lnrpc+itest: expose AMP fields on InvoiceHTLC

We also test that legacy keysend payments are promoted to AMP payments
on the receiver-sdie by asserting basic properties of the fields
returned via the rpc.

lnrpc/invoicesrpc/invoices.swagger.json

@@ -256,6 +256,37 @@
     "invoicesrpcSettleInvoiceResp": {
       "type": "object"
     },
+    "lnrpcAMP": {
+      "type": "object",
+      "properties": {
+        "root_share": {
+          "type": "string",
+          "format": "byte",
+          "description": "An n-of-n secret share of the root seed from which child payment hashes\nand preimages are derived."
+        },
+        "set_id": {
+          "type": "string",
+          "format": "byte",
+          "description": "An identifier for the HTLC set that this HTLC belongs to."
+        },
+        "child_index": {
+          "type": "integer",
+          "format": "int64",
+          "description": "A nonce used to randomize the child preimage and child hash from a given\nroot_share."
+        },
+        "hash": {
+          "type": "string",
+          "format": "byte",
+          "description": "The payment hash of the AMP HTLC."
+        },
+        "preimage": {
+          "type": "string",
+          "format": "byte",
+          "description": "The preimage used to settle this AMP htlc. This field will only be\npopulated if the invoice is in InvoiceState_ACCEPTED or\nInvoiceState_SETTLED."
+        }
+      },
+      "description": "Details specific to AMP HTLCs."
+    },
     "lnrpcFeature": {
       "type": "object",
       "properties": {
@@ -489,6 +520,10 @@
           "type": "string",
           "format": "uint64",
           "description": "The total amount of the mpp payment in msat."
+        },
+        "amp": {
+          "$ref": "#/definitions/lnrpcAMP",
+          "description": "Details relevant to AMP HTLCs, only populated if this is an AMP HTLC."
         }
       },
       "title": "Details of an HTLC that paid to an invoice"

lnrpc/invoicesrpc/utils.go

@@ -116,6 +116,25 @@ func CreateRPCInvoice(invoice *channeldb.Invoice,
 			MppTotalAmtMsat: uint64(htlc.MppTotalAmt),
 		}
 
+		// Populate any fields relevant to AMP payments.
+		if htlc.AMP != nil {
+			rootShare := htlc.AMP.Record.RootShare()
+			setID := htlc.AMP.Record.SetID()
+
+			var preimage []byte
+			if htlc.AMP.Preimage != nil {
+				preimage = htlc.AMP.Preimage[:]
+			}
+
+			rpcHtlc.Amp = &lnrpc.AMP{
+				RootShare:  rootShare[:],
+				SetId:      setID[:],
+				ChildIndex: uint32(htlc.AMP.Record.ChildIndex()),
+				Hash:       htlc.AMP.Hash[:],
+				Preimage:   preimage,
+			}
+		}
+
 		// Only report resolved times if htlc is resolved.
 		if htlc.State != channeldb.HtlcStateAccepted {
 			rpcHtlc.ResolveTime = htlc.ResolveTime.Unix()

lnrpc/rpc.proto

@@ -2954,6 +2954,31 @@ message InvoiceHTLC {
 
     // The total amount of the mpp payment in msat.
     uint64 mpp_total_amt_msat = 10;
+
+    // Details relevant to AMP HTLCs, only populated if this is an AMP HTLC.
+    AMP amp = 11;
+}
+
+// Details specific to AMP HTLCs.
+message AMP {
+    // An n-of-n secret share of the root seed from which child payment hashes
+    // and preimages are derived.
+    bytes root_share = 1;
+
+    // An identifier for the HTLC set that this HTLC belongs to.
+    bytes set_id = 2;
+
+    // A nonce used to randomize the child preimage and child hash from a given
+    // root_share.
+    uint32 child_index = 3;
+
+    // The payment hash of the AMP HTLC.
+    bytes hash = 4;
+
+    // The preimage used to settle this AMP htlc. This field will only be
+    // populated if the invoice is in InvoiceState_ACCEPTED or
+    // InvoiceState_SETTLED.
+    bytes preimage = 5;
 }
 
 message AddInvoiceResponse {

lnrpc/rpc.swagger.json

@@ -2468,6 +2468,37 @@
         }
       }
     },
+    "lnrpcAMP": {
+      "type": "object",
+      "properties": {
+        "root_share": {
+          "type": "string",
+          "format": "byte",
+          "description": "An n-of-n secret share of the root seed from which child payment hashes\nand preimages are derived."
+        },
+        "set_id": {
+          "type": "string",
+          "format": "byte",
+          "description": "An identifier for the HTLC set that this HTLC belongs to."
+        },
+        "child_index": {
+          "type": "integer",
+          "format": "int64",
+          "description": "A nonce used to randomize the child preimage and child hash from a given\nroot_share."
+        },
+        "hash": {
+          "type": "string",
+          "format": "byte",
+          "description": "The payment hash of the AMP HTLC."
+        },
+        "preimage": {
+          "type": "string",
+          "format": "byte",
+          "description": "The preimage used to settle this AMP htlc. This field will only be\npopulated if the invoice is in InvoiceState_ACCEPTED or\nInvoiceState_SETTLED."
+        }
+      },
+      "description": "Details specific to AMP HTLCs."
+    },
     "lnrpcAbandonChannelResponse": {
       "type": "object"
     },
@@ -4186,6 +4217,10 @@
           "type": "string",
           "format": "uint64",
           "description": "The total amount of the mpp payment in msat."
+        },
+        "amp": {
+          "$ref": "#/definitions/lnrpcAMP",
+          "description": "Details relevant to AMP HTLCs, only populated if this is an AMP HTLC."
         }
       },
       "title": "Details of an HTLC that paid to an invoice"

lntest/itest/lnd_single_hop_invoice_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
+	"github.com/stretchr/testify/require"
 )
 
 func testSingleHopInvoice(net *lntest.NetworkHarness, t *harnessTest) {
@@ -162,6 +163,21 @@ func testSingleHopInvoice(net *lntest.NetworkHarness, t *harnessTest) {
 		t.Fatalf(err.Error())
 	}
 
+	// Assert that the invoice has the proper AMP fields set, since the
+	// legacy keysend payment should have been promoted into an AMP payment
+	// internally.
+	ctxt, _ = context.WithTimeout(ctxb, defaultTimeout)
+	keysendInvoice, err := net.Bob.LookupInvoice(
+		ctxt, &lnrpc.PaymentHash{
+			RHash: keySendHash[:],
+		},
+	)
+	require.NoError(t.t, err)
+	require.Equal(t.t, 1, len(keysendInvoice.Htlcs))
+	htlc := keysendInvoice.Htlcs[0]
+	require.Equal(t.t, uint64(0), htlc.MppTotalAmtMsat)
+	require.Nil(t.t, htlc.Amp)
+
 	// Now create an invoice and specify routing hints.
 	// We will test that the routing hints are encoded properly.
 	hintChannel := lnwire.ShortChannelID{BlockHeight: 10}