server: Move to using Backend.Features() instead of Enable* flags

backend.go

@@ -21,6 +21,18 @@ type Backend interface {
 	AnonymousLogin(state *ConnectionState) (Session, error)
 }
 
+type Feature int32
+
+const (
+	FeatureNoSMTPUTF8 Feature = 2 << iota
+	FeatureNoBINARYMIME
+	FeatureNoREQUIRETLS
+)
+
+type FeatureBackend interface {
+	Features() Feature
+}
+
 type BodyType string
 
 const (

conn.go

@@ -255,13 +255,14 @@ func (c *Conn) handleGreet(enhanced bool, arg string) {
 
 			caps = append(caps, authCap)
 		}
-		if c.server.EnableSMTPUTF8 {
+		feat := c.server.backendFeatures()
+		if feat&FeatureNoSMTPUTF8 == 0 {
 			caps = append(caps, "SMTPUTF8")
 		}
-		if _, isTLS := c.TLSConnectionState(); isTLS && c.server.EnableREQUIRETLS {
+		if _, isTLS := c.TLSConnectionState(); isTLS && feat&FeatureNoREQUIRETLS == 0 {
 			caps = append(caps, "REQUIRETLS")
 		}
-		if c.server.EnableBINARYMIME {
+		if feat&FeatureNoBINARYMIME == 0 {
 			caps = append(caps, "BINARYMIME")
 		}
 		if c.server.MaxMessageBytes > 0 {
@@ -346,21 +347,21 @@ func (c *Conn) handleMail(arg string) {
 
 				opts.Size = int(size)
 			case "SMTPUTF8":
-				if !c.server.EnableSMTPUTF8 {
+				if c.server.backendFeatures()&FeatureNoSMTPUTF8 != 0 {
 					c.WriteResponse(504, EnhancedCode{5, 5, 4}, "SMTPUTF8 is not implemented")
 					return
 				}
 				opts.UTF8 = true
 			case "REQUIRETLS":
-				if !c.server.EnableREQUIRETLS {
+				if c.server.backendFeatures()&FeatureNoREQUIRETLS != 0 {
 					c.WriteResponse(504, EnhancedCode{5, 5, 4}, "REQUIRETLS is not implemented")
 					return
 				}
 				opts.RequireTLS = true
 			case "BODY":
 				switch value {
 				case "BINARYMIME":
-					if !c.server.EnableBINARYMIME {
+					if c.server.backendFeatures()&FeatureNoBINARYMIME != 0 {
 						c.WriteResponse(504, EnhancedCode{5, 5, 4}, "BINARYMIME is not implemented")
 						return
 					}

server.go

@@ -45,18 +45,6 @@ type Server struct {
 	ReadTimeout       time.Duration
 	WriteTimeout      time.Duration
 
-	// Advertise SMTPUTF8 (RFC 6531) capability.
-	// Should be used only if backend supports it.
-	EnableSMTPUTF8 bool
-
-	// Advertise REQUIRETLS (RFC 8689) capability.
-	// Should be used only if backend supports it.
-	EnableREQUIRETLS bool
-
-	// Advertise BINARYMIME (RFC 3030) capability.
-	// Should be used only if backend supports it.
-	EnableBINARYMIME bool
-
 	// If set, the AUTH command will not be advertised and authentication
 	// attempts will be rejected. This setting overrides AllowInsecureAuth.
 	AuthDisabled bool
@@ -262,3 +250,11 @@ func (s *Server) ForEachConn(f func(*Conn)) {
 		f(conn)
 	}
 }
+
+func (s *Server) backendFeatures() Feature {
+	fb, ok := s.Backend.(FeatureBackend)
+	if !ok {
+		return 0
+	}
+	return fb.Features()
+}

server_test.go

@@ -40,10 +40,16 @@ type backend struct {
 	// Read N bytes of message before returning dataErr.
 	dataErrOffset int64
 
+	features smtp.Feature
+
 	panicOnMail bool
 	userErr     error
 }
 
+func (be *backend) Features() smtp.Feature {
+	return be.features
+}
+
 func (be *backend) Login(_ *smtp.ConnectionState, username, password string) (smtp.Session, error) {
 	if be.userErr != nil {
 		return &session{}, be.userErr
@@ -339,7 +345,6 @@ func TestServerPanicRecover(t *testing.T) {
 
 func TestServerSMTPUTF8(t *testing.T) {
 	_, s, c, scanner := testServerAuthenticated(t)
-	s.EnableSMTPUTF8 = true
 	defer s.Close()
 	defer c.Close()
 
@@ -353,9 +358,10 @@ func TestServerSMTPUTF8(t *testing.T) {
 }
 
 func TestServerSMTPUTF8_Disabled(t *testing.T) {
-	_, s, c, scanner := testServerAuthenticated(t)
+	be, s, c, scanner := testServerAuthenticated(t)
 	defer s.Close()
 	defer c.Close()
+	be.features = smtp.FeatureNoSMTPUTF8
 
 	io.WriteString(c, "MAIL FROM:<alice@wonderland.book> SMTPUTF8\r\n")
 	scanner.Scan()
@@ -1050,7 +1056,6 @@ func TestServer_Chunking_Binarymime(t *testing.T) {
 	be, s, c, scanner := testServerAuthenticated(t)
 	defer s.Close()
 	defer c.Close()
-	s.EnableBINARYMIME = true
 
 	io.WriteString(c, "MAIL FROM:<root@nsa.gov> BODY=BINARYMIME\r\n")
 	scanner.Scan()