fix(smtp): tolerate bare reply code without text (RFC 5321 section 4.2)

lib/src/smtp/smtp_client.dart

@@ -451,10 +451,12 @@ class SmtpClient extends ClientBase {
         log(responseText, isClient: false);
       }
     }
-    final response = SmtpResponse(responseTexts);
     final cmd = _currentCommand;
     if (cmd != null) {
       try {
+        // parse inside the guard so that a malformed server reply fails the
+        // pending command instead of leaving its completer hanging forever
+        final response = SmtpResponse(responseTexts);
         final next = cmd.next(response);
         final text = next?.text;
         final data = next?.data;

lib/src/smtp/smtp_response.dart

@@ -72,11 +72,18 @@ class SmtpResponseLine {
   const SmtpResponseLine(this.code, this.message);
 
   /// Parses the given response [text].
+  ///
+  /// A reply line is `Reply-code [ SP textstring ]` or
+  /// `Reply-code "-" [ textstring ]` (RFC 5321, section 4.2), so both the
+  /// separator and the text are optional: a bare `250` is a valid line.
   factory SmtpResponseLine.parse(String text) {
-    final code = int.tryParse(text.substring(0, 3));
-    final message = (code == null) ? text : text.substring(4);
+    final code = text.length < 3 ? null : int.tryParse(text.substring(0, 3));
+    if (code == null) {
+      return SmtpResponseLine(500, text);
+    }
+    final message = text.length > 4 ? text.substring(4) : '';
 
-    return SmtpResponseLine(code ?? 500, message);
+    return SmtpResponseLine(code, message);
   }
 
   /// The code of the response

test/smtp/mock_smtp_server.dart

@@ -20,6 +20,9 @@ class MockSmtpServer {
   }
 
   String? nextResponse;
+
+  /// Responses for consecutive requests, consumed before [nextResponse]
+  final List<String> responses = [];
   final Socket _socket;
 
   _MailSendState _sendState = _MailSendState.notStarted;
@@ -35,6 +38,8 @@ class MockSmtpServer {
       return;
     } else if (request == 'QUIT\r\n') {
       writeln('221 2.0.0 Bye');
+    } else if (responses.isNotEmpty) {
+      writeln(responses.removeAt(0));
     } else if (nextResponse == null || nextResponse.isEmpty) {
       // // no supported request found, answer with the pre-defined response:
       writeln('500 Invalid state - define nextResponse for MockSmtpServer');

test/smtp/smtp_client_test.dart

@@ -70,6 +70,31 @@ void main() {
     expect(client.serverInfo.supports('NOTTHERE'), isFalse);
   });
 
+  test('SmtpClient EHLO with bare reply code', () async {
+    // RFC 5321 section 4.2: the last reply line may consist of the code only
+    _mockServer.nextResponse = '250-domain.com Hello\r\n250';
+    final response = await client.ehlo();
+    expect(response.type, SmtpResponseType.success);
+    expect(response.code, 250);
+    expect(response.message, isEmpty);
+  });
+
+  test('SmtpClient AUTH XOAUTH2 with bare 334 challenge', () async {
+    // smtp.yandex.ru answers `AUTH XOAUTH2` (sent without an initial
+    // response) with an empty challenge written as `334<CRLF>` - no space
+    _mockServer.responses.addAll([
+      '334',
+      '235 2.7.0 Authentication successful',
+    ]);
+    final response = await client.authenticate(
+      'user@example.com',
+      'access-token',
+      AuthMechanism.xoauth2,
+    );
+    expect(response.type, SmtpResponseType.success);
+    expect(response.code, 235);
+  });
+
   test('SmtpClient login', () async {
     _mockServer.nextResponse = '235 2.7.0 Authentication successful';
     final response = await client.authenticate(_smtpUser, _smtpPassword);

test/src/smtp/smtp_response_test.dart

@@ -0,0 +1,54 @@
+import 'package:enough_mail/enough_mail.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('SmtpResponseLine.parse', () {
+    test('code with space and text', () {
+      final line = SmtpResponseLine.parse('250 OK');
+      expect(line.code, 250);
+      expect(line.message, 'OK');
+      expect(line.type, SmtpResponseType.success);
+    });
+
+    test('code with hyphen and text (multiline reply)', () {
+      final line = SmtpResponseLine.parse('250-PIPELINING');
+      expect(line.code, 250);
+      expect(line.message, 'PIPELINING');
+    });
+
+    test('bare code without text (RFC 5321 section 4.2)', () {
+      final line = SmtpResponseLine.parse('250');
+      expect(line.code, 250);
+      expect(line.message, isEmpty);
+      expect(line.type, SmtpResponseType.success);
+    });
+
+    test('code followed only by a separator', () {
+      expect(SmtpResponseLine.parse('250 ').message, isEmpty);
+      expect(SmtpResponseLine.parse('250-').message, isEmpty);
+    });
+
+    test('non-numeric prefix is kept as message with code 500', () {
+      final line = SmtpResponseLine.parse('garbage line');
+      expect(line.code, 500);
+      expect(line.message, 'garbage line');
+      expect(line.type, SmtpResponseType.fatalError);
+    });
+
+    test('line shorter than a reply code does not throw', () {
+      final line = SmtpResponseLine.parse('25');
+      expect(line.code, 500);
+      expect(line.message, '25');
+    });
+  });
+
+  group('SmtpResponse', () {
+    test('multiline reply ending with a bare code', () {
+      final response = SmtpResponse(['250-domain.com Hello', '250']);
+      expect(response.responseLines.length, 2);
+      expect(response.code, 250);
+      expect(response.message, isEmpty);
+      expect(response.isOkStatus, isTrue);
+    });
+  });
+}