Allow to reset Deleted flag when exporting messages in Rmail

* lisp/mail/rmailout.el (rmail-output-reset-deleted-flag): New
defcustom.
(rmail-output): When 'rmail-output-reset-deleted-flag' is non-nil,
reset the Deleted flag of the appended messages, and if COUNT is
greater than 1, do not ignore deleted messages.  Update the doc
string accordingly.  (Bug#31271)

* doc/emacs/rmail.texi (Rmail Output): Document
'rmail-output-reset-deleted-flag'.

* etc/NEWS: Mention the new user option.

3 files changed, 82 insertions, 28 deletions

diff --git a/doc/emacs/rmail.texi b/doc/emacs/rmail.texi
index 068111cab1e..a17ef4938e6 100644
--- a/doc/emacs/rmail.texi
+++ b/doc/emacs/rmail.texi
@@ -529,13 +529,18 @@ file name from the message @samp{Subject} header.
 @kindex C-o @r{(Rmail)}
 @findex rmail-output-as-seen
   The commands @kbd{o} and @kbd{C-o} copy the current message into a
-specified file, adding it at the end.  The two commands differ mainly
-in how much to copy: @kbd{o} copies the full message headers, even if
-they are not all visible, while @kbd{C-o} copies exactly the headers
-currently displayed and no more.  @xref{Rmail Display}.  In addition,
-@kbd{o} converts the message to Babyl format (used by Rmail in Emacs
-version 22 and before) if the file is in Babyl format; @kbd{C-o}
-cannot output to Babyl files at all.
+specified file, adding it at the end.  A positive prefix argument
+serves as a repeat count: that many consecutive messages will be
+copied to the specified file, starting with the current one and
+ignoring deleted messages.
+
+The two commands differ mainly in how much to copy: @kbd{o} copies the
+full message headers, even if they are not all visible, while
+@kbd{C-o} copies exactly the headers currently displayed and no more.
+@xref{Rmail Display}.  In addition, @kbd{o} converts the message to
+Babyl format (used by Rmail in Emacs version 22 and before) if the
+file is in Babyl format; @kbd{C-o} cannot output to Babyl files at
+all.
 @c FIXME remove BABYL mention in some future version?
 
   If the output file is currently visited in an Emacs buffer, the
@@ -565,17 +570,29 @@ second says which files in that directory to offer (all those that
 match the regular expression).  If no files match, you cannot select
 this menu item.
 
-@vindex rmail-delete-after-output
   Copying a message with @kbd{o} or @kbd{C-o} gives the original copy
 of the message the @samp{filed} attribute, so that @samp{filed}
 appears in the mode line when such a message is current.
 
+@vindex rmail-delete-after-output
   If you like to keep just a single copy of every mail message, set
 the variable @code{rmail-delete-after-output} to @code{t}; then the
 @kbd{o}, @kbd{C-o} and @kbd{w} commands delete the original message
 after copying it.  (You can undelete it afterward if you wish, see
 @ref{Rmail Deletion}.)
 
+@vindex rmail-output-reset-deleted-flag
+  By default, @kbd{o} will leave the deleted status of a message it
+outputs as it was on the original message; thus, a message deleted
+before it was output will appear as deleted in the output file.
+Setting the variable @code{rmail-output-reset-deleted-flag} to a
+non-@code{nil} value countermands that: the copy of the message will
+have its deleted status reset, so the message will appear as undeleted
+in the output file.  In addition, when this variable is
+non-@code{nil}, specifying a positive argument to @kbd{o} will not
+ignore deleted messages when looking for consecutive messages to
+output.
+
 @vindex rmail-output-file-alist
   The variable @code{rmail-output-file-alist} lets you specify
 intelligent defaults for the output file, based on the contents of the
diff --git a/etc/NEWS b/etc/NEWS
index 3371718473f..4ea34407547 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -482,6 +482,13 @@ It now applies to epg functions as well as epa functions.
 been removed.  Use 'encode-coding-string', 'decode-coding-string', and
 'select-safe-coding-system' instead.
 
