refactor: use new FilterFunc.Apply

instead of constructing temporary Filter renderers.

This also makes clearer what is going on.

app/api_topology.go

@@ -44,13 +44,9 @@ func handleNode(ctx context.Context, renderer render.Renderer, filter render.Fil
 	)
 	// We must not lose the node during filtering. We achieve that by
 	// (1) rendering the report with the base renderer, without
-	// filtering, which gives us the node (if it exists at all), then
-	// (2) performing a normal filtered render of the report. If the
+	// filtering, which gives us the node (if it exists at all), and
+	// then (2) applying the filter separately to that result.  If the
 	// node is lost in the second step, we simply put it back.
-	//
-	// To avoid repeating the work from step (1) in step (2), we
-	// replace the renderer in the latter with a constant renderer of
-	// the result obtained in step (1).
 	nodes := renderer.Render(rc.Report)
 	node, ok := nodes.Nodes[nodeID]
 	if !ok {
@@ -58,7 +54,7 @@ func handleNode(ctx context.Context, renderer render.Renderer, filter render.Fil
 		return
 	}
 	if filter != nil {
-		nodes = render.Render(rc.Report, render.ConstantRenderer{Nodes: nodes}, filter)
+		nodes = filter.Apply(nodes)
 		if filteredNode, ok := nodes.Nodes[nodeID]; ok {
 			node = filteredNode
 		} else { // we've lost the node during filtering; put it back

render/render.go

@@ -31,10 +31,11 @@ func (r Nodes) Merge(o Nodes) Nodes {
 
 // Render renders the report and then applies the filter
 func Render(rpt report.Report, renderer Renderer, filter FilterFunc) Nodes {
+	nodes := renderer.Render(rpt)
 	if filter != nil {
-		renderer = MakeFilterPseudo(filter, renderer)
+		nodes = filter.Apply(nodes)
 	}
-	return renderer.Render(rpt)
+	return nodes
 }
 
 // Reduce renderer is a Renderer which merges together the output of several
@@ -147,16 +148,6 @@ func (cr conditionalRenderer) Render(rpt report.Report) Nodes {
 	return Nodes{}
 }
 
-// ConstantRenderer renders a fixed set of nodes
-type ConstantRenderer struct {
-	Nodes
-}
-
-// Render implements Renderer
-func (c ConstantRenderer) Render(_ report.Report) Nodes {
-	return c.Nodes
-}
-
 // joinResults is used by Renderers that join sets of nodes
 type joinResults struct {
 	nodes  report.Nodes