Add functional options to constructors

This is a preparatory commit that adds the ability to add new
options to any of the constructor functions, without breaking backward
compatibility.
An actual option is going to be added in the next commit, this just
introduces the mechanism.

Author: guggero

migrate.go

@@ -55,12 +55,29 @@ func (e ErrDirty) Error() string {
 	return fmt.Sprintf("Dirty database version %v. Fix and force version.", e.Version)
 }
 
+// options is a set of optional options that can be set when a Migrate instance
+// is created.
+type options struct {
+}
+
+// defaultOptions returns a new options struct with default values.
+func defaultOptions() options {
+	return options{}
+}
+
+// Option is a function that can be used to set options on a Migrate instance.
+type Option func(*options)
+
 type Migrate struct {
 	sourceName   string
 	sourceDrv    source.Driver
 	databaseName string
 	databaseDrv  database.Driver
 
+	// opts is a set of options that can be used to modify the behavior
+	// of the Migrate instance.
+	opts options
+
 	// Log accepts a Logger interface
 	Log Logger
 
@@ -84,8 +101,8 @@ type Migrate struct {
 
 // New returns a new Migrate instance from a source URL and a database URL.
 // The URL scheme is defined by each driver.
-func New(sourceURL, databaseURL string) (*Migrate, error) {
-	m := newCommon()
+func New(sourceURL, databaseURL string, opts ...Option) (*Migrate, error) {
+	m := newCommon(opts)
 
 	sourceName, err := iurl.SchemeFromURL(sourceURL)
 	if err != nil {
@@ -118,8 +135,10 @@ func New(sourceURL, databaseURL string) (*Migrate, error) {
 // and an existing database instance. The source URL scheme is defined by each driver.
 // Use any string that can serve as an identifier during logging as databaseName.
 // You are responsible for closing the underlying database client if necessary.
-func NewWithDatabaseInstance(sourceURL string, databaseName string, databaseInstance database.Driver) (*Migrate, error) {
-	m := newCommon()
+func NewWithDatabaseInstance(sourceURL string, databaseName string,
+	databaseInstance database.Driver, opts ...Option) (*Migrate, error) {
+
+	m := newCommon(opts)
 
 	sourceName, err := iurl.SchemeFromURL(sourceURL)
 	if err != nil {
@@ -144,8 +163,10 @@ func NewWithDatabaseInstance(sourceURL string, databaseName string, databaseInst
 // and a database URL. The database URL scheme is defined by each driver.
 // Use any string that can serve as an identifier during logging as sourceName.
 // You are responsible for closing the underlying source client if necessary.
-func NewWithSourceInstance(sourceName string, sourceInstance source.Driver, databaseURL string) (*Migrate, error) {
-	m := newCommon()
+func NewWithSourceInstance(sourceName string, sourceInstance source.Driver,
+	databaseURL string, opts ...Option) (*Migrate, error) {
+
+	m := newCommon(opts)
 
 	databaseName, err := iurl.SchemeFromURL(databaseURL)
 	if err != nil {
@@ -170,8 +191,11 @@ func NewWithSourceInstance(sourceName string, sourceInstance source.Driver, data
 // database instance. Use any string that can serve as an identifier during logging
 // as sourceName and databaseName. You are responsible for closing down
 // the underlying source and database client if necessary.
-func NewWithInstance(sourceName string, sourceInstance source.Driver, databaseName string, databaseInstance database.Driver) (*Migrate, error) {
-	m := newCommon()
+func NewWithInstance(sourceName string, sourceInstance source.Driver,
+	databaseName string, databaseInstance database.Driver,
+	opts ...Option) (*Migrate, error) {
+
+	m := newCommon(opts)
 
 	m.sourceName = sourceName
 	m.databaseName = databaseName
@@ -182,8 +206,13 @@ func NewWithInstance(sourceName string, sourceInstance source.Driver, databaseNa
 	return m, nil
 }
 
-func newCommon() *Migrate {
+func newCommon(optFunctions []Option) *Migrate {
+	opts := defaultOptions()
+	for _, opt := range optFunctions {
+		opt(&opts)
+	}
 	return &Migrate{
+		opts:               opts,
 		GracefulStop:       make(chan bool, 1),
 		PrefetchMigrations: DefaultPrefetchMigrations,
 		LockTimeout:        DefaultLockTimeout,