htlcswitch: fuzz hop.Payload and route.Hop

hop.Payload and route.Hop are analogs, with onion payloads encoded from
route.Hops and decoded to hop.Payloads. For checking equality of
encoding/decoding, we implement a helper function to convert
hop.Payloads into route.Hops.

htlcswitch/hop/fuzz_test.go

@@ -2,9 +2,11 @@ package hop
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 
 	sphinx "github.com/lightningnetwork/lightning-onion"
+	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/stretchr/testify/require"
 )
 
@@ -78,3 +80,47 @@ func FuzzOnionPacket(f *testing.F) {
 		require.Equal(t, pkt1, pkt2)
 	})
 }
+
+func hopFromPayload(p *Payload) (*route.Hop, uint64) {
+	return &route.Hop{
+		AmtToForward:     p.FwdInfo.AmountToForward,
+		OutgoingTimeLock: p.FwdInfo.OutgoingCTLV,
+		MPP:              p.MPP,
+		AMP:              p.AMP,
+		Metadata:         p.metadata,
+		CustomRecords:    p.customRecords,
+	}, p.FwdInfo.NextHop.ToUint64()
+}
+
+func FuzzPayload(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data []byte) {
+		if len(data) > sphinx.MaxPayloadSize {
+			return
+		}
+
+		r := bytes.NewReader(data)
+
+		payload1, err := NewPayloadFromReader(r)
+		if err != nil {
+			return
+		}
+
+		var b bytes.Buffer
+		hop, nextChanID := hopFromPayload(payload1)
+		err = hop.PackHopPayload(&b, nextChanID)
+		if errors.Is(err, route.ErrAMPMissingMPP) {
+			// PackHopPayload refuses to encode an AMP record
+			// without an MPP record. However, NewPayloadFromReader
+			// does allow decoding an AMP record without an MPP
+			// record, since validation is done at a later stage. Do
+			// not report a bug for this case.
+			return
+		}
+		require.NoError(t, err)
+
+		payload2, err := NewPayloadFromReader(&b)
+		require.NoError(t, err)
+
+		require.Equal(t, payload1, payload2)
+	})
+}