lnrpc: ensure parsing of the Amp flag for payments is consistent

In this commit, we fix an inconsistent in the API related to AMP
payments. When a payment request isn't specified, we require the `--amp`
flag on the CLI to make an AMP payment. However, for payment requests,
we don't require this flag. To fix this inconsistency, we now require
the `--amp` flag to _also_ be set for payment requests.

docs/release-notes/release-notes-0.18.0.md

@@ -368,6 +368,10 @@ bitcoin peers' feefilter values into account](https://github.com/lightningnetwor
   whether through gRPC or the REST API, due to the destructive nature of the
   operation.
 
+* When paying an AMP payment request, [the `--amp` flag is now
+  required](https://github.com/lightningnetwork/lnd/pull/8681) to be consistent
+  w/ the flow when a payment request isn't used. 
+
 ## lncli Updates
 
 * [Documented all available `lncli`
@@ -532,6 +536,7 @@ bitcoin peers' feefilter values into account](https://github.com/lightningnetwor
 * Marcos Fernandez Perez
 * Matt Morehouse
 * Mohamed Awnallah
+* Olaoluwa Osuntokun
 * Ononiwu Maureen Chiamaka
 * Slyghtning
 * Tee8z

itest/lnd_amp_test.go

@@ -129,6 +129,7 @@ func testSendPaymentAMPInvoiceCase(ht *lntest.HarnessTest,
 		PaymentAddr:    externalPayAddr,
 		TimeoutSeconds: 60,
 		FeeLimitMsat:   noFeeLimitMsat,
+		Amp:            true,
 	}
 	payment := ht.SendPaymentAssertSettled(mts.alice, sendReq)
 
@@ -252,6 +253,7 @@ func testSendPaymentAMPInvoiceRepeat(ht *lntest.HarnessTest) {
 	// Now we'll use Carol to pay the invoice that Dave created.
 	ht.CompletePaymentRequests(
 		carol, []string{addInvoiceResp.PaymentRequest},
+		lntest.WithAMP(),
 	)
 
 	// Dave should get a notification that the invoice has been settled.
@@ -274,6 +276,7 @@ func testSendPaymentAMPInvoiceRepeat(ht *lntest.HarnessTest) {
 	// has received another payment.
 	ht.CompletePaymentRequests(
 		carol, []string{addInvoiceResp.PaymentRequest},
+		lntest.WithAMP(),
 	)
 
 	// Dave should get another notification.

lnrpc/routerrpc/router_backend.go

@@ -940,6 +940,14 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 
 		payAddr := payReq.PaymentAddr
 		if payReq.Features.HasFeature(lnwire.AMPOptional) {
+			// The opt-in AMP flag is required to pay an AMP
+			// invoice.
+			if !rpcPayReq.Amp {
+				return nil, fmt.Errorf("the AMP flag (--amp " +
+					"or SendPaymentRequest.Amp) must be " +
+					"set to pay an AMP invoice")
+			}
+
 			// Generate random SetID and root share.
 			var setID [32]byte
 			_, err = rand.Read(setID[:])

lntest/harness.go

@@ -105,6 +105,32 @@ type HarnessTest struct {
 	cleaned bool
 }
 
+// harnessOpts contains functional option to modify the behavior of the various
+// harness calls.
+type harnessOpts struct {
+	useAMP bool
+}
+
+// defaultHarnessOpts returns a new instance of the harnessOpts with default
+// values specified.
+func defaultHarnessOpts() harnessOpts {
+	return harnessOpts{
+		useAMP: false,
+	}
+}
+
+// HarnessOpt is a functional option that can be used to modify the behavior of
+// harness functionality.
+type HarnessOpt func(*harnessOpts)
+
+// WithAMP is a functional option that can be used to enable the AMP feature
+// for sending payments.
+func WithAMP() HarnessOpt {
+	return func(h *harnessOpts) {
+		h.useAMP = true
+	}
+}
+
 // NewHarnessTest creates a new instance of a harnessTest from a regular
 // testing.T instance.
 func NewHarnessTest(t *testing.T, lndBinary string, feeService WebFeeService,
@@ -1438,7 +1464,13 @@ func (h *HarnessTest) FundCoinsP2TR(amt btcutil.Amount,
 // all payment requests. This function does not return until all payments
 // have reached the specified status.
 func (h *HarnessTest) completePaymentRequestsAssertStatus(hn *node.HarnessNode,
-	paymentRequests []string, status lnrpc.Payment_PaymentStatus) {
+	paymentRequests []string, status lnrpc.Payment_PaymentStatus,
+	opts ...HarnessOpt) {
+
+	payOpts := defaultHarnessOpts()
+	for _, opt := range opts {
+		opt(&payOpts)
+	}
 
 	// Create a buffered chan to signal the results.
 	results := make(chan rpc.PaymentClient, len(paymentRequests))
@@ -1449,6 +1481,7 @@ func (h *HarnessTest) completePaymentRequestsAssertStatus(hn *node.HarnessNode,
 			PaymentRequest: payReq,
 			TimeoutSeconds: int32(wait.PaymentTimeout.Seconds()),
 			FeeLimitMsat:   noFeeLimitMsat,
+			Amp:            payOpts.useAMP,
 		}
 		stream := hn.RPC.SendPayment(req)
 
@@ -1477,10 +1510,10 @@ func (h *HarnessTest) completePaymentRequestsAssertStatus(hn *node.HarnessNode,
 // requests. This function does not return until all payments successfully
 // complete without errors.
 func (h *HarnessTest) CompletePaymentRequests(hn *node.HarnessNode,
-	paymentRequests []string) {
+	paymentRequests []string, opts ...HarnessOpt) {
 
 	h.completePaymentRequestsAssertStatus(
-		hn, paymentRequests, lnrpc.Payment_SUCCEEDED,
+		hn, paymentRequests, lnrpc.Payment_SUCCEEDED, opts...,
 	)
 }