htlcswitch/iterator: validate presence/omission of payload types

From BOLT 04:

The writer:
 - MUST include amt_to_forward and outgoing_cltv_value for every node.
 - MUST include short_channel_id for every non-final node.
 - MUST NOT include short_channel_id for the final node.

htlcswitch/hop/payload_test.go

@@ -0,0 +1,89 @@
+package hop_test
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/htlcswitch/hop"
+	"github.com/lightningnetwork/lnd/record"
+)
+
+type decodePayloadTest struct {
+	name    string
+	payload []byte
+	expErr  error
+}
+
+var decodePayloadTests = []decodePayloadTest{
+	{
+		name:    "final hop no amount",
+		payload: []byte{0x04, 0x00},
+		expErr: hop.ErrInvalidPayload{
+			Type:     record.AmtOnionType,
+			Omitted:  true,
+			FinalHop: true,
+		},
+	},
+	{
+		name: "intermediate hop no amount",
+		payload: []byte{0x04, 0x00, 0x06, 0x08, 0x01, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+		},
+		expErr: hop.ErrInvalidPayload{
+			Type:     record.AmtOnionType,
+			Omitted:  true,
+			FinalHop: false,
+		},
+	},
+	{
+		name:    "final hop no expiry",
+		payload: []byte{0x02, 0x00},
+		expErr: hop.ErrInvalidPayload{
+			Type:     record.LockTimeOnionType,
+			Omitted:  true,
+			FinalHop: true,
+		},
+	},
+	{
+		name: "intermediate hop no expiry",
+		payload: []byte{0x02, 0x00, 0x06, 0x08, 0x01, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+		},
+		expErr: hop.ErrInvalidPayload{
+			Type:     record.LockTimeOnionType,
+			Omitted:  true,
+			FinalHop: false,
+		},
+	},
+	{
+		name: "final hop next sid present",
+		payload: []byte{0x02, 0x00, 0x04, 0x00, 0x06, 0x08, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		},
+		expErr: hop.ErrInvalidPayload{
+			Type:     record.NextHopOnionType,
+			Omitted:  false,
+			FinalHop: true,
+		},
+	},
+}
+
+// TestDecodeHopPayloadRecordValidation asserts that parsing the payloads in the
+// tests yields the expected errors depending on whether the proper fields were
+// included or omitted.
+func TestDecodeHopPayloadRecordValidation(t *testing.T) {
+	for _, test := range decodePayloadTests {
+		t.Run(test.name, func(t *testing.T) {
+			testDecodeHopPayloadValidation(t, test)
+		})
+	}
+}
+
+func testDecodeHopPayloadValidation(t *testing.T, test decodePayloadTest) {
+	_, err := hop.NewPayloadFromReader(bytes.NewReader(test.payload))
+	if !reflect.DeepEqual(test.expErr, err) {
+		t.Fatalf("expected error mismatch, want: %v, got: %v",
+			test.expErr, err)
+	}
+}