channeldb: add new AMPInvoiceState field to store AMP sub-invoice metadata

In this commit, we add a new type `AMPInvoiceState` that's used to store
AMP sub-invoice meta data alongside the main invoice. This will be used
to allow changes to be made to an AMP invoices without reading out all
the HTLCs. In addition, callers can use this metadata to look up
information about the current sub-invoice state of AMP HTLCs.

channeldb/invoice_test.go

@@ -1,6 +1,7 @@
 package channeldb
 
 import (
+	"bytes"
 	"crypto/rand"
 	"fmt"
 	"math"
@@ -12,6 +13,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
+	"github.com/lightningnetwork/lnd/tlv"
 	"github.com/stretchr/testify/require"
 )
 
@@ -49,7 +51,8 @@ func randInvoice(value lnwire.MilliSatoshi) (*Invoice, error) {
 			Value:           value,
 			Features:        emptyFeatures,
 		},
-		Htlcs: map[CircuitKey]*InvoiceHTLC{},
+		Htlcs:    map[CircuitKey]*InvoiceHTLC{},
+		AMPState: map[SetID]InvoiceStateAMP{},
 	}
 	i.Memo = []byte("memo")
 
@@ -2474,3 +2477,89 @@ func TestAddInvoiceInvalidFeatureDeps(t *testing.T) {
 		lnwire.PaymentAddrOptional,
 	))
 }
+
+// TestEncodeDecodeAmpInvoiceState asserts that the nested TLV
+// encoding+decoding for the AMPInvoiceState struct works as expected.
+func TestEncodeDecodeAmpInvoiceState(t *testing.T) {
+	t.Parallel()
+
+	setID1 := [32]byte{1}
+	setID2 := [32]byte{2}
+	setID3 := [32]byte{3}
+
+	circuitKey1 := CircuitKey{
+		ChanID: lnwire.NewShortChanIDFromInt(1), HtlcID: 1,
+	}
+	circuitKey2 := CircuitKey{
+		ChanID: lnwire.NewShortChanIDFromInt(2), HtlcID: 2,
+	}
+	circuitKey3 := CircuitKey{
+		ChanID: lnwire.NewShortChanIDFromInt(2), HtlcID: 3,
+	}
+
+	// Make a sample invoice state map that we'll encode then decode to
+	// assert equality of.
+	ampState := AMPInvoiceState{
+		setID1: InvoiceStateAMP{
+			State:       HtlcStateSettled,
+			SettleDate:  testNow,
+			SettleIndex: 1,
+			InvoiceKeys: map[CircuitKey]struct{}{
+				circuitKey1: struct{}{},
+				circuitKey2: struct{}{},
+			},
+		},
+		setID2: InvoiceStateAMP{
+			State:       HtlcStateCanceled,
+			SettleDate:  testNow,
+			SettleIndex: 2,
+			InvoiceKeys: map[CircuitKey]struct{}{
+				circuitKey1: struct{}{},
+			},
+		},
+		setID3: InvoiceStateAMP{
+			State:       HtlcStateAccepted,
+			SettleDate:  testNow,
+			SettleIndex: 3,
+			InvoiceKeys: map[CircuitKey]struct{}{
+				circuitKey1: struct{}{},
+				circuitKey2: struct{}{},
+				circuitKey3: struct{}{},
+			},
+		},
+	}
+
+	// We'll now make a sample invoice stream, and use that to encode the
+	// amp state we created above.
+	tlvStream, err := tlv.NewStream(
+		tlv.MakeDynamicRecord(
+			invoiceAmpStateType, &ampState, ampState.recordSize,
+			ampStateEncoder, ampStateDecoder,
+		),
+	)
+	require.Nil(t, err)
+
+	// Next encode the stream into a set of raw bytes.
+	var b bytes.Buffer
+	err = tlvStream.Encode(&b)
+	require.Nil(t, err)
+
+	// Now create a new blank ampState map, which we'll use to decode the
+	// bytes into.
+	ampState2 := make(AMPInvoiceState)
+
+	// Decode from the raw stream into this blank mpa.
+	tlvStream, err = tlv.NewStream(
+		tlv.MakeDynamicRecord(
+			invoiceAmpStateType, &ampState2, nil,
+			ampStateEncoder, ampStateDecoder,
+		),
+	)
+	require.Nil(t, err)
+
+	err = tlvStream.Decode(&b)
+	require.Nil(t, err)
+
+	// The two states should match.
+	require.Equal(t, ampState, ampState2)
+}