func open(
	controllerModelTag names.ModelTag,
	info *mongo.MongoInfo, opts mongo.DialOpts,
	newPolicy NewPolicyFunc,
	clock clock.Clock,
) (*State, error) {
	logger.Infof("opening state, mongo addresses: %q; entity %v", info.Addrs, info.Tag)
	logger.Debugf("dialing mongo")
	session, err := mongo.DialWithInfo(info.Info, opts)
	if err != nil {
		return nil, maybeUnauthorized(err, "cannot connect to mongodb")
	}
	logger.Debugf("connection established")

	err = mongodbLogin(session, info)
	if err != nil {
		session.Close()
		return nil, errors.Trace(err)
	}
	logger.Debugf("mongodb login successful")

	st, err := newState(controllerModelTag, controllerModelTag, session, info, newPolicy, clock)
	if err != nil {
		return nil, errors.Trace(err)
	}
	return st, nil
}

// dialAndLogin returns a mongo session logged in as a user with administrative
// privileges
func dialAndLogin(mongoInfo *mongo.MongoInfo, callArgs retry.CallArgs) (mgoSession, mgoDb, error) {
	var session *mgo.Session
	opts := mongo.DefaultDialOpts()
	callArgs.Func = func() error {
		// Try to connect, retry a few times until the db comes up.
		var err error
		session, err = mongo.DialWithInfo(mongoInfo.Info, opts)
		if err == nil {
			return nil
		}
		logger.Errorf("cannot open mongo connection: %v", err)
		return err
	}
	if err := retry.Call(callArgs); err != nil {
		return nil, nil, errors.Annotate(err, "error dialing mongo to resume HA")
	}
	admin := session.DB("admin")
	if mongoInfo.Tag != nil {
		if err := admin.Login(mongoInfo.Tag.String(), mongoInfo.Password); err != nil {
			return nil, nil, errors.Annotatef(err, "cannot log in to admin database as %q", mongoInfo.Tag)
		}
	} else if mongoInfo.Password != "" {
		if err := admin.Login(mongo.AdminUser, mongoInfo.Password); err != nil {
			return nil, nil, errors.Annotate(err, "cannot log in to admin database")
		}
	}
	return session, admin, nil
}

func open(tag names.EnvironTag, info *mongo.MongoInfo, opts mongo.DialOpts, policy Policy) (*State, error) {
	logger.Infof("opening state, mongo addresses: %q; entity %v", info.Addrs, info.Tag)
	logger.Debugf("dialing mongo")
	session, err := mongo.DialWithInfo(info.Info, opts)
	if err != nil {
		return nil, maybeUnauthorized(err, "cannot connect to mongodb")
	}
	logger.Debugf("connection established")

	// In rare circumstances, we may be upgrading from pre-1.23, and not have the
	// environment UUID available. In that case we need to infer what it might be;
	// we depend on the assumption that this is the only circumstance in which
	// the the UUID might not be known.
	if tag.Id() == "" {
		logger.Warningf("creating state without environment tag; inferring bootstrap environment")
		ssInfo, err := readRawStateServerInfo(session)
		if err != nil {
			return nil, errors.Trace(err)
		}
		tag = ssInfo.EnvironmentTag
	}

	st, err := newState(tag, session, info, policy)
	if err != nil {
		session.Close()
		return nil, errors.Trace(err)
	}
	return st, nil
}

// initMongoAdminUser adds the admin user with the specified
// password to the admin database in Mongo.
func initMongoAdminUser(info mongo.Info, dialOpts mongo.DialOpts, password string) error {
	session, err := mongo.DialWithInfo(info, dialOpts)
	if err != nil {
		return err
	}
	defer session.Close()
	return mongo.SetAdminMongoPassword(session, mongo.AdminUser, password)
}

func open(info *mongo.MongoInfo, opts mongo.DialOpts, policy Policy) (*State, error) {
	logger.Infof("opening state, mongo addresses: %q; entity %q", info.Addrs, info.Tag)
	logger.Debugf("dialing mongo")
	session, err := mongo.DialWithInfo(info.Info, opts)
	if err != nil {
		return nil, err
	}
	logger.Debugf("connection established")

	st, err := newState(session, info, policy)
	if err != nil {
		session.Close()
		return nil, err
	}
	return st, nil
}

func (s *mongoSuite) testStateWorkerDialSetsWriteMajority(c *gc.C, configureReplset bool) {
	inst := gitjujutesting.MgoInstance{
		Params: []string{"--replSet", "juju"},
	}
	err := inst.Start(coretesting.Certs)
	c.Assert(err, jc.ErrorIsNil)
	defer inst.Destroy()

	var expectedWMode string
	dialOpts := stateWorkerDialOpts
	dialOpts.Timeout = coretesting.LongWait
	if configureReplset {
		info := inst.DialInfo()
		info.Timeout = dialOpts.Timeout
		args := peergrouper.InitiateMongoParams{
			DialInfo:       info,
			MemberHostPort: inst.Addr(),
		}
		err = peergrouper.InitiateMongoServer(args)
		c.Assert(err, jc.ErrorIsNil)
		expectedWMode = "majority"
	} else {
		dialOpts.Direct = true
	}

	mongoInfo := mongo.Info{
		Addrs:  []string{inst.Addr()},
		CACert: coretesting.CACert,
	}
	session, err := mongo.DialWithInfo(mongoInfo, dialOpts)
	c.Assert(err, jc.ErrorIsNil)
	defer session.Close()

	safe := session.Safe()
	c.Assert(safe, gc.NotNil)
	c.Assert(safe.WMode, gc.Equals, expectedWMode)
	c.Assert(safe.J, jc.IsTrue) // always enabled
}