Intercept forward htlc

htlcswitch/interceptable_switch.go

@@ -0,0 +1,170 @@
+package htlcswitch
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/go-errors/errors"
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/htlcswitch/hop"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+var (
+	// ErrFwdNotExists is an error returned when the caller tries to resolve
+	// a forward that doesn't exist anymore.
+	ErrFwdNotExists = errors.New("forward does not exist")
+)
+
+// InterceptableSwitch is an implementation of ForwardingSwitch interface.
+// This implementation is used like a proxy that wraps the switch and
+// intercepts forward requests. A reference to the Switch is held in order
+// to communicate back the interception result where the options are:
+// Resume - forwards the original request to the switch as is.
+// Settle - routes UpdateFulfillHTLC to the originating link.
+// Fail - routes UpdateFailHTLC to the originating link.
+type InterceptableSwitch struct {
+	sync.RWMutex
+
+	// htlcSwitch is the underline switch
+	htlcSwitch *Switch
+
+	// fwdInterceptor is the callback that is called for each forward of
+	// an incoming htlc. It should return true if it is interested in handling
+	// it.
+	fwdInterceptor ForwardInterceptor
+}
+
+// NewInterceptableSwitch returns an instance of InterceptableSwitch.
+func NewInterceptableSwitch(s *Switch) *InterceptableSwitch {
+	return &InterceptableSwitch{htlcSwitch: s}
+}
+
+// SetInterceptor sets the ForwardInterceptor to be used.
+func (s *InterceptableSwitch) SetInterceptor(
+	interceptor ForwardInterceptor) {
+
+	s.Lock()
+	defer s.Unlock()
+	s.fwdInterceptor = interceptor
+}
+
+// ForwardPackets attempts to forward the batch of htlcs through the
+// switch, any failed packets will be returned to the provided
+// ChannelLink. The link's quit signal should be provided to allow
+// cancellation of forwarding during link shutdown.
+func (s *InterceptableSwitch) ForwardPackets(linkQuit chan struct{},
+	packets ...*htlcPacket) error {
+
+	var interceptor ForwardInterceptor
+	s.Lock()
+	interceptor = s.fwdInterceptor
+	s.Unlock()
+
+	// Optimize for the case we don't have an interceptor.
+	if interceptor == nil {
+		return s.htlcSwitch.ForwardPackets(linkQuit, packets...)
+	}
+
+	var notIntercepted []*htlcPacket
+	for _, p := range packets {
+		if !s.interceptForward(p, interceptor, linkQuit) {
+			notIntercepted = append(notIntercepted, p)
+		}
+	}
+	return s.htlcSwitch.ForwardPackets(linkQuit, notIntercepted...)
+}
+
+// interceptForward checks if there is any external interceptor interested in
+// this packet. Currently only htlc type of UpdateAddHTLC that are forwarded
+// are being checked for interception. It can be extended in the future given
+// the right use case.
+func (s *InterceptableSwitch) interceptForward(packet *htlcPacket,
+	interceptor ForwardInterceptor, linkQuit chan struct{}) bool {
+
+	switch htlc := packet.htlc.(type) {
+	case *lnwire.UpdateAddHTLC:
+		// We are not interested in intercepting initated payments.
+		if packet.incomingChanID == hop.Source {
+			return false
+		}
+
+		intercepted := &interceptedForward{
+			linkQuit:   linkQuit,
+			htlc:       htlc,
+			packet:     packet,
+			htlcSwitch: s.htlcSwitch,
+		}
+
+		// If this htlc was intercepted, don't handle the forward.
+		return interceptor(intercepted)
+	default:
+		return false
+	}
+}
+
+// interceptedForward implements the InterceptedForward interface.
+// It is passed from the switch to external interceptors that are interested
+// in holding forwards and resolve them manually.
+type interceptedForward struct {
+	linkQuit   chan struct{}
+	htlc       *lnwire.UpdateAddHTLC
+	packet     *htlcPacket
+	htlcSwitch *Switch
+}
+
+// Packet returns the intercepted htlc packet.
+func (f *interceptedForward) Packet() lnwire.UpdateAddHTLC {
+	return *f.htlc
+}
+
+// CircuitKey returns the circuit key for the intercepted htlc.
+func (f *interceptedForward) CircuitKey() channeldb.CircuitKey {
+	return channeldb.CircuitKey{
+		ChanID: f.packet.incomingChanID,
+		HtlcID: f.packet.incomingHTLCID,
+	}
+}
+
+// Resume resumes the default behavior as if the packet was not intercepted.
+func (f *interceptedForward) Resume() error {
+	return f.htlcSwitch.ForwardPackets(f.linkQuit, f.packet)
+}
+
+// Fail forward a failed packet to the switch.
+func (f *interceptedForward) Fail() error {
+	reason, err := f.packet.obfuscator.EncryptFirstHop(lnwire.NewTemporaryChannelFailure(nil))
+	if err != nil {
+		return fmt.Errorf("failed to encrypt failure reason %v", err)
+	}
+	return f.resolve(&lnwire.UpdateFailHTLC{
+		Reason: reason,
+	})
+}
+
+// Settle forwards a settled packet to the switch.
+func (f *interceptedForward) Settle(preimage lntypes.Preimage) error {
+	if !preimage.Matches(f.htlc.PaymentHash) {
+		return errors.New("preimage does not match hash")
+	}
+	return f.resolve(&lnwire.UpdateFulfillHTLC{
+		PaymentPreimage: preimage,
+	})
+}
+
+// resolve is used for both Settle and Fail and forwards the message to the
+// switch.
+func (f *interceptedForward) resolve(message lnwire.Message) error {
+	pkt := &htlcPacket{
+		incomingChanID: f.packet.incomingChanID,
+		incomingHTLCID: f.packet.incomingHTLCID,
+		outgoingChanID: f.packet.outgoingChanID,
+		outgoingHTLCID: f.packet.outgoingHTLCID,
+		isResolution:   true,
+		circuit:        f.packet.circuit,
+		htlc:           message,
+		obfuscator:     f.packet.obfuscator,
+	}
+	return f.htlcSwitch.mailOrchestrator.Deliver(pkt.incomingChanID, pkt)
+}

