lightningnetwork/lnd/lnwire"
        "github.com/lightningnetwork/lnd/msgmux"
)

const (
        // pollInterval is the interval at which we'll poll the SendWhen
        // predicate if specified.
        pollInterval = time.Millisecond * 100
)

var (
        // ErrStateMachineShutdown occurs when trying to feed an event to a
        // StateMachine that has been asked to Stop.
        ErrStateMachineShutdown = fmt.Errorf("StateMachine is shutting down")
)

// EmittedEvent is a special type that can be emitted by a state transition.
// This can container internal events which are to be routed back to the state,
// or external events which are to be sent to the daemon.
type EmittedEvent[Event any] struct {
        // InternalEvent is an optional internal event that is to be routed
        // back to the target state. This enables state to trigger one or many
        // state transitions without a new external event.
        InternalEvent []Event

        // ExternalEvent is an optional external event that is to be sent to
        // the daemon for dispatch. Usually, this is some form of I/O.
        ExternalEvents DaemonEventSet

        // Outbox is an optional set of events that are accumulated during event
        // processing and returned to the caller for processing into the main
        // state machine. This enables nested state machines to emit events that
        // bubble up to their parent.
        Outbox []Event
}

// StateTransition is a state transition type. It denotes the next state to go
// to, and also the set of events to emit.
type StateTransition[Event any, Env Environment] struct {
        // NextState is the next state to transition to.
        NextState State[Event, Env]

        // NewEvents is the set of events to emit.
        NewEvents fn.Option[EmittedEvent[Event]]
}

// Environment is an abstract interface that represents the environment that
// the state machine will execute using. From the PoV of the main state machine
// executor, we just care about being able to clean up any resources that were
// allocated by the environment.
type Environment interface {
        // Name returns the name of the environment. This is used to uniquely
        // identify the environment of related state machines.
        Name() string
}

// State defines an abstract state along, namely its state transition function
// that takes as input an event and an environment, and returns a state
// transition (next state, and set of events to emit). As state can also either
// be terminal, or not, a terminal event causes state execution to halt.
type State[Event any, Env Environment] interface {
        // ProcessEvent takes an event and an environment, and returns a new
        // state transition. This will be iteratively called until either a
        // terminal state is reached, or no further internal events are
        // emitted.
        ProcessEvent(event Event, env Env) (*StateTransition[Event, Env], error)

        // IsTerminal returns true if this state is terminal, and false
        // otherwise.
        IsTerminal() bool

        // String returns a human readable string that represents the state.
        String() string
}

// DaemonAdapters is a set of methods that server as adapters to bridge the
// pure world of the FSM to the real world of the daemon. These will be used to
// do things like broadcast transactions, or send messages to peers.
type DaemonAdapters interface {
        // SendMessages sends the target set of messages to the target peer.
        SendMessages(btcec.PublicKey, []lnwire.Message) error

        // BroadcastTransaction broadcasts a transaction with the target label.
        BroadcastTransaction(*wire.MsgTx, string) error

        // RegisterConfirmationsNtfn registers an intent to be notified once
        // txid reaches numConfs confirmations. We also pass in the pkScript as
        // the default light client instead needs to match on scripts created
        // in the block. If a nil txid is passed in, then not only should we
        // match on the script, but we should also dispatch once the
        // transaction containing the script reaches numConfs confirmations.
        // This can be useful in instances where we only know the script in
        // advance, but not the transaction containing it.
        //
        // TODO(roasbeef): could abstract further?
        RegisterConfirmationsNtfn(txid *chainhash.Hash, pkScript []byte,
                numConfs, heightHint uint32,
                opts ...chainntnfs.NotifierOption) (
                *chainntnfs.ConfirmationEvent, error)

        // RegisterSpendNtfn registers an intent to be notified once the target
        // outpoint is successfully spent within a transaction. The script that
        // the outpoint creates must also be specified. This allows this
        // interface to be implemented by BIP 158-like filtering.
        RegisterSpendNtfn(outpoint *wire.OutPoint, pkScript []byte,
                heightHint uint32) (*chainntnfs.SpendEvent, error)
}

// stateQuery is used by outside callers to query the internal state of the
// state machine.
type stateQuery[Event any, Env Environment] struct {
        // CurrentState is a channel that will be sent the current state of the
        // state machine.
        CurrentState chan State[Event, Env]
}

// syncEventRequest is used to send an event to the state machine synchronously,
// waiting for the event processing to complete and returning the accumulated
// outbox events.
type syncEventRequest[Event any] struct {
        // event is the event to process.
        event Event

        // promise is used to signal completion and return the accumulated
        // outbox events or an error.
        promise actor.Promise[[]Event]
}

// StateMachine represents an abstract FSM that is able to process new incoming
// events and drive a state machine to termination. This implementation uses
// type params to abstract over the types of events and environment. Events
// trigger new state transitions, that use the environment to perform some
// action.
//
// TODO(roasbeef): terminal check, daemon event execution, init?
type StateMachine[Event any, Env Environment] struct {
        cfg StateMachineCfg[Event, Env]

        log btclog.Logger

        // events is the channel that will be used to send new events to the
        // FSM.
        events chan Event

        // syncEvents is the channel that will be used to send synchronous event
        // requests to the FSM, returning the accumulated outbox events.
        syncEvents chan syncEventRequest[Event]

        // newStateEvents is an EventDistributor that will be used to notify
        // any relevant callers of new state transitions that occur.
        newStateEvents *fn.EventDistributor[State[Event, Env]]

        // stateQuery is a channel that will be used by outside callers to
        // query the internal state machine state.
        stateQuery chan stateQuery[Event, Env]

        gm   fn.GoroutineManager
        quit chan struct{}

        // startOnce and stopOnce are used to ensure that the state machine is
        // only started and stopped once.
        startOnce sync.Once
        stopOnce  sync.Once

        // running is a flag that indicates if the state machine is currently
        // running.
        running atomic.Bool
}

// ErrorReporter is an interface that's used to report errors that occur during
// state machine execution.
type ErrorReporter interface {
        // ReportError is a method that's used to report an error that occurred
        // during state machine execution.
        ReportError(err error)
}

// StateMachineCfg is a configuration struct that's used to create a new state
// machine.
type StateMachineCfg[Event any, Env Environment] struct {
        // ErrorReporter is used to report errors that occur during state
        // transitions.
        ErrorReporter ErrorReporter

        // Daemon is a set of adapters that will be used to bridge the FSM to
        // the daemon.
        Daemon DaemonAdapters

        // InitialState is the initial state of the state machine.
        InitialState State[Event, Env]

        // Env is the environment that the state machine will use to execute.
        Env Env

        // InitEvent is an optional event that will be sent to the state
        // machine as if it was emitted at the onset of the state machine. This
        // can be used to set up tracking state such as a txid confirmation
        // event.
        InitEvent fn.Option[DaemonEvent]

        // MsgMapper is an optional message mapper that can be used to map
        // normal wire messages into FSM events.
        MsgMapper fn.Option[MsgMapper[Event]]

        // CustomPollInterval is an optional custom poll interval that can be
        // used to set a quicker interval for tests.
        CustomPollInterval fn.Option[time.Duration]
}

// NewStateMachine creates a new state machine given a set of daemon adapters,
// an initial state, an environment, and an event to process as if emitted at
// the onset of the state machine. Such an event can be used to set up tracking
// state such as a txid confirmation event.
func NewStateMachine[Event any, Env Environment](
        cfg StateMachineCfg[Event, Env]) StateMachine[Event, Env] {

        return StateMachine[Event, Env]{
                cfg: cfg,
                log: log.WithPrefix(
                        fmt.Sprintf("FSM(%v):", cfg.Env.Name()),
                ),
                events:         make(chan Event, 1),
                syncEvents:     make(chan syncEventRequest[Event], 1),
                stateQuery:     make(chan stateQuery[Event, Env]),
                gm:             *fn.NewGoroutineManager(),
                newStateEvents: fn.NewEventDistributor[State[Event, Env]](),
                quit:           make(chan struct{}),
        }
}

// Start starts the state machine. This will spawn a goroutine that will drive
// the state machine to completion.
func (s *StateMachine[Event, Env]) Start(ctx context.Context) {
        s.startOnce.Do(func() {
                _ = s.gm.Go(ctx, func(ctx context.Context) {
                        s.driveMachine(ctx)
                })

                s.running.Store(true)
        })
}

// Stop stops the state machine. This will block until the state machine has
// reached a stopping point.
func (s *StateMachine[Event, Env]) Stop() {
        s.stopOnce.Do(func() {
                close(s.quit)
                s.gm.Stop()

                s.running.Store(false)
        })
}

// SendEvent sends a new event to the state machine.
//
// TODO(roasbeef): bool if processed?
func (s *StateMachine[Event, Env]) SendEvent(ctx context.Context, event Event) {
        s.log.Debugf("Sending event %T", event)

        select {
        case s.events <- event:
        case <-ctx.Done():
                return
        case <-s.quit:
                return
        }
}

// AskEvent sends a new event to the state machine using the Ask pattern
// (request-response), waiting for the event to be fully processed. It
// returns a Future that will be resolved with the accumulated outbox events
// from all state transitions triggered by this event, including nested
// internal events. The Future's Await method will return fn.Result[[]Event]
// containing either the accumulated outbox events or an error if processing
// failed.
func (s *StateMachine[Event, Env]) AskEvent(ctx context.Context,
        event Event) actor.Future[[]Event] {

        s.log.Debugf("Asking event %T", event)

        // Create a promise to signal completion and return results.
        promise := actor.NewPromise[[]Event]()

        req := syncEventRequest[Event]{
                event:   event,
                promise: promise,
        }

        // Check for context cancellation or shutdown first to avoid races.
        select {
        case <-ctx.Done():
                promise.Complete(
                        fn.Errf[[]Event]("context cancelled: %w",
                                ctx.Err()),
                )

                return promise.Future()

        case <-s.quit:
                promise.Complete(fn.Err[[]Event](ErrStateMachineShutdown))

                return promise.Future()

        default:
        }

        // Send the request to the state machine. If we can't send it due to
        // context cancellation or shutdown, complete the promise with an error.
        select {
        // Successfully sent, the promise will be completed by driveMachine.
        case s.syncEvents <- req:

        case <-ctx.Done():
                promise.Complete(
                        fn.Errf[[]Event]("context cancelled: %w",
                                ctx.Err()),
                )

        case <-s.quit:
                promise.Complete(fn.Err[[]Event](ErrStateMachineShutdown))
        }

        return promise.Future()
}

// CanHandle returns true if the target message can be routed to the state
// machine.
func (s *StateMachine[Event, Env]) CanHandle(msg msgmux.PeerMsg) bool {
        cfgMapper := s.cfg.MsgMapper
        return fn.MapOptionZ(cfgMapper, func(mapper MsgMapper[Event]) bool {
                return mapper.MapMsg(msg).IsSome()
        })
}

// Name returns the name of the state machine's environment.
func (s *StateMachine[Event, Env]) Name() string {
        return s.cfg.Env.Name()
}

// SendMessage attempts to send a wire message to the state machine. If the
// message can be mapped using the default message mapper, then true is
// returned indicating that the message was processed. Otherwise, false is
// returned.
func (s *StateMachine[Event, Env]) SendMessage(ctx context.Context,
        msg msgmux.PeerMsg) bool {

        // If we have no message mapper, then return false as we can't process
        // this message.
        if !s.cfg.MsgMapper.IsSome() {
                return false
        }

        s.log.DebugS(ctx, "Sending msg", "msg", lnutils.SpewLogClosure(msg))

        // Otherwise, try to map the message using the default message mapper.
        // If we can't extract an event, then we'll return false to indicate
        // that the message wasn't processed.
        var processed bool
        s.cfg.MsgMapper.WhenSome(func(mapper MsgMapper[Event]) {
                event := mapper.MapMsg(msg)

                event.WhenSome(func(event Event) {
                        s.SendEvent(ctx, event)

                        processed = true
                })
        })

        return processed
}

// CurrentState returns the current state of the state machine.
func (s *StateMachine[Event, Env]) CurrentState() (State[Event, Env], error) {
        query := stateQuery[Event, Env]{
                CurrentState: make(chan State[Event, Env], 1),
        }

        if !fn.SendOrQuit(s.stateQuery, query, s.quit) {
                return nil, ErrStateMachineShutdown
        }

        return fn.RecvOrTimeout(query.CurrentState, time.Second)
}

// StateSubscriber represents an active subscription to be notified of new
// state transitions.
type StateSubscriber[E any, F Environment] *fn.EventReceiver[State[E, F]]

