Go - Singleton Pattern

main.go

package main

import (
	"fmt"
	"sync"
)

var (
	doOnce   sync.Once
	doLock   = &sync.Mutex{}
	instance *db
)

type db struct {
	Connection string
}

func setInstance(connection string) {
	instance = &db{Connection: connection}
	fmt.Println("Singleton instance has been created.")
}

func (d *db) GetConnection() string {
	return d.Connection
}

func GetInstanceByDoOnce(connection string) *db {
	doOnce.Do(func() {
		setInstance(connection)
	})
	return instance
}

func GetInstanceByDoLock(connection string) *db {
	if instance == nil {
		doLock.Lock()
		defer doLock.Unlock()
		setInstance(connection)package main

import (
	"fmt"
	"io"
	"os"
	"sync"
)

var (
	instance *db
	doOnce   sync.Once
	doLock             = &sync.Mutex{}
	out      io.Writer = os.Stdout
)

type db struct {
	Connection string
}

func setInstance(connection string) {
	instance = &db{Connection: connection}
	fmt.Fprintln(out, "Singleton instance has been created.")
}

func (d *db) GetConnection() string {
	return d.Connection
}

func GetInstanceByDoOnce(connection string) *db {
	doOnce.Do(func() {
		setInstance(connection)
	})
	return instance
}

func GetInstanceByDoLock(connection string) *db {
	if instance == nil {
		doLock.Lock()
		defer doLock.Unlock()
		setInstance(connection)
	}
	return instance
}

func main() {
	connectionString := "myuser@example.com:3306/main-schema"
	GetInstanceByDoOnce(connectionString)
	GetInstanceByDoLock(connectionString)
}

	}
	return instance
}

func main() {
	connectionString := "myuser@example.com:3306/main-schema"
	GetInstanceByDoOnce(connectionString)
	GetInstanceByDoLock(connectionString)
}

main_test.go

package main

import (
	"bytes"
	"testing"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/suite"
)

type SingletonTestSuite struct {
	suite.Suite
}

func (suite *SingletonTestSuite) SetupTest() {
	instance = nil
}

func (suite *SingletonTestSuite) TestDbGetConnection() {
	connectionString := "myuser@example.com:3306/main-schema"
	db := &db{Connection: connectionString}
	assert.Equal(suite.T(), connectionString, db.GetConnection())
}

func (suite *SingletonTestSuite) TestSetInstance() {
	bak := out
	out = new(bytes.Buffer)
	defer func() { out = bak }()
	setInstance("myuser@example.com:3306/main-schema")
	assert.Equal(suite.T(), "Singleton instance has been created.\n", out.(*bytes.Buffer).String())
}

func (suite *SingletonTestSuite) TestGetInstanceByDoOnce() {
	bak := out
	out = new(bytes.Buffer)
	defer func() { out = bak }()
	connectionStringFirst := "myuser@example.com:3306/main-schema-do-one-first"
	connectionStringSecond := "myuser@example.com:3306/main-schema-do-one-second"
	instanceFirst := GetInstanceByDoOnce(connectionStringFirst)
	instanceSecond := GetInstanceByDoOnce(connectionStringSecond)
	assert.Equal(suite.T(), connectionStringFirst, instanceFirst.GetConnection())
	assert.Equal(suite.T(), connectionStringFirst, instanceSecond.GetConnection())
}

func (suite *SingletonTestSuite) TestGetInstanceByDoLock() {
	bak := out
	out = new(bytes.Buffer)
	defer func() { out = bak }()
	connectionStringFirst := "myuser@example.com:3306/main-schema-do-lock-first"
	connectionStringSecond := "myuser@example.com:3306/main-schema-do-lock-second"
	instanceFirst := GetInstanceByDoLock(connectionStringFirst)
	instanceSecond := GetInstanceByDoLock(connectionStringSecond)
	assert.Equal(suite.T(), connectionStringFirst, instanceFirst.GetConnection())
	assert.Equal(suite.T(), connectionStringFirst, instanceSecond.GetConnection())
}

func TestSingletonTestSuite(t *testing.T) {
	suite.Run(t, new(SingletonTestSuite))
}