send-email: implement SMTP bearer authentication

Manually send SMTP AUTH command for auth type OAUTHBEARER and XOAUTH2.
This is necessary since they are currently not supported by the Perls
Authen::SASL module.

The bearer token needs to be passed in as the password. This can be done
with git-credential-oauth[0] after minor modifications[1]. Which will
allow using git send-email with Gmail and oauth2 authentication:

    [credential]
        helper = cache --timeout 7200    # two hours
        helper = oauth
    [sendemail]
        smtpEncryption = tls
        smtpServer = smtp.gmail.com
        smtpUser = [email protected]
        smtpServerPort = 587
        smtpauth = OAUTHBEARER

As well as Office 365 accounts:

    [credential]
        helper = cache --timeout 7200   # two hours
        helper = oauth
    [sendemail]
        smtpEncryption = tls
        smtpServer = smtp.office365.com
        smtpUser = [email protected]
        smtpServerPort = 587
        smtpauth = XOAUTH2

[0] https://github.com/hickford/git-credential-oauth
[1] hickford/git-credential-oauth#48

Tested-by: M Hickford <[email protected]>
Signed-off-by: Julian Swagemakers <[email protected]>
Signed-off-by: Aditya Garg <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: kerhac

Documentation/git-send-email.adoc

@@ -213,7 +213,10 @@ SMTP server and if it is supported by the utilized SASL library, the mechanism
 is used for authentication. If neither 'sendemail.smtpAuth' nor `--smtp-auth`
 is specified, all mechanisms supported by the SASL library can be used. The
 special value 'none' maybe specified to completely disable authentication
-independently of `--smtp-user`
+independently of `--smtp-user`. Specifying `OAUTHBEARER` or `XOAUTH2` will
+bypass SASL negotiation and force bearer authentication. In this case the
+bearer token must be provided with `--smtp-pass` or using a credential helper
+and `--smtp-encryption=tls` must be set.
 
 --smtp-pass[=<password>]::
 	Password for SMTP-AUTH. The argument is optional: If no

git-send-email.perl

@@ -1398,6 +1398,63 @@ sub smtp_host_string {
 	}
 }
 
+sub generate_oauthbearer_string {
+	# This will generate the oauthbearer string used for authentication.
+	#
+	# "n,a=" {User} ",^Ahost=" {Host} "^Aport=" {Port} "^Aauth=Bearer " {Access Token} "^A^A
+	#
+	# The first part `n,a=" {User} ",` is the gs2 header described in RFC5801.
+	# * gs2-cb-flag `n` -> client does not support CB
+	# * gs2-authzid `a=" {User} "`
+	#
+	# The second part are key value pairs containing host, port and auth as
+	# described in RFC7628.
+	#
+	# https://datatracker.ietf.org/doc/html/rfc5801
+	# https://datatracker.ietf.org/doc/html/rfc7628
+	my $username = shift;
+	my $token = shift;
+	return "n,a=$username,\001port=$smtp_server_port\001auth=Bearer $token\001\001";
+}
+
+sub generate_xoauth2_string {
+	# "user=" {User} "^Aauth=Bearer " {Access Token} "^A^A"
+	# https://developers.google.com/gmail/imap/xoauth2-protocol#initial_client_response
+	my $username = shift;
+	my $token = shift;
+	return "user=$username\001auth=Bearer $token\001\001";
+}
+
+sub smtp_bearer_auth {
+	my $username = shift;
+	my $token = shift;
+	my $auth_string;
+	if ($smtp_encryption ne "tls") {
+		# As described in RFC7628 TLS is required and will be enforced
+		# at this point.
+		#
+		# https://datatracker.ietf.org/doc/html/rfc7628#section-3
+		die sprintf(__("For %s TLS is required."), $smtp_auth);
+	}
+	if ($smtp_auth eq "OAUTHBEARER") {
+		$auth_string = generate_oauthbearer_string($username, $token);
+	} elsif ($smtp_auth eq "XOAUTH2") {
+		$auth_string = generate_xoauth2_string($username, $token);
+	}
+	my $encoded_auth_string = MIME::Base64::encode($auth_string, "");
+	$smtp->command("AUTH $smtp_auth $encoded_auth_string\r\n");
+	use Net::Cmd qw(CMD_OK);
+	if ($smtp->response() == CMD_OK){
+		return 1;
+	} else {
+		# Send dummy request on authentication failure according to rfc7628.
+		# https://datatracker.ietf.org/doc/html/rfc7628#section-3.2.3
+		$smtp->command(MIME::Base64::encode("\001"));
+		$smtp->response();
+		return 0;
+	}
+}
+
 # Returns 1 if authentication succeeded or was not necessary
 # (smtp_user was not specified), and 0 otherwise.
 
@@ -1436,7 +1493,12 @@ sub smtp_auth_maybe {
 
 		# catch all SMTP auth error in a unified eval block
 		eval {
-			if ($smtp_auth) {
+			if (defined $smtp_auth && ($smtp_auth eq "OAUTHBEARER" || $smtp_auth eq "XOAUTH2")) {
+				# Since Authen:SASL does not support XOAUTH2 nor OAUTHBEARER we will
+				# manually authenticate for these types. The password field should
+				# contain the auth token at this point.
+				$result = smtp_bearer_auth($cred->{'username'}, $cred->{'password'});
+			} elsif ($smtp_auth) {
 				my $sasl = Authen::SASL->new(
 					mechanism => $smtp_auth,
 					callback => {