org-clock: Query when exiting with running clock

It's annoying to accidentally quit Emacs with a running clock, then
resolve the clock the next time when Emacs is started.

* lisp/org-clock.el (org-clock-kill-emacs-query): New function.
(org-clock-ask-before-exiting): New user option.

[km: added package-version keyword, fixed function name typo]

lisp/org-clock.el

@@ -485,6 +485,17 @@ is added to the user configuration."
 	  (integer :tag "Clock out after Emacs is idle for X seconds")
 	  (const :tag "Never auto clock out" nil)))
 
+(defcustom org-clock-ask-before-exiting t
+  "If non-nil, ask if the user wants to clock out before exiting Emacs.
+This variable only has effect if set with \\[customize]."
+  :set (lambda (symbol value)
+         (if value
+             (add-hook 'kill-emacs-query-functions #'org-clock-kill-emacs-query)
+           (remove-hook 'kill-emacs-query-functions #'org-clock-kill-emacs-query))
+         (set symbol value))
+  :type 'boolean
+  :package-version '(Org . "9.5"))
+
 (defvar org-clock-in-prepare-hook nil
   "Hook run when preparing the clock.
 This hook is run before anything happens to the task that
@@ -3100,6 +3111,17 @@ The details of what will be saved are regulated by the variable
 	       (when (org-invisible-p) (org-show-context))))))
 	(_ nil)))))
 
+(defun org-clock-kill-emacs-query ()
+  "Query user when killing Emacs.
+This function is added to `kill-emacs-query-functions'."
+  (let ((buf (org-clocking-buffer)))
+    (when (and buf (yes-or-no-p "Clock out and save? "))
+      (with-current-buffer buf
+        (org-clock-out)
+        (save-buffer))))
+  ;; Unconditionally return t for `kill-emacs-query-functions'.
+  t)
+
 ;; Suggested bindings
 (org-defkey org-mode-map "\C-c\C-x\C-e" 'org-clock-modify-effort-estimate)