code: func (client *cniClient) ReleaseIPResource(cfg *Config) error {
  	runtimeConfig := libcni.RuntimeConf{
  		ContainerID: cfg.ContainerID,
  		NetNS:       fmt.Sprintf(netnsFormat, cfg.ContainerPID),
  	}
  
  	seelog.Debugf("[ECSCNI] Releasing the ip resource from ipam db, id: [%s], ip: [%v]", cfg.ID, cfg.IPAMV4Address)
  	os.Setenv("ECS_CNI_LOGLEVEL", logger.GetLevel())
  	defer os.Unsetenv("ECS_CNI_LOGLEVEL")
  	return client.del(runtimeConfig, cfg, client.createIPAMNetworkConfig)
  }
docs: // ReleaseIPResource marks the ip available in the ipam db