mu4e/ical: Compose the response attaching the calendar

This composes the response before any hook is run which is important
so that, if those hooks modify the message, they are aware that some
MML is present.  In particular, this is needed to have the
compatibility with the org-msg package; see
https://github.com/djcb/mu/issues/1956

mu4e/mu4e-icalendar.el

@@ -76,6 +76,13 @@
               ("Decline" mu4e-icalendar-reply (,handle declined ,event)))))
       (cl-call-next-method event handle))))
 
+(defun mu4e~icalendar-has-email (email list)
+  "Check that EMAIL is in LIST."
+  (let ((email (downcase email)))
+    (cl-find-if (lambda (c) (let ((e (cdr c)))
+                              (and (stringp e) (string= email (downcase e)))))
+                list)))
+
 (defun mu4e-icalendar-reply (data)
   "Reply to the text/calendar event present in DATA."
   ;; Based on `gnus-icalendar-reply'.
@@ -98,13 +105,44 @@
               (replace-match "\\1\n \\2")
               (goto-char (line-beginning-position)))))
 
-        (with-current-buffer (get-buffer-create gnus-icalendar-reply-bufname)
-          (delete-region (point-min) (point-max))
-          (insert reply)
-          (fold-icalendar-buffer)
-          (when (and charset (string= (downcase charset) "utf-8"))
-            (decode-coding-region (point-min) (point-max) 'utf-8))
-          (mu4e-icalendar-reply-ical msg event status (buffer-name)))
+        (let ((ical-name  gnus-icalendar-reply-bufname))
+          (with-current-buffer (get-buffer-create ical-name)
+            (delete-region (point-min) (point-max))
+            (insert reply)
+            (fold-icalendar-buffer)
+            (when (and charset (string= (downcase charset) "utf-8"))
+              (decode-coding-region (point-min) (point-max) 'utf-8)))
+          ;; Compose the reply message.
+          (save-excursion
+            (let ((message-signature nil)
+                  (mu4e-compose-cite-function #'mu4e~icalendar-delete-citation)
+                  (mu4e-sent-messages-behavior 'delete)
+                  (mu4e-compose-reply-recipients 'sender)
+                  (ical-msg (cl-copy-list msg)))
+              ;; Make sure the reply is sent to the organiser.
+              (let* ((organizer (gnus-icalendar-event:organizer event))
+                     (reply-to (plist-get msg :reply-to))
+                     (name (or (caar reply-to)
+                               (caar (plist-get msg :from))))
+                     (email (cons name organizer)))
+                (unless (or (string= organizer "")
+                            (mu4e~icalendar-has-email organizer reply-to))
+                  (plist-put ical-msg :reply-to (cons email reply-to))))
+              (plist-put ical-msg :subject
+                         (concat (capitalize (symbol-name status))
+                                 ": " (gnus-icalendar-event:summary event)))
+              (mu4e~compose-handler
+               'reply ical-msg
+               `((:buffer-name ,ical-name
+                  :mime-type "text/calendar; method=REPLY; charset=utf-8")))
+              (message-goto-body)
+              (set-buffer-modified-p nil); not yet modified by user
+              (when mu4e-icalendar-trash-after-reply
+                ;; Override `mu4e-sent-handler' set by `mu4e-compose-mode' to
+                ;; also trash the message (thus must be appended to hooks).
+                (add-hook 'message-sent-hook
+                          (mu4e~icalendar-trash-message-hook msg)
+                          90 t)))))
 
         ;; Back in article buffer
         (setq-local gnus-icalendar-reply-status status)
@@ -144,40 +182,6 @@
   (lambda () (setq mu4e-sent-func
                    (mu4e~icalendar-trash-message original-msg))))
 
-(defun mu4e-icalendar-reply-ical (original-msg event status buffer-name)
-  "Reply to ORIGINAL-MSG containing invitation EVENT with STATUS.
-See `gnus-icalendar-event-reply-from-buffer' for the possible
-STATUS values.  BUFFER-NAME is the name of the buffer holding the
-response in icalendar format."
-  (let ((message-signature nil))
-    (let ((mu4e-compose-cite-function #'mu4e~icalendar-delete-citation)
-          (mu4e-sent-messages-behavior 'delete)
-          (mu4e-compose-reply-recipients 'sender))
-      (mu4e~compose-handler 'reply original-msg))
-    ;; Make sure the recipient is the organizer
-    (let ((organizer (gnus-icalendar-event:organizer event)))
-      (unless (string= organizer "")
-        (message-remove-header "To")
-        (message-goto-to)
-        (insert organizer)))
-    ;; Not (message-goto-body) to possibly skip mll sign directive
-    ;; inserted by `mu4e-compose-mode-hook':
-    (goto-char (point-max))
-    (mml-attach-buffer buffer-name "text/calendar; method=REPLY; charset=utf-8")
-    (message-remove-header "Subject")
-    (message-goto-subject)
-    (insert (capitalize (symbol-name status))
-            ": " (gnus-icalendar-event:summary event))
-    (message-goto-body)
-    (set-buffer-modified-p nil); not yet modified by user
-    (when mu4e-icalendar-trash-after-reply
-      ;; Override `mu4e-sent-handler' set by `mu4e-compose-mode' to
-      ;; also trash the message (thus must be appended to hooks).
-      (add-hook
-       'message-sent-hook
-       (mu4e~icalendar-trash-message-hook original-msg)
-       90 t))))
-
 (defun mu4e~icalendar-insert-diary (event reply-status filename)
   "Insert a diary entry for the EVENT in file named FILENAME.
 REPLY-STATUS is the status of the reply.  The possible values are