multi: add TLV awareness to htlcswitch, pass extra EOB to the invoice registry

In this commit, we update the `HopIterator` to gain awareness of the new
TLV hop payload. The default `HopIterator` will now hide the details of
the TLV from the caller, and return the same `ForwardingInfo` struct in
a uniform manner. We also add a new method: `ExtraOnionBlob` to allow
the caller to obtain the raw EOB (the serialized TLV stream) to pass
around.

Within the link, we'll now pass the EOB information into the invoice
registry. This allows the registry to parse out any additional
information from the EOB that it needs to settle the payment, such as a
preimage shard in the AMP case.

htlcswitch/link.go

@@ -2627,8 +2627,9 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			// If we're unable to process the onion blob than we
 			// should send the malformed htlc error to payment
 			// sender.
-			l.sendMalformedHTLCError(pd.HtlcIndex, failureCode,
-				onionBlob[:], pd.SourceRef)
+			l.sendMalformedHTLCError(
+				pd.HtlcIndex, failureCode, onionBlob[:], pd.SourceRef,
+			)
 			needUpdate = true
 
 			log.Errorf("unable to decode onion "+
@@ -2638,11 +2639,29 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 
 		heightNow := l.cfg.Switch.BestHeight()
 
-		fwdInfo := chanIterator.ForwardingInstructions()
+		fwdInfo, err := chanIterator.ForwardingInstructions()
+		if err != nil {
+			// If we're unable to process the onion payload, or we
+			// we received malformed TLV stream, then we should
+			// send an error back to the caller so the HTLC can be
+			// cancelled.
+			l.sendHTLCError(
+				pd.HtlcIndex,
+				lnwire.NewInvalidOnionVersion(onionBlob[:]),
+				obfuscator, pd.SourceRef,
+			)
+			needUpdate = true
+
+			log.Errorf("Unable to decode forwarding "+
+				"instructions: %v", err)
+			continue
+		}
+
 		switch fwdInfo.NextHop {
 		case exitHop:
 			updated, err := l.processExitHop(
 				pd, obfuscator, fwdInfo, heightNow,
+				chanIterator.ExtraOnionBlob(),
 			)
 			if err != nil {
 				l.fail(LinkFailureError{code: ErrInternalError},
@@ -2814,8 +2833,8 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 // processExitHop handles an htlc for which this link is the exit hop. It
 // returns a boolean indicating whether the commitment tx needs an update.
 func (l *channelLink) processExitHop(pd *lnwallet.PaymentDescriptor,
-	obfuscator ErrorEncrypter, fwdInfo ForwardingInfo, heightNow uint32) (
-	bool, error) {
+	obfuscator ErrorEncrypter, fwdInfo ForwardingInfo,
+	heightNow uint32, eob []byte) (bool, error) {
 
 	// If hodl.ExitSettle is requested, we will not validate the final hop's
 	// ADD, nor will we settle the corresponding invoice or respond with the
@@ -2861,7 +2880,7 @@ func (l *channelLink) processExitHop(pd *lnwallet.PaymentDescriptor,
 
 	event, err := l.cfg.Registry.NotifyExitHopHtlc(
 		invoiceHash, pd.Amount, pd.Timeout, int32(heightNow),
-		l.hodlQueue.ChanIn(),
+		l.hodlQueue.ChanIn(), eob,
 	)
 
 	switch err {