plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/metadata/metadata.go

@@ -27,17 +27,18 @@ func ContextWithMetadata(ctx context.Context) context.Context {
 
 // ServeDNS implements the plugin.Handler interface.
 func (m *Metadata) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
-	ctx = ContextWithMetadata(ctx)
+	rcode, err := plugin.NextOrFailure(m.Name(), m.Next, ctx, w, r)
+	return rcode, err
+}
 
-	state := request.Request{W: w, Req: r}
+// Collect will retrieve metadata functions from each metadata provider and update the context
+func (m *Metadata) Collect(ctx context.Context, state request.Request) context.Context {
+	ctx = ContextWithMetadata(ctx)
 	if plugin.Zones(m.Zones).Matches(state.Name()) != "" {
 		// Go through all Providers and collect metadata.
 		for _, p := range m.Providers {
 			ctx = p.Metadata(ctx, state)
 		}
 	}
-
-	rcode, err := plugin.NextOrFailure(m.Name(), m.Next, ctx, w, r)
-
-	return rcode, err
+	return ctx
 }

plugin/metadata/metadata_test.go

@@ -47,7 +47,10 @@ func TestMetadataServeDNS(t *testing.T) {
 	}
 
 	ctx := context.TODO()
-	m.ServeDNS(ctx, &test.ResponseWriter{}, new(dns.Msg))
+	w := &test.ResponseWriter{}
+	r := new(dns.Msg)
+	ctx = m.Collect(ctx, request.Request{W: w, Req: r})
+	m.ServeDNS(ctx, w, r)
 	nctx := next.ctx
 
 	for _, expected := range expectedMetadata {