Break out of Map.Render loop if context cancelled

These can be long-running operations, and if the client retries we get
the cancelled one running in parallel with the retry, slowing both down
and making it likely the next one will time out too.

render/render.go

@@ -113,9 +113,6 @@ func MakeMap(f MapFunc, r Renderer) Renderer {
 // Render transforms a set of Nodes produces by another Renderer.
 // using a map function
 func (m Map) Render(ctx context.Context, rpt report.Report) Nodes {
-	if ctx.Err() != nil {
-		return Nodes{}
-	}
 	span, ctx := opentracing.StartSpanFromContext(ctx, "Map.Render:"+functionName(m.MapFunc))
 	defer span.Finish()
 	var (
@@ -125,6 +122,9 @@ func (m Map) Render(ctx context.Context, rpt report.Report) Nodes {
 
 	// Rewrite all the nodes according to the map function
 	for _, inRenderable := range input.Nodes {
+		if ctx.Err() != nil { // check if cancelled
+			return Nodes{}
+		}
 		outRenderable := m.MapFunc(inRenderable)
 		if outRenderable.ID != "" {
 			output.add(inRenderable.ID, outRenderable)