routing: Fix possible infinite loop on first hop misleading hint

Add ignore condition to additional edges that connect to self. These
edges are already known and avoiding these hints protect the payment
from malformed channel ids which could lead to infinite loop.

Fixes lightningnetwork#6169.

Co-authored-by: lsunsi <lsunsi@pm.me>

routing/router_test.go

@@ -361,6 +361,79 @@ func TestSendPaymentRouteFailureFallback(t *testing.T) {
 	)
 }
 
+// TestSendPaymentRouteInfiniteLoopWithBadHopHint tests that when sending
+// a payment with a malformed hop hint in the first hop, the hint is ignored
+// and the payment succeeds without an infinite loop of retries.
+func TestSendPaymentRouteInfiniteLoopWithBadHopHint(t *testing.T) {
+	t.Parallel()
+
+	const startingBlockHeight = 101
+	ctx := createTestCtxFromFile(t, startingBlockHeight, basicGraphFilePath)
+
+	source := ctx.aliases["roasbeef"]
+	sourceNodeID, err := btcec.ParsePubKey(source[:])
+	require.NoError(t, err)
+
+	actualChannelID := ctx.getChannelIDFromAlias(t, "roasbeef", "songoku")
+	badChannelID := uint64(66666)
+
+	// Craft a LightningPayment struct that'll send a payment from roasbeef
+	// to songoku for 1000 satoshis.
+	var payHash lntypes.Hash
+	paymentAmt := lnwire.NewMSatFromSatoshis(1000)
+	payment := LightningPayment{
+		Target:      ctx.aliases["songoku"],
+		Amount:      paymentAmt,
+		FeeLimit:    noFeeLimit,
+		paymentHash: &payHash,
+		RouteHints: [][]zpay32.HopHint{{
+			zpay32.HopHint{
+				NodeID:          sourceNodeID,
+				ChannelID:       badChannelID,
+				FeeBaseMSat:     uint32(50),
+				CLTVExpiryDelta: uint16(200),
+			},
+		}},
+	}
+
+	var preImage [32]byte
+	copy(preImage[:], bytes.Repeat([]byte{9}, 32))
+
+	// Mock a payment result that always fails with FailUnknownNextPeer when
+	// the bad channel is the first hop.
+	badShortChanID := lnwire.NewShortChanIDFromInt(badChannelID)
+	newFwdError := htlcswitch.NewForwardingError(
+		&lnwire.FailUnknownNextPeer{}, 0,
+	)
+
+	payer, ok := ctx.router.cfg.Payer.(*mockPaymentAttemptDispatcherOld)
+	require.Equal(t, ok, true, "failed Payer cast")
+
+	payer.setPaymentResult(
+		func(firstHop lnwire.ShortChannelID) ([32]byte, error) {
+			// Returns a FailUnknownNextPeer if it's trying
+			// to pay an invalid channel.
+			if firstHop == badShortChanID {
+				return [32]byte{}, newFwdError
+			}
+
+			return preImage, nil
+		})
+
+	// Send off the payment request to the router, should succeed
+	// ignoring the bad channel id hint.
+	paymentPreImage, route, paymentErr := ctx.router.SendPayment(&payment)
+	require.NoError(t, paymentErr, "payment returned an error")
+
+	// The preimage should match up with the one created above.
+	require.Equal(t, preImage[:], paymentPreImage[:], "incorrect preimage")
+
+	// The route should have songoku as the first hop.
+	require.Equal(t, actualChannelID, route.Hops[0].ChannelID,
+		"route should go through the correct channel id",
+	)
+}
+
 // TestChannelUpdateValidation tests that a failed payment with an associated
 // channel update will only be applied to the graph when the update contains a
 // valid signature.