graph: implement index for disabled channels.

This commit adds an index bucket, disabledEdgePolicyBucket, for those
ChannelEdgePolicy with disabled bit on.
The main purpose is to be able to iterate over these fast when prune is
needed without the need for iterating the whole graph.

The entry points for accessing this index are:
1. When updating ChannelEdgePolicy - insert an entry.
2. When deleting ChannelEdge - delete the associated entries.
3. When querying for disabled channels - implemented DisabledChannelIDs
function

channeldb/graph_test.go

@@ -2680,6 +2680,102 @@ func TestNodeIsPublic(t *testing.T) {
 	)
 }
 
+// TestDisabledChannelIDs ensures that the disabled channels within the
+// disabledEdgePolicyBucket are managed properly and the list returned from
+// DisabledChannelIDs is correct.
+func TestDisabledChannelIDs(t *testing.T) {
+	t.Parallel()
+
+	db, cleanUp, err := makeTestDB()
+	if err != nil {
+		t.Fatalf("unable to make test database: %v", err)
+	}
+	defer cleanUp()
+
+	graph := db.ChannelGraph()
+
+	// Create first node and add it to the graph.
+	node1, err := createTestVertex(db)
+	if err != nil {
+		t.Fatalf("unable to create test node: %v", err)
+	}
+	if err := graph.AddLightningNode(node1); err != nil {
+		t.Fatalf("unable to add node: %v", err)
+	}
+
+	// Create second node and add it to the graph.
+	node2, err := createTestVertex(db)
+	if err != nil {
+		t.Fatalf("unable to create test node: %v", err)
+	}
+	if err := graph.AddLightningNode(node2); err != nil {
+		t.Fatalf("unable to add node: %v", err)
+	}
+
+	// Adding a new channel edge to the graph.
+	edgeInfo, edge1, edge2 := createChannelEdge(db, node1, node2)
+	if err := graph.AddLightningNode(node2); err != nil {
+		t.Fatalf("unable to add node: %v", err)
+	}
+
+	if err := graph.AddChannelEdge(edgeInfo); err != nil {
+		t.Fatalf("unable to create channel edge: %v", err)
+	}
+
+	// Ensure no disabled channels exist in the bucket on start.
+	disabledChanIds, err := graph.DisabledChannelIDs()
+	if err != nil {
+		t.Fatalf("unable to get disabled channel ids: %v", err)
+	}
+	if len(disabledChanIds) > 0 {
+		t.Fatalf("expected empty disabled channels, got %v disabled channels",
+			len(disabledChanIds))
+	}
+
+	// Add one disabled policy and ensure the channel is still not in the
+	// disabled list.
+	edge1.ChannelFlags |= lnwire.ChanUpdateDisabled
+	if err := graph.UpdateEdgePolicy(edge1); err != nil {
+		t.Fatalf("unable to update edge: %v", err)
+	}
+	disabledChanIds, err = graph.DisabledChannelIDs()
+	if err != nil {
+		t.Fatalf("unable to get disabled channel ids: %v", err)
+	}
+	if len(disabledChanIds) > 0 {
+		t.Fatalf("expected empty disabled channels, got %v disabled channels",
+			len(disabledChanIds))
+	}
+
+	// Add second disabled policy and ensure the channel is now in the
+	// disabled list.
+	edge2.ChannelFlags |= lnwire.ChanUpdateDisabled
+	if err := graph.UpdateEdgePolicy(edge2); err != nil {
+		t.Fatalf("unable to update edge: %v", err)
+	}
+	disabledChanIds, err = graph.DisabledChannelIDs()
+	if err != nil {
+		t.Fatalf("unable to get disabled channel ids: %v", err)
+	}
+	if len(disabledChanIds) != 1 || disabledChanIds[0] != edgeInfo.ChannelID {
+		t.Fatalf("expected disabled channel with id %v, "+
+			"got %v", edgeInfo.ChannelID, disabledChanIds)
+	}
+
+	// Delete the channel edge and ensure it is removed from the disabled list.
+	if err = graph.DeleteChannelEdges(edgeInfo.ChannelID); err != nil {
+		t.Fatalf("unable to delete channel edge: %v", err)
+	}
+	disabledChanIds, err = graph.DisabledChannelIDs()
+	if err != nil {
+		t.Fatalf("unable to get disabled channel ids: %v", err)
+	}
+	if len(disabledChanIds) > 0 {
+		t.Fatalf("expected empty disabled channels, got %v disabled channels",
+			len(disabledChanIds))
+	}
+}
+
 // TestEdgePolicyMissingMaxHtcl tests that if we find a ChannelEdgePolicy in
 // the DB that indicates that it should support the htlc_maximum_value_msat
 // field, but it is not part of the opaque data, then we'll handle it as it is