Include an In-Reply-To: <Message-ID of triggering e-mail>, if any, in the new_ticket notification. Again

In commit 8a3c9b63b9 ("Include an
 In-Reply-To: <Message-ID of triggering e-mail>, if any,
 in the new_ticket notification") on 2020-07-17,
I made it so the return mail for a ticket submission
is a reply to the submission.

commit dee9e75762 ("todosrht-lmtp:
 Use GraphQL for ticket submission") on 2022-04-05 deletes this.

Why? Unclear. The patch submission in
  https://lists.sr.ht/~sircmpwn/sr.ht-dev/%3C20220405161354.11100-1-me%40adnano.co%3E
says this is the case but doesn't try to justify or rectify it.

Why? Unclear. Add it back because this is basic functionality.

api/graph/events.go

@@ -90,6 +90,7 @@ type EventBuilder struct {
 	submitterID int
 	tracker     *model.Tracker
 	ticket      *model.Ticket
+	inReplyTo   *string
 
 	mentionedParticipants []int
 	mentions              *Mentions
@@ -272,6 +273,11 @@ func (builder *EventBuilder) InsertNotifications(eventID int, commentID *int) {
 	}
 }
 
+// Sets the given message ID to be added to In-Reply-To/References headers
+func (builder *EventBuilder) InReplyTo(messageId *string) {
+	builder.inReplyTo = messageId
+}
+
 func sendEmail(ctx context.Context, address, message string) error {
 	return client.Do(ctx, "", "meta.sr.ht", client.GraphQLQuery{
 		Query: `
@@ -363,9 +369,17 @@ func (builder *EventBuilder) SendEmails(subject string,
 	header.SetAddressList("Reply-To", []*mail.Address{&ticketAddress})
 	header.SetAddressList("Sender", []*mail.Address{&sender})
 	if builder.eventType == model.EVENT_CREATED {
+		if builder.inReplyTo != nil {
+			header.SetMsgIDList("In-Reply-To", []string{*builder.inReplyTo})
+		}
 		header.SetMessageID(ticketRef)
 	} else {
+		refs := []string{ticketRef}
+		if builder.inReplyTo != nil {
+			refs = append(refs, *builder.inReplyTo)
+		}
 		header.SetMsgIDList("In-Reply-To", []string{ticketRef})
+		header.SetMsgIDList("References", refs)
 	}
 	header.SetSubject(subject)
 	msgBodyWriter, err := mail.CreateSingleInlineWriter(&message, header)

api/graph/schema.resolvers.go

@@ -1010,7 +1010,7 @@ func (r *mutationResolver) SubmitTicketEmail(ctx context.Context, trackerID int,
 
 		builder.InsertNotifications(eventID, nil)
 
-		// TODO: In-Reply-To: {{input.MessageID}}
+		builder.InReplyTo(&input.MessageID)
 
 		details := NewTicketDetails{
 			Body: ticket.Body,

todosrht-lmtp

@@ -145,7 +145,7 @@ class MailHandler:
             "subject": mail["Subject"],
             "body": body,
             "senderId": sender.id,
-            "messageId": mail["Message-ID"],
+            "messageId": mail["Message-ID"].removeprefix("<").removesuffix(">"),
         }
 
         resp = exec_gql("todo.sr.ht", """