engine, lang: core: Match exported resources properly

I inverted the logic for complex setups and forgot to handle the zero
cases. I also didn't notice my loop continue error. This cleans all this
up so that we can have proper exported resource matching.

engine/world.go

@@ -181,6 +181,19 @@ func (obj *ResFilter) Match(kind, name, host string) error {
 	return nil // match!
 }
 
+// MatchFilters is a simple helper function to avoid duplicating this loop here.
+// If any filter matches, it returns nil. Otherwise we error.
+func MatchFilters(filters []*ResFilter, kind, name, host string) error {
+	// TODO: I'd love to avoid this O(N^2) matching if possible...
+	for _, filter := range filters {
+		if err := filter.Match(kind, name, host); err == nil {
+			return nil
+		}
+	}
+
+	return fmt.Errorf("not matches found") // did not match
+}
+
 // ResOutput represents a record of exported resource data which we have read
 // out from the world storage system. The Data field contains an encoded version
 // of the resource, and even though decoding it will get you a Kind and Name, we

etcd/client/resources/resources.go

@@ -146,10 +146,8 @@ func GetResources(ctx context.Context, client interfaces.Client, hostname string
 		}
 
 		// TODO: I'd love to avoid this O(N^2) matching if possible...
-		for _, filter := range filters {
-			if err := filter.Match(kind, name, hostnameFrom); err != nil {
-				continue // did not match
-			}
+		if err := engine.MatchFilters(filters, kind, name, hostnameFrom); err != nil {
+			continue // did not match any of these
 		}
 
 		ro := &engine.ResOutput{

lang/core/collect.go

@@ -456,12 +456,21 @@ func (obj *CollectFunc) Call(ctx context.Context, args []types.Value) (types.Val
 	if obj.init == nil {
 		return nil, funcs.ErrCantSpeculate
 	}
+
+	list := types.NewList(obj.Info().Sig.Out) // collectFuncOutType
+
+	if len(filters) == 0 {
+		// If we have no filters, it means we're matching on nothing,
+		// which happens if we've pre-filtered away all the resources
+		// that we'd want to collect, so here we return absolutely zero!
+		return list, nil
+	}
+
 	resOutput, err := obj.init.World.ResCollect(ctx, filters)
 	if err != nil {
 		return nil, err
 	}
 
-	list := types.NewList(obj.Info().Sig.Out) // collectFuncOutType
 	for _, x := range resOutput {
 		// programming error if any of these error...
 		if x.Kind != kind {