From 696aa1bbd20f5b9b6557a8db9259e2995843fd50 Mon Sep 17 00:00:00 2001
From: "R.B. Boyer" <4903+rboyer@users.noreply.github.com>
Date: Mon, 18 Sep 2023 16:19:54 -0500
Subject: [PATCH] mesh: update xds controller to synthesize empty endpoints
 when no endpoints ref is found (#18835)

---
 .../internal/controllers/xds/controller.go    | 54 ++++++++++++-------
 1 file changed, 34 insertions(+), 20 deletions(-)

diff --git a/internal/mesh/internal/controllers/xds/controller.go b/internal/mesh/internal/controllers/xds/controller.go
index f2aac840c1..604181a49e 100644
--- a/internal/mesh/internal/controllers/xds/controller.go
+++ b/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 {
@@ -158,39 +159,52 @@ 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.
-		serviceEndpoints, err := getServiceEndpoints(ctx, rt, endpointRef.Id)
-		if err != nil {
-			rt.Logger.Error("error reading service endpoint", "id", endpointRef.Id, "error", err)
-			// Set the status.
-			statusCondition = status.ConditionRejectedErrorReadingEndpoints(status.KeyFromID(endpointRef.Id), err.Error())
-			status.WriteStatusIfChanged(ctx, rt, pstResource, statusCondition)
+		//
+		// 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)
+				// Set the status.
+				statusCondition = status.ConditionRejectedErrorReadingEndpoints(status.KeyFromID(endpointRef.Id), err.Error())
+				status.WriteStatusIfChanged(ctx, rt, pstResource, statusCondition)
 
-			return err
-		}
+				return err
+			}
 
-		// Step 2: Translate it into pbproxystate.Endpoints.
-		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)
+			// Step 2: Translate it into pbproxystate.Endpoints.
+			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)
 
-			// Set the status.
-			statusCondition = status.ConditionRejectedCreatingProxyStateEndpoints(status.KeyFromID(endpointRef.Id), err.Error())
-			status.WriteStatusIfChanged(ctx, rt, pstResource, statusCondition)
+				// Set the status.
+				statusCondition = status.ConditionRejectedCreatingProxyStateEndpoints(status.KeyFromID(endpointRef.Id), err.Error())
+				status.WriteStatusIfChanged(ctx, rt, pstResource, statusCondition)
 
-			return err
+				return err
+			}
+		} else {
+			psEndpoints = &pbproxystate.Endpoints{}
 		}
 
 		// Step 3: Add the endpoints to ProxyState.
 		proxyStateTemplate.Template.ProxyState.Endpoints[xdsClusterName] = psEndpoints
 
-		// 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)
-
+		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: