add Oauth2 support

.cirrus.yml

@@ -6,15 +6,12 @@ freebsd_task:
   name: FreeBSD
 
   matrix:
-    - name: FreeBSD 13.1
+    - name: FreeBSD 13.2
       freebsd_instance:
-        image_family: freebsd-13-1
+        image_family: freebsd-13-2
     - name: FreeBSD 12.4
       freebsd_instance:
         image_family: freebsd-12-4
-    - name: FreeBSD 12.3
-      freebsd_instance:
-        image_family: freebsd-12-3
 
   env:
     TEST_ALLOW_SEND: 0

auth.go

@@ -19,6 +19,10 @@ const (
 
 	// SMTPAuthCramMD5 is the "CRAM-MD5" SASL authentication mechanism as described in RFC 4954
 	SMTPAuthCramMD5 SMTPAuthType = "CRAM-MD5"
+
+	// SMTPAuthXOAUTH2 is the "XOAUTH2" SASL authentication mechanism.
+	// https://developers.google.com/gmail/imap/xoauth2-protocol
+	SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"
 )
 
 // SMTP Auth related static errors
@@ -31,4 +35,7 @@ var (
 
 	// ErrCramMD5AuthNotSupported should be used if the target server does not support the "CRAM-MD5" schema
 	ErrCramMD5AuthNotSupported = errors.New("server does not support SMTP AUTH type: CRAM-MD5")
+
+	// ErrXOauth2AuthNotSupported should be used if the target server does not support the "XOAUTH2" schema
+	ErrXOauth2AuthNotSupported = errors.New("server does not support SMTP AUTH type: XOAUTH2")
 )

client.go

@@ -658,6 +658,11 @@ func (c *Client) auth() error {
 				return ErrCramMD5AuthNotSupported
 			}
 			c.sa = smtp.CRAMMD5Auth(c.user, c.pass)
+		case SMTPAuthXOAUTH2:
+			if !strings.Contains(sat, string(SMTPAuthXOAUTH2)) {
+				return ErrXOauth2AuthNotSupported
+			}
+			c.sa = smtp.XOAuth2Auth(c.user, c.pass)
 		default:
 			return fmt.Errorf("unsupported SMTP AUTH type %q", c.satype)
 		}

smtp/auth_xoauth2.go

@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: Copyright (c) 2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package smtp
+
+type xoauth2Auth struct {
+	username, token string
+}
+
+func XOAuth2Auth(username, token string) Auth {
+	return &xoauth2Auth{username, token}
+}
+
+func (a *xoauth2Auth) Start(_ *ServerInfo) (string, []byte, error) {
+	return "XOAUTH2", []byte("user=" + a.username + "\x01" + "auth=Bearer " + a.token + "\x01\x01"), nil
+}
+
+func (a *xoauth2Auth) Next(_ []byte, more bool) ([]byte, error) {
+	if more {
+		return []byte(""), nil
+	}
+	return nil, nil
+}

smtp/smtp.go

@@ -234,8 +234,10 @@ func (c *Client) Auth(a Auth) error {
 			resp, err = a.Next(msg, code == 334)
 		}
 		if err != nil {
-			// abort the AUTH
-			_, _, _ = c.cmd(501, "*")
+			if mech != "XOAUTH2" {
+				// abort the AUTH. Not required for XOAUTH2
+				_, _, _ = c.cmd(501, "*")
+			}
 			_ = c.Quit()
 			break
 		}

smtp/smtp_test.go

@@ -69,6 +69,13 @@ var authTests = []authTest{
 		[]string{"", "user 287eb355114cf5c471c26a875f1ca4ae"},
 		[]bool{false, false},
 	},
+	{
+		XOAuth2Auth("username", "token"),
+		[]string{""},
+		"XOAUTH2",
+		[]string{"user=username\x01auth=Bearer token\x01\x01", ""},
+		[]bool{false},
+	},
 }
 
 func TestAuth(t *testing.T) {
@@ -193,6 +200,86 @@ func TestAuthLogin(t *testing.T) {
 	}
 }
 
+func TestXOAuthOK(t *testing.T) {
+	server := []string{
+		"220 Fake server ready ESMTP",
+		"250-fake.server",
+		"250-AUTH XOAUTH2",
+		"250 8BITMIME",
+		"235 2.7.0 Accepted",
+	}
+	var wrote strings.Builder
+	var fake faker
+	fake.ReadWriter = struct {
+		io.Reader
+		io.Writer
+	}{
+		strings.NewReader(strings.Join(server, "\r\n")),
+		&wrote,
+	}
+
+	c, err := NewClient(fake, "fake.host")
+	if err != nil {
+		t.Fatalf("NewClient: %v", err)
+	}
+	defer c.Close()
+
+	auth := XOAuth2Auth("user", "token")
+	err = c.Auth(auth)
+	if err != nil {
+		t.Fatalf("XOAuth2 error: %v", err)
+	}
+	// the Next method returns a nil response. It must not be sent.
+	// The client request must end with the authentication.
+	if !strings.HasSuffix(wrote.String(), "AUTH XOAUTH2 dXNlcj11c2VyAWF1dGg9QmVhcmVyIHRva2VuAQE=\r\n") {
+		t.Fatalf("got %q; want AUTH XOAUTH2 dXNlcj11c2VyAWF1dGg9QmVhcmVyIHRva2VuAQE=\r\n", wrote.String())
+	}
+}
+
+func TestXOAuth2Error(t *testing.T) {
+	serverResp := []string{
+		"220 Fake server ready ESMTP",
+		"250-fake.server",
+		"250-AUTH XOAUTH2",
+		"250 8BITMIME",
+		"334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==",
+		"535 5.7.8 Username and Password not accepted",
+		"221 2.0.0 closing connection",
+	}
+	var wrote strings.Builder
+	var fake faker
+	fake.ReadWriter = struct {
+		io.Reader
+		io.Writer
+	}{
+		strings.NewReader(strings.Join(serverResp, "\r\n")),
+		&wrote,
+	}
+
+	c, err := NewClient(fake, "fake.host")
+	if err != nil {
+		t.Fatalf("NewClient: %v", err)
+	}
+	defer c.Close()
+
+	auth := XOAuth2Auth("user", "token")
+	err = c.Auth(auth)
+	if err == nil {
+		t.Fatal("expected auth error, got nil")
+	}
+	client := strings.Split(wrote.String(), "\r\n")
+	if len(client) != 5 {
+		t.Fatalf("unexpected number of client requests got %d; want 5", len(client))
+	}
+	if client[1] != "AUTH XOAUTH2 dXNlcj11c2VyAWF1dGg9QmVhcmVyIHRva2VuAQE=" {
+		t.Fatalf("got %q; want AUTH XOAUTH2 dXNlcj11c2VyAWF1dGg9QmVhcmVyIHRva2VuAQE=", client[1])
+	}
+	// the Next method returns an empty response. It must be sent
+	if client[2] != "" {
+		t.Fatalf("got %q; want empty response", client[2])
+	}
+}
+
 // Issue 17794: don't send a trailing space on AUTH command when there's no password.
 func TestClientAuthTrimSpace(t *testing.T) {
 	server := "220 hello world\r\n" +