invoices: add invoice settlement interceptor service

This commit introduces a new invoice settlement interceptor service
that intercepts invoices during their settlement phase. It forwards
invoices to subscribed clients to determine their settlement outcomes.

This commit also introduces an interface to facilitate integrating the
interceptor with other packages.

invoices/interface.go

@@ -198,3 +198,42 @@ type InvoiceUpdater interface {
 	// Finalize finalizes the update before it is written to the database.
 	Finalize(updateType UpdateType) error
 }
+
+// InterceptClientRequest is the request that is passed to the client via
+// callback during an interceptor session. The request contains the invoice that
+// is being intercepted and supporting information.
+type InterceptClientRequest struct {
+	// ExitHtlcCircuitKey is the circuit key that identifies the HTLC which
+	// is involved in the invoice settlement.
+	ExitHtlcCircuitKey CircuitKey
+
+	// ExitHtlcAmt is the amount of the HTLC which is involved in the
+	// invoice settlement.
+	ExitHtlcAmt lnwire.MilliSatoshi
+
+	// ExitHtlcExpiry is the expiry time of the HTLC which is involved in
+	// the invoice settlement.
+	ExitHtlcExpiry uint32
+
+	// CurrentHeight is the current block height.
+	CurrentHeight uint32
+
+	// Invoice is the invoice that is being intercepted.
+	Invoice Invoice
+}
+
+// InterceptorClientCallback is a function that is called when an invoice is
+// intercepted by the invoice interceptor.
+type InterceptorClientCallback func(InterceptClientRequest) error
+
+// SettlementInterceptorInterface is an interface that allows the caller to
+// intercept and specify invoice settlement outcomes.
+type SettlementInterceptorInterface interface {
+	// SetClientCallback sets the client callback function that is called
+	// when an invoice is intercepted.
+	SetClientCallback(InterceptorClientCallback)
+
+	// Resolve is called by the caller to settle an invoice with the
+	// corresponding resolution.
+	Resolve(lntypes.Hash, bool) error
+}

