lnrpc/routerrpc/router_backend: populate and unmarshal mpp fields

This commit parses mpp_total_amt_msat and mpp_payment_addr from the
SendToRoute rpc and populates an MPP record on the internal hop
reprsentation. When the router goes to encode the onion packet, these
fields will be serialized for the destination. We also populate the mpp
fields when marshalling routes in rpc responses.

lnrpc/routerrpc/router_backend_test.go

@@ -180,3 +180,123 @@ func (m *mockMissionControl) GetPairHistorySnapshot(fromNode,
 
 	return routing.TimedPairResult{}
 }
+
+type mppOutcome byte
+
+const (
+	valid mppOutcome = iota
+	invalid
+	nompp
+)
+
+type unmarshalMPPTest struct {
+	name    string
+	mpp     *lnrpc.MPPRecord
+	outcome mppOutcome
+}
+
+// TestUnmarshalMPP checks both positive and negative cases of UnmarshalMPP to
+// assert that an MPP record is only returned when both fields are properly
+// specified. It also asserts that zero-values for both inputs is also valid,
+// but returns a nil record.
+func TestUnmarshalMPP(t *testing.T) {
+	tests := []unmarshalMPPTest{
+		{
+			name:    "nil record",
+			mpp:     nil,
+			outcome: nompp,
+		},
+		{
+			name: "invalid total or addr",
+			mpp: &lnrpc.MPPRecord{
+				PaymentAddr:  nil,
+				TotalAmtMsat: 0,
+			},
+			outcome: invalid,
+		},
+		{
+			name: "valid total only",
+			mpp: &lnrpc.MPPRecord{
+				PaymentAddr:  nil,
+				TotalAmtMsat: 8,
+			},
+			outcome: invalid,
+		},
+		{
+			name: "valid addr only",
+			mpp: &lnrpc.MPPRecord{
+				PaymentAddr:  bytes.Repeat([]byte{0x02}, 32),
+				TotalAmtMsat: 0,
+			},
+			outcome: invalid,
+		},
+		{
+			name: "valid total and invalid addr",
+			mpp: &lnrpc.MPPRecord{
+				PaymentAddr:  []byte{0x02},
+				TotalAmtMsat: 8,
+			},
+			outcome: invalid,
+		},
+		{
+			name: "valid total and valid addr",
+			mpp: &lnrpc.MPPRecord{
+				PaymentAddr:  bytes.Repeat([]byte{0x02}, 32),
+				TotalAmtMsat: 8,
+			},
+			outcome: valid,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			testUnmarshalMPP(t, test)
+		})
+	}
+}
+
+func testUnmarshalMPP(t *testing.T, test unmarshalMPPTest) {
+	mpp, err := UnmarshalMPP(test.mpp)
+	switch test.outcome {
+
+	// Valid arguments should result in no error, a non-nil MPP record, and
+	// the fields should be set correctly.
+	case valid:
+		if err != nil {
+			t.Fatalf("unable to parse mpp record: %v", err)
+		}
+		if mpp == nil {
+			t.Fatalf("mpp payload should be non-nil")
+		}
+		if int64(mpp.TotalMsat()) != test.mpp.TotalAmtMsat {
+			t.Fatalf("incorrect total msat")
+		}
+		addr := mpp.PaymentAddr()
+		if !bytes.Equal(addr[:], test.mpp.PaymentAddr) {
+			t.Fatalf("incorrect payment addr")
+		}
+
+	// Invalid arguments should produce a failure and nil MPP record.
+	case invalid:
+		if err == nil {
+			t.Fatalf("expected failure for invalid mpp")
+		}
+		if mpp != nil {
+			t.Fatalf("mpp payload should be nil for failure")
+		}
+
+	// Arguments that produce no MPP field should return no error and no MPP
+	// record.
+	case nompp:
+		if err != nil {
+			t.Fatalf("failure for args resulting for no-mpp")
+		}
+		if mpp != nil {
+			t.Fatalf("mpp payload should be nil for no-mpp")
+		}
+
+	default:
+		t.Fatalf("test case has non-standard outcome")
+	}
+}