code: private void unregisterHandle(ContainerHandle handle) {
          synchronized (this.handles) {
              assert this.handles.containsKey(handle.getContainerId()) : "found unregistered handle " + handle.getContainerId();
              this.handles.remove(handle.getContainerId());
          }
  
          log.info("Container {} has been unregistered.", handle.getContainerId());
      }
docs: region Helpers