routing+channeldb: send payment metadata from invoice

record/hop.go

@@ -16,6 +16,10 @@ const (
 	// NextHopOnionType is the type used in the onion to reference the ID
 	// of the next hop.
 	NextHopOnionType tlv.Type = 6
+
+	// MetadataOnionType is the type used in the onion for the payment
+	// metadata.
+	MetadataOnionType tlv.Type = 16
 )
 
 // NewAmtToFwdRecord creates a tlv.Record that encodes the amount_to_forward
@@ -45,3 +49,15 @@ func NewLockTimeRecord(lockTime *uint32) tlv.Record {
 func NewNextHopIDRecord(cid *uint64) tlv.Record {
 	return tlv.MakePrimitiveRecord(NextHopOnionType, cid)
 }
+
+// NewMetadataRecord creates a tlv.Record that encodes the metadata (type 10)
+// for an onion payload.
+func NewMetadataRecord(metadata *[]byte) tlv.Record {
+	return tlv.MakeDynamicRecord(
+		MetadataOnionType, metadata,
+		func() uint64 {
+			return uint64(len(*metadata))
+		},
+		tlv.EVarBytes, tlv.DVarBytes,
+	)
+}