mesh: update xds controller to synthesize empty endpoints when no endpoints ref is found (#18835)

internal/mesh/internal/controllers/xds/controller.go

@@ -24,6 +24,7 @@ import (
 )
 
 const ControllerName = "consul.io/xds-controller"
+
 const defaultTenancy = "default"
 
 func Controller(endpointsMapper *bimapper.Mapper, updater ProxyUpdater, fetcher TrustBundleFetcher, leafCertManager *leafcert.Manager, leafMapper *LeafMapper, leafCancels *LeafCancels, datacenter string) controller.Controller {
@@ -158,6 +159,12 @@ func (r *xdsReconciler) Reconcile(ctx context.Context, rt controller.Runtime, re
 
 		// Step 1: Resolve the reference by looking up the ServiceEndpoints.
 		// serviceEndpoints will not be nil unless there is an error.
+		//
+		// TODO(rb/v2): note we should expose a flag on the endpointRef indicating if the user
+		// wants the absence of an Endpoints to imply returning a slice of no data, vs failing outright.
+		// In xdsv1 we call this the "allowEmpty" semantic. Here we are assuming "allowEmpty=true"
+		var psEndpoints *pbproxystate.Endpoints
+		if endpointRef.Id != nil {
 			serviceEndpoints, err := getServiceEndpoints(ctx, rt, endpointRef.Id)
 			if err != nil {
 				rt.Logger.Error("error reading service endpoint", "id", endpointRef.Id, "error", err)
@@ -169,7 +176,7 @@ func (r *xdsReconciler) Reconcile(ctx context.Context, rt controller.Runtime, re
 			}
 
 			// Step 2: Translate it into pbproxystate.Endpoints.
-		psEndpoints, err := generateProxyStateEndpoints(serviceEndpoints, endpointRef.Port)
+			psEndpoints, err = generateProxyStateEndpoints(serviceEndpoints, endpointRef.Port)
 			if err != nil {
 				rt.Logger.Error("error translating service endpoints to proxy state endpoints", "endpoint", endpointRef.Id, "error", err)
 
@@ -179,18 +186,25 @@ func (r *xdsReconciler) Reconcile(ctx context.Context, rt controller.Runtime, re
 
 				return err
 			}
+		} else {
+			psEndpoints = &pbproxystate.Endpoints{}
+		}
 
 		// Step 3: Add the endpoints to ProxyState.
 		proxyStateTemplate.Template.ProxyState.Endpoints[xdsClusterName] = psEndpoints
 
+		if endpointRef.Id != nil {
 			// Track all the endpoints that are used by this ProxyStateTemplate, so we can use this for step 4.
 			endpointResourceRef := resource.Reference(endpointRef.Id, "")
 			endpointsInProxyStateTemplate = append(endpointsInProxyStateTemplate, endpointResourceRef)
-
+		}
 	}
 
 	// Step 4: Track relationships between ProxyStateTemplates and ServiceEndpoints.
 	r.endpointsMapper.TrackItem(req.ID, endpointsInProxyStateTemplate)
+	if len(endpointsInProxyStateTemplate) == 0 {
+		r.endpointsMapper.UntrackItem(req.ID)
+	}
 
 	// Iterate through leaf certificate references.
 	// For each leaf certificate reference, the controller should: