[evolution-data-server] I#326 - libecal: Move Due date relative to the Start date on a recurring task completion

[Milan Crha <mcrha src gnome org>]

commit 869c9a802a983267f65bd44ccada386b358878fc
Author: Milan Crha <mcrha redhat com>
Date:   Tue Apr 20 19:28:00 2021 +0200

    I#326 - libecal: Move Due date relative to the Start date on a recurring task completion
    
    Closes https://gitlab.gnome.org/GNOME/evolution-data-server/-/issues/326

 src/calendar/libecal/e-cal-util.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)
---
diff --git a/src/calendar/libecal/e-cal-util.c b/src/calendar/libecal/e-cal-util.c
index 3381d423c..e1b6dbb61 100644
--- a/src/calendar/libecal/e-cal-util.c
+++ b/src/calendar/libecal/e-cal-util.c
@@ -2674,22 +2674,31 @@ e_cal_util_mark_task_complete_sync (ICalComponent *vtodo,
                        }
 
                        if (new_dtstart && !i_cal_time_is_null_time (new_dtstart) && i_cal_time_is_valid_time 
(new_dtstart)) {
-                               ICalTime *old_due;
+                               ICalTime *old_dtstart, *old_due;
 
+                               old_dtstart = i_cal_component_get_dtstart (vtodo);
                                old_due = i_cal_component_get_due (vtodo);
 
-                               /* When the previous DUE is before new DTSTART, then move relatively also the 
DUE
-                                  date, to keep the difference... */
-                               if (old_due && !i_cal_time_is_null_time (old_due) && i_cal_time_is_valid_time 
(old_due) &&
-                                   i_cal_time_compare (old_due, new_dtstart) < 0) {
-                                       if (!e_cal_util_find_next_occurrence (vtodo, old_due, &new_due, 
cal_client, cancellable, error)) {
+                               /* Move relatively also the DUE date, to keep the difference... */
+                               if (old_due && !i_cal_time_is_null_time (old_due) && i_cal_time_is_valid_time 
(old_due)) {
+                                       if (old_dtstart && !i_cal_time_is_null_time (old_dtstart) && 
i_cal_time_is_valid_time (old_dtstart)) {
+                                               gint64 diff;
+
+                                               diff = i_cal_time_as_timet (old_due) - i_cal_time_as_timet 
(old_dtstart);
+                                               new_due = i_cal_time_clone (new_dtstart);
+                                               i_cal_time_adjust (new_due, diff / (24 * 60 * 60), (diff / 
(60 * 60)) % 24,
+                                                       (diff / 60) % 60, diff % 60);
+                                       } else if (!e_cal_util_find_next_occurrence (vtodo, old_due, 
&new_due, cal_client, cancellable, error)) {
                                                g_clear_object (&new_dtstart);
                                                g_clear_object (&new_due);
+                                               g_clear_object (&old_dtstart);
                                                g_clear_object (&old_due);
                                                return FALSE;
                                        }
                                }
 
+                               g_clear_object (&old_dtstart);
+
                                /* ...  otherwise set the new DUE as the next-next-DTSTART ... */
                                if (!new_due || i_cal_time_is_null_time (new_due) || 
!i_cal_time_is_valid_time (new_due)) {
                                        g_clear_object (&new_due);