+** Rmail
+
++++
+*** New user option 'rmail-output-reset-deleted-flag'.
+If this option is non-nil, messages appended to an output file by the
+'rmail-output' command have their Deleted flag reset.
+
 * New Modes and Packages in Emacs 27.1
 
 +++
diff --git a/lisp/mail/rmailout.el b/lisp/mail/rmailout.el
index eee8805ab4c..824b1a59fb9 100644
--- a/lisp/mail/rmailout.el
+++ b/lisp/mail/rmailout.el
@@ -56,6 +56,13 @@ The function `rmail-delete-unwanted-fields' uses this, ignoring case."
                  regexp)
   :group 'rmail-output)
 
+(defcustom rmail-output-reset-deleted-flag nil
+  "Non-nil means reset the \"deleted\" flag when outputting a message to a file."
+  :type '(choice (const :tag "Output with the \"deleted\" flag reset" t)
+                 (const :tag "Output with the \"deleted\" flag intact" nil))
+  :version "27.1"
+  :group 'rmail-output)
+
 (defun rmail-output-read-file-name ()
   "Read the file name to use for `rmail-output'.
 Set `rmail-default-file' to this name as well as returning it.
@@ -472,9 +479,15 @@ buffer, updates it accordingly.
 This command always outputs the complete message header, even if
 the header display is currently pruned.
 
+If `rmail-output-reset-deleted-flag' is non-nil, the message's
+deleted flag is reset in the message appended to the destination
+file.  Otherwise, the appended message will remain marked as
+deleted if it was deleted before invoking this command.
+
 Optional prefix argument COUNT (default 1) says to output that
 many consecutive messages, starting with the current one (ignoring
-deleted messages).  If `rmail-delete-after-output' is non-nil, deletes
+deleted messages, unless `rmail-output-reset-deleted-flag' is
+non-nil).  If `rmail-delete-after-output' is non-nil, deletes
 messages after output.
 
 The optional third argument NOATTRIBUTE, if non-nil, says not to
@@ -533,30 +546,47 @@ from a non-Rmail buffer.  In this case, COUNT is ignored."
       (if (zerop rmail-total-messages)
           (error "No messages to output"))
       (let ((orig-count count)
-            beg end)
+            beg end delete-attr-reset-p)
         (while (> count 0)
-          (setq beg (rmail-msgbeg rmail-current-message)
-                end (rmail-msgend rmail-current-message))
-          ;; All access to the buffer's local variables is now finished...
-          (save-excursion
-            ;; ... so it is ok to go to a different buffer.
-            (if (rmail-buffers-swapped-p) (set-buffer rmail-view-buffer))
-            (setq cur (current-buffer))
-            (save-restriction
-              (widen)
-              (with-temp-buffer
-                (insert-buffer-substring cur beg end)
-                (if babyl-format
-                    (rmail-output-as-babyl file-name noattribute)
-                  (rmail-output-as-mbox file-name noattribute)))))
+          (when (and rmail-output-reset-deleted-flag
+                     (rmail-message-deleted-p rmail-current-message))
+            (rmail-set-attribute rmail-deleted-attr-index nil)
+            (setq delete-attr-reset-p t))
+          ;; Make sure we undo our messing with the DELETED attribute.
+          (unwind-protect
+              (progn
+                (setq beg (rmail-msgbeg rmail-current-message)
+                      end (rmail-msgend rmail-current-message))
+                ;; All access to the buffer's local variables is now finished...
+                (save-excursion
+                  ;; ... so it is ok to go to a different buffer.
+                  (if (rmail-buffers-swapped-p) (set-buffer rmail-view-buffer))
+                  (setq cur (current-buffer))
+                  (save-restriction
+                    (widen)
+                    (with-temp-buffer
+                      (insert-buffer-substring cur beg end)
+                      (if babyl-format
+                          (rmail-output-as-babyl file-name noattribute)
+                        (rmail-output-as-mbox file-name noattribute))))))
+            (if delete-attr-reset-p
+                (rmail-set-attribute rmail-deleted-attr-index t)))
           (or noattribute               ; mark message as "filed"
               (rmail-set-attribute rmail-filed-attr-index t))
           (setq count (1- count))
           (let ((next-message-p
-                 (if rmail-delete-after-output
-                     (rmail-delete-forward)
-                   (if (> count 0)
-                       (rmail-next-undeleted-message 1))))
+                 (if rmail-output-reset-deleted-flag
+                     (progn
+                       (if rmail-delete-after-output
+                           (rmail-delete-message))
+                       (if (> count 0)
+                           (let ((msgnum rmail-current-message))
+                             (rmail-next-message 1)
+                             (eq rmail-current-message (1+ msgnum)))))
+                   (if rmail-delete-after-output
+                       (rmail-delete-forward)
+                     (if (> count 0)
+                         (rmail-next-undeleted-message 1)))))
                 (num-appended (- orig-count count)))
             (if (and (> count 0) (not next-message-p))
                 (error "Only %d message%s appended" num-appended