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: manifold.go
    path of file: ./repos/juju/worker/modelcache
    
    the code of the file until where you have to start completion: // Copyright 2018 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package modelcache

import (
	"github.com/juju/errors"
	"github.com/juju/pubsub/v2"
	"github.com/juju/worker/v3"
	"github.com/juju/worker/v3/dependency"
	"github.com/prometheus/client_golang/prometheus"

	"github.com/juju/juju/core/cache"
	"github.com/juju/juju/core/multiwatcher"
	"github.com/juju/juju/worker/gate"
	workerstate "github.com/juju/juju/worker/state"
)

// Logger describes the logging methods used in this package by the worker.
type Logger interface {
	IsTraceEnabled() bool
	Tracef(string, ...interface{})
	Errorf(string, ...interface{})
	Criticalf(string, ...interface{})
}

// ManifoldConfig holds the information necessary to run a model cache worker in
// a dependency.Engine.
type ManifoldConfig struct {
	StateName           string
	CentralHubName      string
	MultiwatcherName    string
	InitializedGateName string
	Logger              Log
    
func (config ManifoldConfig) start(context dependency.Context) (worker.Worker, error) {
	if err := config.Validate(); err != nil {
		return nil, errors.Trace(err)
	}
	// Get the hub.
	var hub *pubsub.StructuredHub
	if err := context.Get(config.CentralHubName, &hub); err != nil {
		config.Logger.Tracef("hub dependency not available")
		return nil, err
	}
	var unlocker gate.Unlocker
	if err := context.Get(config.InitializedGateName, &unlocker); err != nil {
		return nil, errors.Trace(err)
	}
	var factory multiwatcher.Factory
	if err := context.Get(config.MultiwatcherName, &factory); err != nil {
		return nil, errors.Trace(err)
	}

	var stTracker workerstate.StateTracker
	if err := context.Get(config.StateName, &stTracker); err != nil {
		return nil, errors.Trace(err)
	}

	pool, err := stTracker.Use()
	if err != nil {
		return nil, errors.Trace(err)
	}

	w, err := config.NewWorker(Config{
		StatePool:            pool,
		Hub:                  hub,
		InitializedGate:      unlocker,
		Logger:               config.Logger,
		WatcherFactory:       factory.WatchController,
		PrometheusRegisterer: config.PrometheusRegisterer,
		Cleanup:              func() { _ = stTracker.Done() },
	}.WithDefaultRestartStrategy())
	if err != nil {
		_ = stTracker.Done()
		return nil, errors.Trace(err)
	}
	return w, nil
}