code: func (pl *PackageLoader) Ensure(c context.Context, root string, packages []*vpython.Spec_Package) error {
  	pins, err := packagesToPins(packages)
  	if err != nil {
  		return errors.Annotate(err, "failed to convert packages to CIPD pins").Err()
  	}
  	pinSlice := common.PinSliceBySubdir{
  		"": pins,
  	}
  
  	// Generate a CIPD client. Use the supplied root.
  	opts := pl.Options
  	opts.Root = root
  	client, err := cipd.NewClient(opts)
  	if err != nil {
  		return errors.Annotate(err, "failed to generate CIPD client").Err()
  	}
  
  	// Start a CIPD client batch.
  	client.BeginBatch(c)
  	defer client.EndBatch(c)
  
  	actionMap, err := client.EnsurePackages(c, pinSlice, cipd.NotParanoid, false)
  	if err != nil {
  		return errors.Annotate(err, "failed to install CIPD packages").Err()
  	}
  	if len(actionMap) > 0 {
  		errorCount := 0
  		for root, action := range actionMap {
  			errorCount += len(action.Errors)
  			for _, err := range action.Errors {
  				logging.Errorf(c, "CIPD root %q action %q for pin %q encountered error: %s", root, err.Action, err.Pin, err)
  			}
  		}
  		if errorCount > 0 {
  			return errors.Reason("CIPD package installation encountered %d error(s)", errorCount).Err()
  		}
  	}
  	return nil
  }
docs: // Ensure implement venv.PackageLoader.
  //
  // The packages must be valid (PackageIsComplete). If they aren't, Ensure will
  // panic.
  //
  // The CIPD client that is used for the operation is generated from the supplied
  // options, opts.