You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: download.go
    path of file: ./repos/juju/worker/caasoperator
    
    the code of the file until where you have to start completion: // Copyright 2017 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package caasoperator

import (
	"github.com/juju/errors"

	"github.com/juju/juju/core/status"
	"github.com/juju/juju/downloader"
	jujucharm "github.com/juju/juju/worker/uniter/charm"
)

// Downloader provides an interface for downloading files to disk.
type Downloader interface {
	// Download downloads a file to a local directory, and
	// returns the local path to the file.
	Download(downloader.Request) (string, error)
}

type charmInfo struct {
	curl   string
    
func (op *caasOperator) ensureCharm(localState *LocalState) error {
	dbCharmInfo, err := op.config.CharmGetter.Charm(op.config.Application)
	if err != nil {
		return errors.Trace(err)
	}
	op.deploymentMode = dbCharmInfo.DeploymentMode
	localState.CharmModifiedVersion = dbCharmInfo.CharmModifiedVersion
	curl := dbCharmInfo.URL
	if localState.CharmURL == curl {
		op.config.Logger.Debugf("charm %s already downloaded", curl)
		return nil
	}
	if err := op.setStatus(status.Maintenance, "downloading charm (%s)", curl); err != nil {
		return errors.Trace(err)
	}

	info := &charmInfo{curl: curl.String(), sha256: dbCharmInfo.SHA256}
	if err := op.deployer.Stage(info, op.catacomb.Dying()); err != nil {
		return errors.Trace(err)
	}

	if err := op.deployer.Deploy(); err != nil {
		if err == jujucharm.ErrConflict {
			err = op.setStatus(status.Error, "upgrade failed")
		}
		return errors.Trace(err)
	}
	localState.CharmURL = curl
	return op.stateFile.Write(localState)
}