Undo: fix multi-level undo of delete-dive and remove-dive-from-trip

The original undo-code was fundamentally broken. Not only did it leak
resources (copied trips were never freed), it also kept references
to trips or dives that could be changed by other commands. Thus,
anything more than a single undo could lead to crashes.

Two ways of fixing this were considered
1) Don't store pointers, but unique dive-ids and trip-ids.
   Whereas such unique ids exist for dives, they would have to be
   implemented for trips.
2) Don't free objects in the backend.
   Instead, take ownership of deleted objects in the undo-object.
   Thus, all references in previous undo-objects are guaranteed to
   still exist (unless the objects are deleted elsewhere).

After some contemplation, the second method was chosen, because
it is significantly less intrusive. While touching the undo-objects,
clearly separate backend from ui-code, such that they can ultimately
be reused for mobile.

Note that if other parts of the code delete dives, crashes can still
be provoked. Notable examples are split/merge dives. These will have
to be fixed later. Nevertheless, the new code is a significant
improvement over the old state.

While touching the code, implement proper translation string based
on Qt's plural-feature (using %n).

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 3 insertions, 2 deletions

diff --git a/core/dive.h b/core/dive.h
index b9ec48824..f80412a03 100644
--- a/core/dive.h
+++ b/core/dive.h
@@ -416,12 +416,13 @@ extern bool autogroup;
 
 extern void add_dive_to_trip(struct dive *, dive_trip_t *);
 
+struct dive *unregister_dive(int idx);
 extern void delete_single_dive(int idx);
 extern void add_single_dive(int idx, struct dive *dive);
 
 extern void insert_trip(dive_trip_t **trip);
-extern struct dive_trip *clone_empty_trip(struct dive_trip *trip);
-
+extern void unregister_trip(dive_trip_t *trip);
+extern void free_trip(dive_trip_t *trip);
 
 extern const struct units SI_units, IMPERIAL_units;
 extern struct units xml_parsing_units;