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/pathfind_test.go

@@ -773,6 +773,9 @@ func TestPathFinding(t *testing.T) {
 	}, {
 		name: "path finding with additional edges",
 		fn:   runPathFindingWithAdditionalEdges,
+	}, {
+		name: "path finding with redundant additional edges",
+		fn:   runPathFindingWithRedundantAdditionalEdges,
 	}, {
 		name: "new route path too long",
 		fn:   runNewRoutePathTooLong,
@@ -1290,6 +1293,58 @@ func runPathFindingWithAdditionalEdges(t *testing.T, useCache bool) {
 	assertExpectedPath(t, graph.aliasMap, path, "songoku", "doge")
 }
 
+// runPathFindingWithRedundantAdditionalEdges asserts that we are able to find
+// paths to nodes ignoring additional edges that are already known by self node.
+func runPathFindingWithRedundantAdditionalEdges(t *testing.T, useCache bool) {
+	t.Helper()
+
+	var realChannelID uint64 = 3145
+	var hintChannelID uint64 = 1618
+
+	testChannels := []*testChannel{
+		symmetricTestChannel("alice", "bob", 100000, &testChannelPolicy{
+			Expiry:  144,
+			FeeRate: 400,
+			MinHTLC: 1,
+			MaxHTLC: 100000000,
+		}, realChannelID),
+	}
+
+	ctx := newPathFindingTestContext(t, useCache, testChannels, "alice")
+
+	target := ctx.keyFromAlias("bob")
+	paymentAmt := lnwire.NewMSatFromSatoshis(100)
+
+	// Create the channel edge going from alice to bob and include it in
+	// our map of additional edges.
+	aliceToBob := &channeldb.CachedEdgePolicy{
+		ToNodePubKey: func() route.Vertex {
+			return target
+		},
+		ToNodeFeatures:            lnwire.EmptyFeatureVector(),
+		ChannelID:                 hintChannelID,
+		FeeBaseMSat:               1,
+		FeeProportionalMillionths: 1000,
+		TimeLockDelta:             9,
+	}
+
+	additionalEdges := map[route.Vertex][]*channeldb.CachedEdgePolicy{
+		ctx.source: {aliceToBob},
+	}
+
+	path, err := dbFindPath(
+		ctx.graph, additionalEdges, ctx.bandwidthHints,
+		&ctx.restrictParams, &ctx.pathFindingConfig, ctx.source, target,
+		paymentAmt, ctx.timePref, 0,
+	)
+
+	require.NoError(t, err, "unable to find path to bob")
+	require.Len(t, path, 1)
+
+	require.Equal(t, realChannelID, path[0].ChannelID,
+		"additional edge for known edge wasn't ignored")
+}
+
 // TestNewRoute tests whether the construction of hop payloads by newRoute
 // is executed correctly.
 func TestNewRoute(t *testing.T) {