service: make global services validation backward compatible

2819d7c ("service: add extra validation for global services")
introduced a validation function to prevent unmarshalling invalid global
service objects retrieved from remote clusters. In particular, it
flags a ClusterID=0 as invalid, unless it matches the one of the local
cluster (as that case is supported for external workloads).

Yet, the ClusterID field was not present as part of the ClusterService
struct in v1.13, causing a failure when the v1.14 agents connect to
a cluster still running the v1.13 clustermesh-apiserver. Hence, let's
relax this validation to support ClusterID=0 and prevent issues during
upgrades.

Fixes: 2819d7c ("service: add extra validation for global services")
Reported-by:  Luke <lifeixiangchina@gmail.com>
Signed-off-by: Marco Iorio <marco.iorio@isovalent.com>

pkg/service/store/store.go

@@ -14,7 +14,6 @@ import (
 	"github.com/cilium/cilium/pkg/loadbalancer"
 	"github.com/cilium/cilium/pkg/lock"
 	"github.com/cilium/cilium/pkg/logging/logfields"
-	"github.com/cilium/cilium/pkg/option"
 )
 
 var (
@@ -124,9 +123,9 @@ func (s *ClusterService) Unmarshal(_ string, data []byte) error {
 }
 
 func (s *ClusterService) validate() error {
-	// Skip the ClusterID check if it matches the local one, as we assume that
-	// it has already been validated, and to allow it to be zero.
-	if s.ClusterID != option.Config.ClusterID {
+	// Explicitly allow the ClusterID to be zero for backward compatibility,
+	// as this field was not present in v1.13.
+	if s.ClusterID != 0 {
 		if err := cmtypes.ValidateClusterID(s.ClusterID); err != nil {
 			return err
 		}