Use variables to store marking state

Bug#78967.

(calendar-mode): Init new variables with user options.
* lisp/calendar/calendar.el (calendar-generate-window)
(calendar-unmark):
* lisp/calendar/diary-lib.el (diary-mark-entries)
(diary-redraw-calendar):
* lisp/calendar/holidays.el (calendar-mark-holidays): Use new
variables.
* etc/NEWS: Document the change.

etc/NEWS

@@ -2451,6 +2451,14 @@ modifier, it scrolls by year.
 The month and year navigation key bindings 'M-}', 'M-{', 'C-x ]' and
 'C-x [' now have the alternative keys '}', '{', ']' and '['.
 
+---
+*** Avoid modifying user options.
+The user options `calendar-mark-holidays-flag' and
+`calendar-mark-diary-entries-flag' are not modified anymore when
+changing the marking state in the calendar buffer.  We now use variables
+that are reset to the user option values whenever we generate a new
+calendar.
+
 ** Calc
 
 *** New user option 'calc-string-maximum-character'.

lisp/calendar/calendar.el

@@ -456,6 +456,12 @@ LEFT is the leftmost column associated with month segment N,
 FIRST and LAST are the first and last columns with day digits in,
 and LAST is the rightmost column.")
 
+(defvar calendar-mark-holidays nil
+  "Variable version of the user option `calendar-mark-holidays-flag'.")
+
+(defvar calendar-mark-diary-entries nil
+  "Variable version of the user option `calendar-mark-diary-entries-flag'.")
+
 (defun calendar-month-edges (segment)
   "Compute the month edge columns for month SEGMENT.
 Returns a list (LEFT FIRST LAST RIGHT), where LEFT is the
@@ -1436,11 +1442,11 @@ Optional integers MON and YR are used instead of today's date."
 	;; For a full height window or a window that is horizontally
 	;; combined don't fit height to that of its buffer.
 	(set-window-vscroll nil 0)))
-    (and calendar-mark-holidays-flag
+    (and calendar-mark-holidays
          ;; (calendar-date-is-valid-p today) ; useful for BC dates
          (calendar-mark-holidays))
     (unwind-protect
-        (if calendar-mark-diary-entries-flag (diary-mark-entries))
+        (if calendar-mark-diary-entries (diary-mark-entries))
       (run-hooks (if today-visible
                      'calendar-today-visible-hook
                    'calendar-today-invisible-hook)))))
@@ -1833,6 +1839,9 @@ For a complete description, see the info node `Calendar/Diary'.
   (make-local-variable 'calendar-mark-ring)
   (make-local-variable 'displayed-month) ; month in middle of window
   (make-local-variable 'displayed-year)  ; year in middle of window
+  ;; Init with user options.
+  (setq calendar-mark-holidays calendar-mark-holidays-flag
+        calendar-mark-diary-entries calendar-mark-diary-entries-flag)
   ;; Most functions only work if displayed-month and displayed-year are set,
   ;; so let's make sure they're always set.  Most likely, this will be reset
   ;; soon in calendar-generate, but better safe than sorry.
@@ -2431,8 +2440,8 @@ interpreted as BC; -1 being 1 BC, and so on."
 (defun calendar-unmark ()
   "Delete all diary/holiday marks/highlighting from the calendar."
   (interactive)
-  (setq calendar-mark-holidays-flag nil
-        calendar-mark-diary-entries-flag nil)
+  (setq calendar-mark-holidays nil
+        calendar-mark-diary-entries nil)
   (with-current-buffer calendar-buffer
     (mapc #'delete-overlay (overlays-in (point-min) (point-max)))))
 

lisp/calendar/diary-lib.el

@@ -1395,8 +1395,8 @@ marks.  This is intended to deal with deleted diary entries."
   ;; ii) called via calendar-redraw (since calendar has already been
   ;; erased).
   ;; Use of REDRAW handles both of these cases.
-  (when (and redraw calendar-mark-diary-entries-flag)
-    (setq calendar-mark-diary-entries-flag nil)
+  (when (and redraw calendar-mark-diary-entries)
+    (setq calendar-mark-diary-entries nil)
     (calendar-redraw))
   (let ((diary-marking-entries-flag t)
         (diary-buffer (find-buffer-visiting diary-file))
@@ -1417,7 +1417,7 @@ marks.  This is intended to deal with deleted diary entries."
               ;; If including, caller has already verified it is readable.
               (insert-file-contents diary-file)
             (if (eq major-mode (default-value 'major-mode)) (diary-mode)))
-          (setq calendar-mark-diary-entries-flag t)
+          (setq calendar-mark-diary-entries t)
           (setq file-glob-attrs (nth 1 (diary-pull-attrs nil '())))
           (with-syntax-table diary-syntax-table
             (save-excursion
@@ -2243,7 +2243,7 @@ Prefix argument ARG makes the entry nonmarking."
 
 (defun diary-redraw-calendar ()
   "If `calendar-buffer' is live and diary entries are marked, redraw it."
-  (and calendar-mark-diary-entries-flag
+  (and calendar-mark-diary-entries
        (save-excursion
          (calendar-redraw)))
   ;; Return value suitable for `write-contents-functions'.

lisp/calendar/holidays.el

@@ -585,7 +585,7 @@ use instead of point."
   (with-current-buffer
       (if event (calendar-event-buffer event)
         (current-buffer))
-    (setq calendar-mark-holidays-flag t)
+    (setq calendar-mark-holidays t)
     (message "Marking holidays...")
     (dolist (holiday (calendar-holiday-list))
       (calendar-mark-visible-date (car holiday) calendar-holiday-marker))