Add CE version of Gateway Upstream Disambiguation (#19860)

* Add CE version of gateway-upstream-disambiguation

* Use NamespaceOrDefault and PartitionOrDefault

* Add Changelog entry

* Remove the unneeded reassignment

* Use c.ID()

.changelog/19860.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+connect: Solves an issue where two upstream services with the same name in different namespaces were not getting routed to correctly by API Gateways.
+```

agent/consul/discoverychain/gateway.go

@@ -257,7 +257,7 @@ func targetForResolverNode(nodeName string, chains []*structs.CompiledDiscoveryC
 	splitterName := splitterPrefix + strings.TrimPrefix(nodeName, resolverPrefix)
 
 	for _, c := range chains {
-		targetChainPrefix := resolverPrefix + c.ServiceName + "."
+		targetChainPrefix := resolverPrefix + c.ID()
 		if strings.HasPrefix(nodeName, targetChainPrefix) && len(c.Nodes) == 1 {
 			// we have a virtual resolver that just maps to another resolver, return
 			// the given node name

agent/consul/discoverychain/gateway_test.go

@@ -712,6 +712,7 @@ func TestGatewayChainSynthesizer_Synthesize(t *testing.T) {
 			chain: &structs.CompiledDiscoveryChain{
 				ServiceName: "foo",
 				Namespace:   "default",
+				Partition:   "default",
 				Datacenter:  "dc1",
 				StartNode:   "resolver:foo-2.default.default.dc2",
 				Nodes: map[string]*structs.DiscoveryGraphNode{

agent/xds/routes.go

@@ -445,6 +445,8 @@ func (s *ResourceGenerator) routesForAPIGateway(cfgSnap *proxycfg.ConfigSnapshot
 	readyListeners := getReadyListeners(cfgSnap)
 	listenerNames := maps.Keys(readyListeners)
 	sort.Strings(listenerNames)
+
+	// Iterate over all listeners that are ready and configure their routes.
 	for _, listenerName := range listenerNames {
 		readyListener, ok := readyListeners[listenerName]
 		if !ok {
@@ -486,6 +488,7 @@ func (s *ResourceGenerator) routesForAPIGateway(cfgSnap *proxycfg.ConfigSnapshot
 				continue
 			}
 
+			consolidatedRoute := consolidatedRoute // Reassignment to avoid closure issues with the loop variable.
 			domains := generateUpstreamAPIsDomains(readyListener.listenerKey, upstream, consolidatedRoute.Hostnames)
 
 			filterBuilder := perRouteFilterBuilder{providerMap: cfgSnap.JWTProviders, listener: &readyListener.listenerCfg, route: &consolidatedRoute}