// RegisterStateEvents registers a new event listener that will be notified of
// new state transitions.
func (s *StateMachine[Event, Env]) RegisterStateEvents() StateSubscriber[
        Event, Env] {

        subscriber := fn.NewEventReceiver[State[Event, Env]](10)

        // TODO(roasbeef): instead give the state and the input event?

        s.newStateEvents.RegisterSubscriber(subscriber)

        return subscriber
}

// RemoveStateSub removes the target state subscriber from the set of active
// subscribers.
func (s *StateMachine[Event, Env]) RemoveStateSub(sub StateSubscriber[
        Event, Env]) {

        _ = s.newStateEvents.RemoveSubscriber(sub)
}

// IsRunning returns true if the state machine is currently running.
func (s *StateMachine[Event, Env]) IsRunning() bool {
        return s.running.Load()
}

// executeDaemonEvent executes a daemon event, which is a special type of event
// that can be emitted as part of the state transition function of the state
// machine. An error is returned if the type of event is unknown.
func (s *StateMachine[Event, Env]) executeDaemonEvent(ctx context.Context,
        event DaemonEvent) error {

        switch daemonEvent := event.(type) {
        // This is a send message event, so we'll send the event, and also mind
        // any preconditions as well as post-send events.
        case *SendMsgEvent[Event]:
                sendAndCleanUp := func() error {
                        s.log.DebugS(ctx, "Sending message:",
                                btclog.Hex6("target", daemonEvent.TargetPeer.SerializeCompressed()),
                                "messages", lnutils.SpewLogClosure(daemonEvent.Msgs))

                        err := s.cfg.Daemon.SendMessages(
                                daemonEvent.TargetPeer, daemonEvent.Msgs,
                        )
                        if err != nil {
                                return fmt.Errorf("unable to send msgs: %w",
                                        err)
                        }

                        // If a post-send event was specified, then we'll funnel
                        // that back into the main state machine now as well.
                        //nolint:ll
                        return fn.MapOptionZ(daemonEvent.PostSendEvent, func(event Event) error {
                                launched := s.gm.Go(
                                        ctx, func(ctx context.Context) {
                                                s.log.DebugS(ctx, "Sending post-send event",
                                                        "event", lnutils.SpewLogClosure(event))

                                                s.SendEvent(ctx, event)
                                        },
                                )

                                if !launched {
                                        return ErrStateMachineShutdown
                                }

                                return nil
                        })
                }

                canSend := func() bool {
                        return fn.MapOptionZ(
                                daemonEvent.SendWhen,
                                func(pred SendPredicate) bool {
                                        return pred()
                                },
                        )
                }

                // If this doesn't have a SendWhen predicate, or if it's already
                // true, then we can just send it off right away.
                if !daemonEvent.SendWhen.IsSome() || canSend() {
                        return sendAndCleanUp()
                }

                // Otherwise, this has a SendWhen predicate, so we'll need
                // launch a goroutine to poll the SendWhen, then send only once
                // the predicate is true.
                launched := s.gm.Go(ctx, func(ctx context.Context) {
                        predicateTicker := time.NewTicker(
                                s.cfg.CustomPollInterval.UnwrapOr(pollInterval),
                        )
                        defer predicateTicker.Stop()

                        s.log.InfoS(ctx, "Waiting for send predicate to be true")

                        for {
                                select {
                                case <-predicateTicker.C:
                                        if canSend() {
                                                s.log.InfoS(ctx, "Send active predicate")

                                                err := sendAndCleanUp()
                                                if err != nil {
                                                        s.log.ErrorS(ctx, "Unable to send message", err)
                                                }

