Do not return an error if requested service is not a gateway

This commit converts the previous error into just a Warn-level log
message. By returning an error when the requested service was not a
gateway, we did not appropriately update envoy because the cache Fetch
returned an error and thus did not propagate the update through proxycfg
and xds packages.

agent/consul/internal_endpoint.go

@@ -414,8 +414,15 @@ func (m *Internal) GatewayServices(args *structs.ServiceSpecificRequest, reply *
 				}
 			}
 
+			// We log a warning here to indicate that there is a potential
+			// misconfiguration. We explicitly do NOT return an error because this
+			// can occur in the course of normal operation by deleting a
+			// configuration entry or starting the proxy before registering the
+			// config entry.
 			if !found {
-				return fmt.Errorf("service %q is not a configured terminating-gateway or ingress-gateway", args.ServiceName)
+				m.logger.Warn("no terminating-gateway or ingress-gateway associated with this gateway",
+					"gateway", args.ServiceName,
+				)
 			}
 
 			index, services, err = state.GatewayServices(ws, args.ServiceName, &args.EnterpriseMeta)

agent/consul/internal_endpoint_test.go

@@ -954,8 +954,11 @@ func TestInternal_GatewayServices_BothGateways(t *testing.T) {
 	}
 	var resp structs.IndexedGatewayServices
 	err := msgpackrpc.CallWithCodec(codec, "Internal.GatewayServices", &req, &resp)
-	assert.Error(t, err)
-	assert.Contains(t, err.Error(), `service "api" is not a configured terminating-gateway or ingress-gateway`)
+	assert.NoError(t, err)
+	assert.Empty(t, resp.Services)
+	// Ensure that the index is not zero so that a blocking query still gets the
+	// latest GatewayServices index
+	assert.NotEqual(t, 0, resp.Index)
 }
 
 func TestInternal_GatewayServices_ACLFiltering(t *testing.T) {