invoices/settlement_interceptor.go

@@ -0,0 +1,229 @@
+package invoices
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/lightningnetwork/lnd/fn"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/lnutils"
+)
+
+// SafeCallback is a thread safe wrapper around an InterceptorClientCallback.
+type SafeCallback struct {
+	// mu is a mutex that protects the callback field.
+	mu sync.Mutex
+
+	// callback is the client callback function that is called when an
+	// invoice is intercepted. This function gives the client the ability to
+	// determine how the invoice should be settled.
+	callback InterceptorClientCallback
+}
+
+// Set sets the client callback function.
+func (sc *SafeCallback) Set(callback InterceptorClientCallback) {
+	sc.mu.Lock()
+	defer sc.mu.Unlock()
+
+	sc.callback = callback
+}
+
+// Exec calls the client callback function.
+func (sc *SafeCallback) Exec(req InterceptClientRequest) error {
+	sc.mu.Lock()
+	defer sc.mu.Unlock()
+
+	if sc.callback == nil {
+		return fmt.Errorf("client callback not set")
+	}
+
+	return sc.callback(req)
+}
+
+// IsSet returns true if the client callback function is set.
+func (sc *SafeCallback) IsSet() bool {
+	sc.mu.Lock()
+	defer sc.mu.Unlock()
+
+	return sc.callback != nil
+}
+
+// InterceptClientResponse is the response that is sent from the client during
+// an interceptor session. The response contains modifications to the invoice
+// settlement process.
+type InterceptClientResponse struct {
+	// SkipAmountCheck is a flag that indicates whether the amount check
+	// should be skipped during the invoice settlement process.
+	SkipAmountCheck bool
+}
+
+// InterceptSession creates a session that is returned to the caller when an
+// invoice is submitted to this service. This session allows the caller to block
+// until the invoice is processed.
+type InterceptSession struct {
+	InterceptClientRequest
+
+	// ClientResponseChannel is a channel that is populated with the
+	// client's interceptor response during an interceptor session.
+	ClientResponseChannel chan InterceptClientResponse
+
+	// Quit is a channel that is closed when the session is no longer
+	// needed.
+	Quit chan struct{}
+}
+
+// SettlementInterceptor is a service that intercepts invoices during the
+// settlement phase, enabling a subscribed client to determine the settlement
+// outcome.
+type SettlementInterceptor struct {
+	wg sync.WaitGroup
+
+	// callback is a client defined function that is called when an invoice
+	// is intercepted. This function gives the client the ability to
+	// determine the settlement outcome.
+	clientCallback SafeCallback
+
+	// activeSessions is a map of active intercept sessions that are used to
+	// manage the client query/response for a given invoice payment hash.
+	activeSessions lnutils.SyncMap[lntypes.Hash, InterceptSession]
+}
+
+// NewSettlementInterceptor creates a new SettlementInterceptor.
+func NewSettlementInterceptor() *SettlementInterceptor {
+	return &SettlementInterceptor{
+		activeSessions: lnutils.SyncMap[
+			lntypes.Hash, InterceptSession,
+		]{},
+	}
+}
+
+// Intercept generates a new intercept session for the given invoice. The
+// session is returned to the caller so that they can block until the
+// client resolution is received.
+func (s *SettlementInterceptor) Intercept(
+	clientRequest InterceptClientRequest) fn.Option[InterceptSession] {
+
+	// If there is no client callback set we will not handle the invoice
+	// further.
+	if !s.clientCallback.IsSet() {
+		return fn.None[InterceptSession]()
+	}
+
+	// Create and store a new intercept session for the invoice. We will use
+	// the payment hash as the storage/retrieval key for the session.
+	paymentHash := clientRequest.Invoice.Terms.PaymentPreimage.Hash()
+	session := InterceptSession{
+		InterceptClientRequest: clientRequest,
+		ClientResponseChannel:  make(chan InterceptClientResponse, 1),
+		Quit:                   make(chan struct{}, 1),
+	}
+	s.activeSessions.Store(paymentHash, session)
+
+	// The callback function will block at the client's discretion. We will
+	// therefore execute it in a separate goroutine.
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+
+		// By this point, we've already checked that the client callback
+		// is set. However, if the client callback has been set to nil
+		// since that check then Exec will return an error.
+		err := s.clientCallback.Exec(clientRequest)
+		if err != nil {
+			log.Errorf("client callback failed: %v", err)
+		}
+	}()
+
+	// Return the session to the caller so that they can block until the
+	// resolution is received.
+	return fn.Some(session)
+}
+
+// Resolve passes a client specified resolution to the session resolution
+// channel associated with the given invoice payment hash.
+func (s *SettlementInterceptor) Resolve(invoicePaymentHash lntypes.Hash,
+	skipAmountCheck bool) error {
+
+	// Retrieve the intercept session for the invoice payment hash.
+	session, ok := s.activeSessions.LoadAndDelete(
+		invoicePaymentHash,
+	)
+	if !ok {
+		return fmt.Errorf("invoice intercept session not found "+
+			"(payment_hash=%s)", invoicePaymentHash.String())
+	}
+
+	// Send the resolution to the session resolution channel.
+	resolution := InterceptClientResponse{
+		SkipAmountCheck: skipAmountCheck,
+	}
+	sendSuccessful := fn.SendOrQuit(
+		session.ClientResponseChannel, resolution, session.Quit,
+	)
+	if !sendSuccessful {
+		return fmt.Errorf("failed to send resolution to client")
+	}
+
+	return nil
+}
+
+// SetClientCallback sets the client callback function that will be called when
+// an invoice is intercepted.
+func (s *SettlementInterceptor) SetClientCallback(
+	callback InterceptorClientCallback) {
+
+	s.clientCallback.Set(callback)
+}
+
+// QuitSession closes the quit channel for the session associated with the
+// given invoice. This signals to the client that the session has ended.
+func (s *SettlementInterceptor) QuitSession(session InterceptSession) error {
+	// Retrieve the intercept session and delete it from the local cache.
+	paymentHash := session.Invoice.Terms.PaymentPreimage.Hash()
+	session, ok := s.activeSessions.LoadAndDelete(paymentHash)
+	if !ok {
+		// If the session is not found, no further action is necessary.
+		return nil
+	}
+
+	// Send to the quit channel to signal the client that the session has
+	// ended.
+	session.Quit <- struct{}{}
+
+	return nil
+}
+
+// QuitActiveSessions quits all active sessions by sending on each session quit
+// channel.
+func (s *SettlementInterceptor) QuitActiveSessions() error {
+	s.activeSessions.Range(func(_ lntypes.Hash, session InterceptSession) bool { //nolint:lll
+		session.Quit <- struct{}{}
+
+		return true
+	})
+
+	// Empty the intercept sessions map.
+	s.activeSessions = lnutils.SyncMap[lntypes.Hash, InterceptSession]{}
+
+	return nil
+}
+
+// Start starts the service.
+func (s *SettlementInterceptor) Start() error {
+	return nil
+}
+
+// Stop stops the service.
+func (s *SettlementInterceptor) Stop() error {
+	// If the service is stopping, we will quit all active sessions.
+	err := s.QuitActiveSessions()
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Ensure that SettlementInterceptor implements the HtlcResolutionInterceptor
+// interface.
+var _ SettlementInterceptorInterface = (*SettlementInterceptor)(nil)