htlcswitch/interfaces.go

@@ -185,6 +185,46 @@ type TowerClient interface {
 	BackupState(*lnwire.ChannelID, *lnwallet.BreachRetribution, bool) error
 }
 
+// InterceptableHtlcForwarder is the interface to set the interceptor
+// implementation that intercepts htlc forwards.
+type InterceptableHtlcForwarder interface {
+	// SetInterceptor sets a ForwardInterceptor.
+	SetInterceptor(interceptor ForwardInterceptor)
+}
+
+// ForwardInterceptor is a function that is invoked from the switch for every
+// incoming htlc that is intended to be forwarded. It is passed with the
+// InterceptedForward that contains the information about the packet and a way
+// to resolve it manually later in case it is held.
+// The return value indicates if this handler will take control of this forward
+// and resolve it later or let the switch execute its default behavior.
+type ForwardInterceptor func(InterceptedForward) bool
+
+// InterceptedForward is passed to the ForwardInterceptor for every forwarded
+// htlc. It contains all the information about the packet which accordingly
+// the interceptor decides if to hold or not.
+// In addition this interface allows a later resolution by calling either
+// Resume, Settle or Fail.
+type InterceptedForward interface {
+	// CircuitKey returns the intercepted packet.
+	CircuitKey() channeldb.CircuitKey
+
+	// Packet returns the intercepted packet.
+	Packet() lnwire.UpdateAddHTLC
+
+	// Resume notifies the intention to resume an existing hold forward. This
+	// basically means the caller wants to resume with the default behavior for
+	// this htlc which usually means forward it.
+	Resume() error
+
+	// Settle notifies the intention to settle an existing hold
+	// forward with a given preimage.
+	Settle(lntypes.Preimage) error
+
+	// Fails notifies the intention to fail an existing hold forward
+	Fail() error
+}
+
 // htlcNotifier is an interface which represents the input side of the
 // HtlcNotifier which htlc events are piped through. This interface is intended
 // to allow for mocking of the htlcNotifier in tests, so is unexported because

