lnrpc: provide hop hints when adding an invoice with no amount

With this change we allow adding hop hints when adding an invoice, even
if its amount is zero.

A couple of new unit test case have been added, and the
`testInvoiceRoutingHints` itest was expanded to account for this
scenario.

lnrpc/invoicesrpc/addinvoice.go

@@ -655,9 +655,9 @@ func newSelectHopHintsCfg(invoicesCfg *AddInvoiceConfig,
 // sufficientHints checks whether we have sufficient hop hints, based on the
 // any of the following criteria:
 //   - Hop hint count: the number of hints have reach our max target.
-//   - Total incoming capacity: the sum of the remote balance amount in the
-//     hints is bigger of equal than our target (currently twice the invoice
-//     amount)
+//   - Total incoming capacity (for non-zero invoice amounts): the sum of the
+//     remote balance amount in the hints is bigger of equal than our target
+//     (currently twice the invoice amount)
 //
 // We limit our number of hop hints like this to keep our invoice size down,
 // and to avoid leaking all our private channels when we don't need to.
@@ -669,7 +669,7 @@ func sufficientHints(nHintsLeft int, currentAmount,
 		return true
 	}
 
-	if currentAmount >= targetAmount {
+	if targetAmount != 0 && currentAmount >= targetAmount {
 		log.Debugf("Total hint amount: %v has reached target hint "+
 			"bandwidth: %v", currentAmount, targetAmount)
 		return true

lnrpc/invoicesrpc/addinvoice_test.go

@@ -478,6 +478,12 @@ var sufficientHintsTestCases = []struct {
 	currentAmount: 200,
 	targetAmount:  200,
 	done:          true,
+}, {
+	name:          "no amount provided",
+	nHintsLeft:    1,
+	currentAmount: 100,
+	targetAmount:  0,
+	done:          false,
 }}
 
 func TestSufficientHints(t *testing.T) {
@@ -719,6 +725,93 @@ var populateHopHintsTestCases = []struct {
 			},
 		},
 	},
+}, {
+	name: "consider all the open channels when amount is zero",
+	setupMock: func(h *hopHintsConfigMock) {
+		chanID1, chanID2 := setupMockTwoChannels(h)
+
+		// Prepare the mock for the first channel.
+		h.Mock.On(
+			"IsChannelActive", chanID1,
+		).Once().Return(true)
+
+		h.Mock.On(
+			"IsPublicNode", mock.Anything,
+		).Once().Return(true, nil)
+
+		h.Mock.On(
+			"FetchChannelEdgesByID", mock.Anything,
+		).Once().Return(
+			&channeldb.ChannelEdgeInfo{},
+			&channeldb.ChannelEdgePolicy{},
+			&channeldb.ChannelEdgePolicy{}, nil,
+		)
+
+		// Prepare the mock for the second channel.
+		h.Mock.On(
+			"IsChannelActive", chanID2,
+		).Once().Return(true)
+
+		h.Mock.On(
+			"IsPublicNode", mock.Anything,
+		).Once().Return(true, nil)
+
+		h.Mock.On(
+			"FetchChannelEdgesByID", mock.Anything,
+		).Once().Return(
+			&channeldb.ChannelEdgeInfo{},
+			&channeldb.ChannelEdgePolicy{},
+			&channeldb.ChannelEdgePolicy{}, nil,
+		)
+	},
+	maxHopHints: 10,
+	amount:      0,
+	expectedHopHints: [][]zpay32.HopHint{
+		{
+			{
+				NodeID:    getTestPubKey(),
+				ChannelID: 9,
+			},
+		}, {
+			{
+				NodeID:    getTestPubKey(),
+				ChannelID: 2,
+			},
+		},
+	},
+}, {
+	name: "consider all the open channels when amount is zero" +
+		" up to maxHopHints",
+	setupMock: func(h *hopHintsConfigMock) {
+		chanID1, _ := setupMockTwoChannels(h)
+
+		// Prepare the mock for the first channel.
+		h.Mock.On(
+			"IsChannelActive", chanID1,
+		).Once().Return(true)
+
+		h.Mock.On(
+			"IsPublicNode", mock.Anything,
+		).Once().Return(true, nil)
+
+		h.Mock.On(
+			"FetchChannelEdgesByID", mock.Anything,
+		).Once().Return(
+			&channeldb.ChannelEdgeInfo{},
+			&channeldb.ChannelEdgePolicy{},
+			&channeldb.ChannelEdgePolicy{}, nil,
+		)
+	},
+	maxHopHints: 1,
+	amount:      0,
+	expectedHopHints: [][]zpay32.HopHint{
+		{
+			{
+				NodeID:    getTestPubKey(),
+				ChannelID: 9,
+			},
+		},
+	},
 }}
 
 func setupMockTwoChannels(h *hopHintsConfigMock) (lnwire.ChannelID,

lntest/itest/lnd_routing_test.go

@@ -1233,6 +1233,15 @@ func testInvoiceRoutingHints(net *lntest.NetworkHarness, t *harnessTest) {
 	}
 	checkInvoiceHints(invoice)
 
+	// Create another invoice for Alice with no value and ensure it still
+	// populates routing hints.
+	invoice = &lnrpc.Invoice{
+		Memo:    "routing hints with no amount",
+		Value:   0,
+		Private: true,
+	}
+	checkInvoiceHints(invoice)
+
 	// Now that we've confirmed the routing hints were added correctly, we
 	// can close all the channels and shut down all the nodes created.
 	closeChannelAndAssert(t, net, net.Alice, chanPointBob, false)