refactor+htlcswitch: method for TLV payload parsing logic

In preparation for calling the TLV payload parsing logic recursively for
when we need to peel dummy hops from an onion, this commit creates a new
extractTLVPayload function. This is a pure refactor.

htlcswitch/hop/iterator.go

@@ -173,53 +173,7 @@ func (r *sphinxHopIterator) HopPayload() (*Payload, RouteRole, error) {
 	// Otherwise, if this is the TLV payload, then we'll make a new stream
 	// to decode only what we need to make routing decisions.
 	case sphinx.PayloadTLV:
-		isFinal := r.processedPacket.Action == sphinx.ExitNode
-		payload, parsed, err := ParseTLVPayload(
-			bytes.NewReader(r.processedPacket.Payload.Payload),
-		)
-		if err != nil {
-			// If we couldn't even parse our payload then we do
-			// a best-effort of determining our role in a blinded
-			// route, accepting that we can't know whether we
-			// were the introduction node (as the payload
-			// is not parseable).
-			routeRole := RouteRoleCleartext
-			if r.blindingKit.UpdateAddBlinding.IsSome() {
-				routeRole = RouteRoleRelaying
-			}
-
-			return nil, routeRole, err
-		}
-
-		// Now that we've parsed our payload we can determine which
-		// role we're playing in the route.
-		_, payloadBlinding := parsed[record.BlindingPointOnionType]
-		routeRole := NewRouteRole(
-			r.blindingKit.UpdateAddBlinding.IsSome(),
-			payloadBlinding,
-		)
-
-		if err := ValidateTLVPayload(
-			parsed, isFinal,
-			r.blindingKit.UpdateAddBlinding.IsSome(),
-		); err != nil {
-			return nil, routeRole, err
-		}
-
-		// If we had an encrypted data payload present, pull out our
-		// forwarding info from the blob.
-		if payload.encryptedData != nil {
-			fwdInfo, err := r.blindingKit.DecryptAndValidateFwdInfo(
-				payload, isFinal, parsed,
-			)
-			if err != nil {
-				return nil, routeRole, err
-			}
-
-			payload.FwdInfo = *fwdInfo
-		}
-
-		return payload, routeRole, nil
+		return extractTLVPayload(r)
 
 	default:
 		return nil, RouteRoleCleartext,
@@ -228,6 +182,73 @@ func (r *sphinxHopIterator) HopPayload() (*Payload, RouteRole, error) {
 	}
 }
 
+// extractTLVPayload parses the hop payload and assumes that it uses the TLV
+// format. It returns the parsed payload along with the RouteRole that this hop
+// plays given the contents of the payload.
+func extractTLVPayload(r *sphinxHopIterator) (*Payload, RouteRole, error) {
+	isFinal := r.processedPacket.Action == sphinx.ExitNode
+
+	// Extract TLVs from the packet constructor (the sender).
+	payload, parsed, err := ParseTLVPayload(
+		bytes.NewReader(r.processedPacket.Payload.Payload),
+	)
+	if err != nil {
+		// If we couldn't even parse our payload then we do a
+		// best-effort of determining our role in a blinded route,
+		// accepting that we can't know whether we were the introduction
+		// node (as the payload is not parseable).
+		routeRole := RouteRoleCleartext
+		if r.blindingKit.UpdateAddBlinding.IsSome() {
+			routeRole = RouteRoleRelaying
+		}
+
+		return nil, routeRole, err
+	}
+
+	// Now that we've parsed our payload we can determine which role we're
+	// playing in the route.
+	_, payloadBlinding := parsed[record.BlindingPointOnionType]
+	routeRole := NewRouteRole(
+		r.blindingKit.UpdateAddBlinding.IsSome(), payloadBlinding,
+	)
+
+	// Validate the presence of the various payload fields we received from
+	// the sender.
+	if err := ValidateTLVPayload(
+		parsed, isFinal, r.blindingKit.UpdateAddBlinding.IsSome(),
+	); err != nil {
+		return nil, routeRole, err
+	}
+
+	// If there is no encrypted data from the receiver then return the
+	// payload as is since the forwarding info would have been received
+	// from the sender.
+	if payload.encryptedData != nil {
+		return payload, routeRole, nil
+	}
+
+	// Validate the presence of various fields in the sender payload given
+	// that we now know that this is a hop with instructions from the
+	// recipient.
+	err = ValidatePayloadWithBlinded(isFinal, parsed)
+	if err != nil {
+		return payload, routeRole, err
+	}
+
+	// If we had an encrypted data payload present, pull out our forwarding
+	// info from the blob.
+	fwdInfo, err := r.blindingKit.DecryptAndValidateFwdInfo(
+		payload, isFinal,
+	)
+	if err != nil {
+		return nil, routeRole, err
+	}
+
+	payload.FwdInfo = *fwdInfo
+
+	return payload, routeRole, nil
+}
+
 // ExtractErrorEncrypter decodes and returns the ErrorEncrypter for this hop,
 // along with a failure code to signal if the decoding was successful. The
 // ErrorEncrypter is used to encrypt errors back to the sender in the event that
@@ -327,8 +348,7 @@ func (b *BlindingKit) getBlindingPoint(payloadBlinding *btcec.PublicKey) (
 // DecryptAndValidateFwdInfo performs all operations required to decrypt and
 // validate a blinded route.
 func (b *BlindingKit) DecryptAndValidateFwdInfo(payload *Payload,
-	isFinalHop bool, payloadParsed map[tlv.Type][]byte) (
-	*ForwardingInfo, error) {
+	isFinalHop bool) (*ForwardingInfo, error) {
 
 	// We expect this function to be called when we have encrypted data
 	// present, and expect validation to already have ensured that a
@@ -354,13 +374,6 @@ func (b *BlindingKit) DecryptAndValidateFwdInfo(payload *Payload,
 			ErrDecodeFailed, err)
 	}
 
-	// Validate the contents of the payload against the values we've
-	// just pulled out of the encrypted data blob.
-	err = ValidatePayloadWithBlinded(isFinalHop, payloadParsed)
-	if err != nil {
-		return nil, err
-	}
-
 	// Validate the data in the blinded route against our incoming htlc's
 	// information.
 	if err := ValidateBlindedRouteData(

htlcswitch/hop/iterator_test.go

@@ -287,7 +287,6 @@ func TestDecryptAndValidateFwdInfo(t *testing.T) {
 					encryptedData: testCase.data,
 					blindingPoint: testCase.payloadBlinding,
 				}, false,
-				make(map[tlv.Type][]byte),
 			)
 			require.ErrorIs(t, err, testCase.expectedErr)
 		})