htlcswitch/link.go

@@ -135,10 +135,10 @@ type ChannelLinkConfig struct {
 	Switch *Switch
 
 	// ForwardPackets attempts to forward the batch of htlcs through the
-	// switch, any failed packets will be returned to the provided
-	// ChannelLink. The link's quit signal should be provided to allow
+	// switch. The function returns and error in case it fails to send one or
+	// more packets. The link's quit signal should be provided to allow
 	// cancellation of forwarding during link shutdown.
-	ForwardPackets func(chan struct{}, ...*htlcPacket) chan error
+	ForwardPackets func(chan struct{}, ...*htlcPacket) error
 
 	// DecodeHopIterators facilitates batched decoding of HTLC Sphinx onion
 	// blobs, which are then used to inform how to forward an HTLC.
@@ -2971,8 +2971,10 @@ func (l *channelLink) forwardBatch(packets ...*htlcPacket) {
 		filteredPkts = append(filteredPkts, pkt)
 	}
 
-	errChan := l.cfg.ForwardPackets(l.quit, filteredPkts...)
-	go handleBatchFwdErrs(errChan, l.log)
+	if err := l.cfg.ForwardPackets(l.quit, filteredPkts...); err != nil {
+		log.Errorf("Unhandled error while reforwarding htlc "+
+			"settle/fail over htlcswitch: %v", err)
+	}
 }
 
 // sendHTLCError functions cancels HTLC and send cancel message back to the

htlcswitch/mailbox.go

@@ -86,7 +86,7 @@ type mailBoxConfig struct {
 	// forwardPackets send a varidic number of htlcPackets to the switch to
 	// be routed. A quit channel should be provided so that the call can
 	// properly exit during shutdown.
-	forwardPackets func(chan struct{}, ...*htlcPacket) chan error
+	forwardPackets func(chan struct{}, ...*htlcPacket) error
 
 	// clock is a time source for the mailbox.
 	clock clock.Clock
@@ -680,8 +680,10 @@ func (m *memoryMailBox) FailAdd(pkt *htlcPacket) {
 		},
 	}
 
-	errChan := m.cfg.forwardPackets(m.quit, failPkt)
-	go handleBatchFwdErrs(errChan, log)
+	if err := m.cfg.forwardPackets(m.quit, failPkt); err != nil {
+		log.Errorf("Unhandled error while reforwarding packets "+
+			"settle/fail over htlcswitch: %v", err)
+	}
 }
 
 // MessageOutBox returns a channel that any new messages ready for delivery
@@ -734,7 +736,7 @@ type mailOrchConfig struct {
 	// forwardPackets send a varidic number of htlcPackets to the switch to
 	// be routed. A quit channel should be provided so that the call can
 	// properly exit during shutdown.
-	forwardPackets func(chan struct{}, ...*htlcPacket) chan error
+	forwardPackets func(chan struct{}, ...*htlcPacket) error
 
 	// fetchUpdate retreives the most recent channel update for the channel
 	// this mailbox belongs to.

htlcswitch/mailbox_test.go

@@ -218,16 +218,13 @@ func newMailboxContext(t *testing.T, startTime time.Time,
 }
 
 func (c *mailboxContext) forward(_ chan struct{},
-	pkts ...*htlcPacket) chan error {
+	pkts ...*htlcPacket) error {
 
 	for _, pkt := range pkts {
 		c.forwards <- pkt
 	}
 
-	errChan := make(chan error)
-	close(errChan)
-
-	return errChan
+	return nil
 }
 
 func (c *mailboxContext) sendAdds(start, num int) []*htlcPacket {
@@ -555,12 +552,8 @@ func TestMailOrchestrator(t *testing.T) {
 			}, nil
 		},
 		forwardPackets: func(_ chan struct{},
-			pkts ...*htlcPacket) chan error {
-			// Close the channel immediately so the goroutine
-			// logging errors can exit.
-			errChan := make(chan error)
-			close(errChan)
-			return errChan
+			pkts ...*htlcPacket) error {
+			return nil
 		},
 		clock:  clock.NewTestClock(time.Now()),
 		expiry: testExpiry,