                                                return
                                        }

                                case <-ctx.Done():
                                        return
                                }
                        }
                })

                if !launched {
                        return ErrStateMachineShutdown
                }

                return nil

        // If this is a broadcast transaction event, then we'll broadcast with
        // the label attached.
        case *BroadcastTxn:
                s.log.DebugS(ctx, "Broadcasting txn",
                        "txid", daemonEvent.Tx.TxHash())

                err := s.cfg.Daemon.BroadcastTransaction(
                        daemonEvent.Tx, daemonEvent.Label,
                )
                if err != nil {
                        log.Errorf("unable to broadcast txn: %v", err)
                }

                return nil

        // The state machine has requested a new event to be sent once a
        // transaction spending a specified outpoint has confirmed.
        case *RegisterSpend[Event]:
                s.log.DebugS(ctx, "Registering spend",
                        "outpoint", daemonEvent.OutPoint)

                spendEvent, err := s.cfg.Daemon.RegisterSpendNtfn(
                        &daemonEvent.OutPoint, daemonEvent.PkScript,
                        daemonEvent.HeightHint,
                )
                if err != nil {
                        return fmt.Errorf("unable to register spend: %w", err)
                }

                launched := s.gm.Go(ctx, func(ctx context.Context) {
                        for {
                                select {
                                case spend, ok := <-spendEvent.Spend:
                                        if !ok {
                                                return
                                        }

                                        // If there's a post-send event, then
                                        // we'll send that into the current
                                        // state now.
                                        postSpend := daemonEvent.PostSpendEvent
                                        postSpend.WhenSome(func(f SpendMapper[Event]) { //nolint:ll
                                                customEvent := f(spend)
                                                s.SendEvent(ctx, customEvent)
                                        })

                                        return

                                case <-ctx.Done():
                                        return
                                }
                        }
                })

                if !launched {
                        return ErrStateMachineShutdown
                }

                return nil

        // The state machine has requested a new event to be sent once a
        // specified txid+pkScript pair has confirmed.
        case *RegisterConf[Event]:
                s.log.DebugS(ctx, "Registering conf",
                        "txid", daemonEvent.Txid)

                var opts []chainntnfs.NotifierOption
                if daemonEvent.FullBlock {
                        opts = append(opts, chainntnfs.WithIncludeBlock())
                }

                numConfs := daemonEvent.NumConfs.UnwrapOr(1)
                confEvent, err := s.cfg.Daemon.RegisterConfirmationsNtfn(
                        &daemonEvent.Txid, daemonEvent.PkScript,
                        numConfs, daemonEvent.HeightHint, opts...,
                )
                if err != nil {
                        return fmt.Errorf("unable to register conf: %w", err)
                }

                launched := s.gm.Go(ctx, func(ctx context.Context) {
                        for {
                                select {
                                //nolint:ll
                                case conf, ok := <-confEvent.Confirmed:
                                        if !ok {
                                                return
                                        }

                                        // If there's a post-conf mapper, then
                                        // we'll send that into the current
                                        // state now.
                                        postConfMapper := daemonEvent.PostConfMapper
                                        postConfMapper.WhenSome(func(f ConfMapper[Event]) {
                                                customEvent := f(conf)
                                                s.SendEvent(ctx, customEvent)
                                        })

                                        return

                                case <-ctx.Done():
                                        return
                                }
                        }
                })

                if !launched {
                        return ErrStateMachineShutdown
                }

                return nil
        }

        return fmt.Errorf("unknown daemon event: %T", event)
}

// applyEvents applies a new event to the state machine. This will continue
// until no further events are emitted by the state machine. Along the way,
// we'll also ensure to execute any daemon events that are emitted. The
// function returns the final state, any accumulated outbox events, and an
// error if one occurred.
func (s *StateMachine[Event, Env]) applyEvents(ctx context.Context,
        currentState State[Event, Env], newEvent Event) (State[Event, Env],
        []Event, error) {

        eventQueue := fn.NewQueue(newEvent)

        // outbox accumulates all outbox events from state transitions during
        // the entire event processing chain.
        var outbox []Event

        // Given the next event to handle, we'll process the event, then add
        // any new emitted internal events to our event queue. This continues
        // until we reach a terminal state, or we run out of internal events to
        // process.
        //
        //nolint:ll
        for nextEvent := eventQueue.Dequeue(); nextEvent.IsSome(); nextEvent = eventQueue.Dequeue() {
                err := fn.MapOptionZ(nextEvent, func(event Event) error {
                        s.log.DebugS(ctx, "Processing event",
                                "event", lnutils.SpewLogClosure(event))

                        // Apply the state transition function of the current
                        // state given this new event and our existing env.
                        transition, err := currentState.ProcessEvent(
                                event, s.cfg.Env,
                        )
                        if err != nil {
                                return err
                        }

