Drop Session.AuthPlain

Server backends now need to implement the AuthSession interface
if they want to enable SASL PLAIN authentication.

backend.go

@@ -55,9 +55,6 @@ type Session interface {
 	// Free all resources associated with session.
 	Logout() error
 
-	// Authenticate the user using SASL PLAIN.
-	AuthPlain(username, password string) error
-
 	// Set return path for currently processed message.
 	Mail(from string, opts *MailOptions) error
 	// Add recipient for currently processed message.

backendutil/transform.go

@@ -3,6 +3,7 @@ package backendutil
 import (
 	"io"
 
+	"github.com/emersion/go-sasl"
 	"github.com/emersion/go-smtp"
 )
 
@@ -33,8 +34,18 @@ func (s *transformSession) Reset() {
 	s.Session.Reset()
 }
 
-func (s *transformSession) AuthPlain(username, password string) error {
-	return s.Session.AuthPlain(username, password)
+func (s *transformSession) AuthMechanisms() []string {
+	if authSession, ok := s.Session.(smtp.AuthSession); ok {
+		return authSession.AuthMechanisms()
+	}
+	return nil
+}
+
+func (s *transformSession) Auth(mech string) (sasl.Server, error) {
+	if authSession, ok := s.Session.(smtp.AuthSession); ok {
+		return authSession.Auth(mech)
+	}
+	return nil, smtp.ErrAuthUnsupported
 }
 
 func (s *transformSession) Mail(from string, opts *smtp.MailOptions) error {

backendutil/transform_test.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/emersion/go-sasl"
 	"github.com/emersion/go-smtp"
 	"github.com/emersion/go-smtp/backendutil"
 )
@@ -48,12 +49,21 @@ func (s *session) Logout() error {
 	return nil
 }
 
-func (s *session) AuthPlain(username, password string) error {
-	if username != "username" || password != "password" {
-		return errors.New("Invalid username or password")
-	}
-	s.anonymous = false
-	return nil
+func (s *session) AuthMechanisms() []string {
+	return []string{sasl.Plain}
+}
+
+func (s *session) Auth(mech string) (sasl.Server, error) {
+	return sasl.NewPlainServer(func(identity, username, password string) error {
+		if identity != "" && identity != username {
+			return errors.New("Invalid identity")
+		}
+		if username != "username" || password != "password" {
+			return errors.New("Invalid username or password")
+		}
+		s.anonymous = false
+		return nil
+	}), nil
 }
 
 func (s *session) Mail(from string, opts *smtp.MailOptions) error {

conn.go

@@ -856,30 +856,14 @@ func (c *Conn) authMechanisms() []string {
 	if authSession, ok := c.Session().(AuthSession); ok {
 		return authSession.AuthMechanisms()
 	}
-	return []string{sasl.Plain}
+	return nil
 }
 
 func (c *Conn) auth(mech string) (sasl.Server, error) {
 	if authSession, ok := c.Session().(AuthSession); ok {
 		return authSession.Auth(mech)
 	}
-
-	if mech != sasl.Plain {
-		return nil, ErrAuthUnknownMechanism
-	}
-
-	return sasl.NewPlainServer(func(identity, username, password string) error {
-		if identity != "" && identity != username {
-			return errors.New("identities not supported")
-		}
-
-		sess := c.Session()
-		if sess == nil {
-			panic("No session when AUTH is called")
-		}
-
-		return sess.AuthPlain(username, password)
-	}), nil
+	return nil, ErrAuthUnknownMechanism
 }
 
 func (c *Conn) handleStartTLS() {

server_test.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/emersion/go-sasl"
 	"github.com/emersion/go-smtp"
 )
 
@@ -66,12 +67,23 @@ type session struct {
 	msg *message
 }
 
-func (s *session) AuthPlain(username, password string) error {
-	if username != "username" || password != "password" {
-		return errors.New("Invalid username or password")
-	}
-	s.anonymous = false
-	return nil
+var _ smtp.AuthSession = (*session)(nil)
+
+func (s *session) AuthMechanisms() []string {
+	return []string{sasl.Plain}
+}
+
+func (s *session) Auth(mech string) (sasl.Server, error) {
+	return sasl.NewPlainServer(func(identity, username, password string) error {
+		if identity != "" && identity != username {
+			return errors.New("Invalid identity")
+		}
+		if username != "username" || password != "password" {
+			return errors.New("Invalid username or password")
+		}
+		s.anonymous = false
+		return nil
+	}), nil
 }
 
 func (s *session) Reset() {