chore: optimize lock in discov.etcd (#4275)

core/discov/internal/registry.go

@@ -30,7 +30,7 @@ var (
 // A Registry is a registry that manages the etcd client connections.
 type Registry struct {
  clusters map[string]*cluster
- lock sync.Mutex
+ lock sync.RWMutex
 }
 
 // GetRegistry returns a global Registry.
@@ -60,12 +60,19 @@ func (r *Registry) Monitor(endpoints []string, key string, l UpdateListener) err
 
 func (r *Registry) getCluster(endpoints []string) (c *cluster, exists bool) {
  clusterKey := getClusterKey(endpoints)
- r.lock.Lock()
- defer r.lock.Unlock()
+ r.lock.RLock()
  c, exists = r.clusters[clusterKey]
+ r.lock.RUnlock()
+
  if !exists {
- c = newCluster(endpoints)
- r.clusters[clusterKey] = c
+ r.lock.Lock()
+ defer r.lock.Unlock()
+ // double-check locking
+ c, exists = r.clusters[clusterKey]
+ if !exists {
+ c = newCluster(endpoints)
+ r.clusters[clusterKey] = c
+ }
  }
 
  return