patch, 2 partssend-email: add oauth2 support and fix outlook breaking threads

Documentation/git-send-email.adoc, git-send-email.perl

diff --git a/Documentation/git-send-email.adoc b/Documentation/git-send-email.adoc
index 7f223db42d..1bf75c060d 100644
--- a/Documentation/git-send-email.adoc
+++ b/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
diff --git a/git-send-email.perl b/git-send-email.perl
index 1f613fa979..aa6aad596f 100755
--- a/git-send-email.perl
+++ b/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 __("For $smtp_auth TLS is required.")
+	}
+	if ($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 => {

diff --git a/git-send-email.perl b/git-send-email.perl
index aa6aad596f..f2a926872d 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -1799,6 +1799,17 @@ sub send_message {
 			$smtp->datasend("$line") or die $smtp->message;
 		}
 		$smtp->dataend() or die $smtp->message;
+
+		# Retrieve the Message-ID from the server response in case of Outlook
+		if ($smtp_server eq 'smtp.office365.com' || $smtp_server eq 'smtp-mail.outlook.com') {
+			if ($smtp->message =~ /<([^>]+)>/) {
+				$message_id = "<$1>";
+				print __("Outlook: Retrieved Message-ID: $message_id\n");
+			} else {
+				warn __("Warning: Could not retrieve Message-ID from server response.\n");
+			}
+		}
+
 		$smtp->code =~ /250|200/ or die sprintf(__("Failed to send %s\n"), $subject).$smtp->message;
 	}
 	if ($quiet) {