                        newEvents := transition.NewEvents
                        err = fn.MapOptionZ(newEvents, func(events EmittedEvent[Event]) error {
                                // With the event processed, we'll process any
                                // new daemon events that were emitted as part
                                // of this new state transition.
                                for _, dEvent := range events.ExternalEvents {
                                        err := s.executeDaemonEvent(
                                                ctx, dEvent,
                                        )
                                        if err != nil {
                                                return err
                                        }
                                }

                                // Next, we'll add any new emitted events to our
                                // event queue.
                                for _, inEvent := range events.InternalEvent {
                                        s.log.DebugS(ctx, "Adding new internal event to queue",
                                                "event", lnutils.SpewLogClosure(inEvent))

                                        eventQueue.Enqueue(inEvent)
                                }

                                // Accumulate any outbox events from this state
                                // transition.
                                outbox = append(outbox, events.Outbox...)

                                return nil
                        })
                        if err != nil {
                                return err
                        }

                        s.log.InfoS(ctx, "State transition",
                                btclog.Fmt("from_state", "%v", currentState),
                                btclog.Fmt("to_state", "%v", transition.NextState))

                        // With our events processed, we'll now update our
                        // internal state.
                        currentState = transition.NextState

                        // Notify our subscribers of the new state transition.
                        //
                        // TODO(roasbeef): will only give us the outer state?
                        //  * let FSMs choose which state to emit?
                        s.newStateEvents.NotifySubscribers(currentState)

                        return nil
                })
                if err != nil {
                        return currentState, nil, err
                }
        }

        return currentState, outbox, nil
}

// driveMachine is the main event loop of the state machine. It accepts any new
// incoming events, and then drives the state machine forward until it reaches
// a terminal state.
func (s *StateMachine[Event, Env]) driveMachine(ctx context.Context) {
        s.log.DebugS(ctx, "Starting state machine")

        currentState := s.cfg.InitialState

        // Before we start, if we have an init daemon event specified, then
        // we'll handle that now.
        err := fn.MapOptionZ(s.cfg.InitEvent, func(event DaemonEvent) error {
                return s.executeDaemonEvent(ctx, event)
        })
        if err != nil {
                s.log.ErrorS(ctx, "Unable to execute init event", err)
                return
        }

        // We just started driving the state machine, so we'll notify our
        // subscribers of this starting state.
        s.newStateEvents.NotifySubscribers(currentState)

        for {
                select {
                // We have a new external event, so we'll drive the state
                // machine forward until we either run out of internal events,
                // or we reach a terminal state.
                case newEvent := <-s.events:
                        newState, _, err := s.applyEvents(
                                ctx, currentState, newEvent,
                        )
                        if err != nil {
                                s.cfg.ErrorReporter.ReportError(err)

                                s.log.ErrorS(ctx, "Unable to apply event", err)

                                // An error occurred, so we'll tear down the
                                // entire state machine as we can't proceed.
                                go s.Stop()

                                return
                        }

                        currentState = newState

                // We have a synchronous event request that expects the
                // accumulated outbox events to be returned via the promise.
                case syncReq := <-s.syncEvents:
                        newState, outbox, err := s.applyEvents(
                                ctx, currentState, syncReq.event,
                        )
                        if err != nil {
                                s.cfg.ErrorReporter.ReportError(err)

                                s.log.ErrorS(ctx, "Unable to apply sync event",
                                        err)

                                // Complete the promise with the error.
                                //
                                // TODO(roasbeef): distinguish between error
                                // types? state vs processing
                                syncReq.promise.Complete(fn.Err[[]Event](err))

                                // An error occurred, so we'll tear down the
                                // entire state machine as we can't proceed.
                                go s.Stop()

                                return
                        }

                        currentState = newState

                        // Complete the promise with the accumulated outbox
                        // events.
                        syncReq.promise.Complete(fn.Ok(outbox))

                // An outside caller is querying our state, so we'll return the
                // latest state.
                case stateQuery := <-s.stateQuery:
                        if !fn.SendOrQuit(
                                stateQuery.CurrentState, currentState, s.quit,
                        ) {

                                return
                        }

                case <-s.gm.Done():
                        return
                }
        }
}

lightningnetwork / lnd / 19087480965

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