In-Reply-To id for outgoing emails (#7950)

Author: isoos

app/lib/fake/backend/fake_email_sender.dart

@@ -28,7 +28,7 @@ class FakeEmailSender implements EmailSender {
 
   @override
   Future<void> sendMessage(EmailMessage message) async {
-    message.verifyLocalMessageId();
+    message.verifyLocalMessageIds();
     if (failNextMessageCount > 0) {
       failNextMessageCount--;
       throw SmtpClientCommunicationException('fake network problem');

app/lib/frontend/email_sender.dart

@@ -67,9 +67,17 @@ class _LoggingEmailSender implements EmailSender {
 final loggingEmailSender = _LoggingEmailSender();
 
 Message _toMessage(EmailMessage input) {
-  input.verifyLocalMessageId();
+  input.verifyLocalMessageIds();
+  final inReplyToMessageId = input.inReplyToLocalMessageId == null
+      ? null
+      : '<${input.inReplyToLocalMessageId}@pub.dev>';
+  final headers = {
+    'Message-ID': '<${input.localMessageId}@pub.dev>',
+    if (inReplyToMessageId != null) 'In-Reply-To': inReplyToMessageId,
+    if (inReplyToMessageId != null) 'References': inReplyToMessageId,
+  };
   return Message()
-    ..headers = {'Message-ID': '<${input.localMessageId}@pub.dev>'}
+    ..headers = headers
     ..from = _toAddress(input.from)
     ..recipients = input.recipients.map(_toAddress).toList()
     ..ccRecipients = input.ccRecipients.map(_toAddress).toList()

app/lib/shared/email.dart

@@ -17,6 +17,11 @@ final _lenientEmailRegExp = RegExp(r'^\S+@\S+\.\S+$');
 final _strictEmailRegExp = RegExp(
     r'''[a-zA-Z0-9.!#$%&'*+\/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$''');
 
+/// Matches the local part of the email's message-id field, with relaxed rules:
+/// - checks that only valid characters are present
+/// - checks for minimum and maximum length
+final _messageIdLocalPartRegExp = RegExp(r'^[0-9a-zA-Z\-]{19,36}$');
+
 final _invitesFrom = EmailAddress(
   _invitesAtPubDev,
   name: 'Dart package site invites',
@@ -100,9 +105,15 @@ class EmailMessage {
   ///
   /// [localMessageId] is not required while in the message construction phase,
   /// but a must have when sending out the actual email. `EmailSender`
-  /// implementation must call [verifyLocalMessageId] before accepting the email
+  /// implementation must call [verifyLocalMessageIds] before accepting the email
   /// for delivery.
   final String? localMessageId;
+
+  /// The local part of a previous email sent by pub.dev, and to which the current
+  /// email is a reply-to. This will become the local part of the `In-Reply-To` and
+  /// the `References` SMTP headers.
+  final String? inReplyToLocalMessageId;
+
   final EmailAddress from;
   final List<EmailAddress> recipients;
   final List<EmailAddress> ccRecipients;
@@ -114,19 +125,27 @@ class EmailMessage {
     this.recipients,
     this.subject,
     String bodyText, {
+    this.inReplyToLocalMessageId,
     this.localMessageId,
     this.ccRecipients = const <EmailAddress>[],
   }) : bodyText = reflowBodyText(bodyText);
 
-  /// Throws [ArgumentError] if the [localMessageId] field doesn't look like
-  /// UUID or ULID.
+  /// Throws [ArgumentError] if the [localMessageId] or the
+  /// [inReplyToLocalMessageId] field doesn't look like an approved ID.
   ///
   /// TODO: double-check that we follow https://www.jwz.org/doc/mid.html
-  void verifyLocalMessageId() {
-    final uuid = localMessageId;
-    if (uuid == null || uuid.length < 25 || uuid.length > 36) {
-      throw ArgumentError('Invalid uuid: `$uuid`');
+  void verifyLocalMessageIds() {
+    void verifyId(String? id) {
+      if (id != null && !_messageIdLocalPartRegExp.hasMatch(id)) {
+        throw ArgumentError('Invalid message-id local part: `$id`');
+      }
+    }
+
+    if (localMessageId == null) {
+      throw ArgumentError('`localMessageId` must be initialized.');
     }
+    verifyId(localMessageId);
+    verifyId(inReplyToLocalMessageId);
   }
 
   Map<String, Object?> toJson() {