org-clock-out-removed-last-clock: New variable to be used by `org-clock-hook'

* lisp/org-clock.el (org-clock-out-removed-last-clock): Add new global
variable.
(org-clock-out): Set the variable to non-nil when the CLOCK line has
been removed before running `org-clock-out-hook'.
(org-clock-out-hook): Add reference to the new variable.
* etc/ORG-NEWS (New variable ~org-clock-out-removed-last-clock~):
Announce the change.

etc/ORG-NEWS

@@ -308,6 +308,13 @@ The change is breaking when ~org-use-property-inheritance~ is set to ~t~.
 
 The =TEST= parameter is better served by Emacs debugging tools.
 ** New and changed options
+*** New variable ~org-clock-out-removed-last-clock~
+
+The variable is intended to be used by ~org-clock-out-hook~.  It is a
+flag used to signal when the =CLOCK= line has been removed.  This can
+happen when ~org-clock-out-remove-zero-time-clocks~ is customized to
+be non-nil.
+
 *** ~org-info-other-documents~ is now a custom option
 
 Users can now extend the value of ~org-info-other-documents~ to

lisp/org-clock.el

@@ -515,7 +515,10 @@ to add an effort property.")
   "Hook run when starting the clock.")
 (defvar org-clock-out-hook nil
   "Hook run when stopping the current clock.
-The point is at the current clock line when the hook is executed.")
+The point is at the current clock line when the hook is executed.
+
+The hook functions can access `org-clock-out-removed-last-clock' to
+check whether the latest CLOCK line has been cleared.")
 
 (defvar org-clock-cancel-hook nil
   "Hook run when canceling the current clock.")
@@ -1698,6 +1701,11 @@ and current `frame-title-format' is equal to `org-clock-frame-title-format'."
 	     (equal frame-title-format org-clock-frame-title-format))
     (setq frame-title-format org-frame-title-format-backup)))
 
+(defvar org-clock-out-removed-last-clock nil
+  "When non-nil, the last `org-clock-out' removed the clock line.
+This can happen when `org-clock-out-remove-zero-time-clocks' is set to
+non-nil and the latest clock took 0 minutes.")
+
 ;;;###autoload
 (defun org-clock-out (&optional switch-to-state fail-quietly at-time)
   "Stop the currently running clock.
@@ -1788,6 +1796,7 @@ to, overriding the existing value of `org-clock-out-switch-to-state'."
 		   te (org-duration-from-minutes (+ (* 60 h) m)))
           (unless (org-clocking-p)
 	    (setq org-clock-current-task nil))
+          (setq org-clock-out-removed-last-clock remove)
           (run-hooks 'org-clock-out-hook)
           ;; Add a note, but only if we didn't remove the clock line.
           (when (and org-log-note-clock-out (not remove))