lnwire: preserve unknown odd zero-length final hop TLVs

When decoding an onion message payload, the loop that forwards
unrecognized final hop TLVs to higher layers skipped any entry with a
zero-length value. DecodeWithParsedTypesP2P marks a recognized type with
a nil map entry but records the raw bytes for an unknown type, and an
unknown odd TLV with an empty value is valid. Keying the skip off a
length check therefore dropped such a TLV instead of passing it through.

Test the recognized-type skip against a nil entry so a valid unknown odd
zero-length TLV is preserved.

lnwire/onion_msg_payload.go

@@ -179,9 +179,13 @@ func (o *OnionMessagePayload) Decode(r io.Reader) (map[tlv.Type][]byte, error) {
 			continue
 		}
 
-		// Skip any tlvs that have been recognized in our decoding (a
-		// zero entry means that we recognized the entry).
-		if len(tlvBytes) == 0 {
+		// Skip any tlvs that have been recognized in our decoding.
+		// DecodeWithParsedTypesP2P stores a nil entry for known types
+		// that it decoded into a dedicated field above, and the raw
+		// bytes for unknown types. A nil check (rather than a length
+		// check) is required so that a valid unknown odd tlv with a
+		// zero-length value is not mistaken for a recognized type.
+		if tlvBytes == nil {
 			continue
 		}
 
@@ -199,7 +203,7 @@ func (o *OnionMessagePayload) Decode(r io.Reader) (map[tlv.Type][]byte, error) {
 	// If we read out an invoice, invoice error or invoice request tlv
 	// sub-namespace, add it to our set of final payloads. This value won't
 	// have been added in the loop above, because we recognized the TLV so
-	// len(tlvMap[invoiceType].tlvBytes) will be zero (thus, skipped above).
+	// tlvMap[invoiceType].tlvBytes will be nil (thus, skipped above).
 	if _, ok := tlvMap[InvoiceNamespaceType]; ok {
 		o.FinalHopTLVs = append(
 			o.FinalHopTLVs, invoicePayload,

lnwire/onion_msg_payload_test.go

@@ -294,6 +294,28 @@ func TestOnionMessagePayloadRoundTrip(t *testing.T) {
 			decoded.FinalHopTLVs[0].Value,
 		)
 	})
+
+	t.Run("odd unknown zero-length final hop TLV", func(t *testing.T) {
+		t.Parallel()
+
+		// A valid unknown odd tlv with a zero-length value must be
+		// preserved rather than mistaken for a recognized type, which
+		// is why decode keys off a nil entry instead of an empty one.
+		original := &OnionMessagePayload{
+			FinalHopTLVs: []*FinalHopTLV{
+				{
+					TLVType: 65,
+					Value:   []byte{},
+				},
+			},
+		}
+
+		decoded := encodeAndDecode(t, original)
+
+		require.Len(t, decoded.FinalHopTLVs, 1)
+		require.Equal(t, tlv.Type(65), decoded.FinalHopTLVs[0].TLVType)
+		require.Empty(t, decoded.FinalHopTLVs[0].Value)
+	})
 }
 
 // TestFinalHopTLVValidate tests that FinalHopTLV.Validate correctly rejects