"Fix" crash when deleting linked object which has indirect usages.

This is in fact very hairy situation here... Objects are only refcounted by scenes,
any other usage is 'free', which means once all object instanciations are gone Blender
considers it can delete it.

There is a trap here though: indirect usages. Typically, we should never modify linked data
(because it is essencially useless, changes would be ignored and ost on next reload or
even undo/redo). This means indirect usages are not affected by default 'safe' remapping/unlinking.

For unlinking preceeding deletion however, this is not acceptable - we are likely to end with
a zero-user ID (aka deletable one) which is still actually used by other linked data.

Solution choosen here is double:
I) From 'user-space' (i.e. outliner, operators...), we check for cases where deleting datablocks
should not be allowed (indirect data or indirectly used data), and abort (with report) if needed.
II) From 'lower' level (BKE_library_remap and RNA), we also unlink from linked data,
which makes actual deletion possible and safe.

Note that with previous behavior (2.77 one), linked object would be deleted, including from linked data -
but then, once file is saved and reloaded, indirect usage would link back the deleted object,
without any instanciation in scene, which made it somehow virtual and unreachable...

With new behavior, this is no more possible, but on the other hand it means that in situations of dependency cycles
(two linked objects using each other), linked objects become impossible to delete (from user space).

Not sure what's best here, behavior with those corner cases of library linking is very poorly defined... :(

1 files changed, 14 insertions, 5 deletions

diff --git a/source/blender/blenkernel/intern/library_remap.c b/source/blender/blenkernel/intern/library_remap.c
index 10dd123a121..81543fad0fe 100644
--- a/source/blender/blenkernel/intern/library_remap.c
+++ b/source/blender/blenkernel/intern/library_remap.c
@@ -175,6 +175,11 @@ static int foreach_libblock_remap_callback(void *user_data, ID *UNUSED(id_self),
 		                            (id_remap_data->flag & ID_REMAP_FORCE_NEVER_NULL_USAGE) == 0);
 		const bool skip_never_null = (id_remap_data->flag & ID_REMAP_SKIP_NEVER_NULL_USAGE) != 0;
 
+#ifdef DEBUG_PRINT
+		printf("In %s: Remapping %s (%p) to %s (%p) (skip_indirect: %d)\n",
+		       id->name, old_id->name, old_id, new_id ? new_id->name : "<NONE>", new_id, skip_indirect);
+#endif
+
 		if ((id_remap_data->flag & ID_REMAP_FLAG_NEVER_NULL_USAGE) && (cb_flag & IDWALK_NEVER_NULL)) {
 			id->tag |= LIB_TAG_DOIT;
 		}
@@ -184,11 +189,14 @@ static int foreach_libblock_remap_callback(void *user_data, ID *UNUSED(id_self),
 		    (is_obj_editmode && (((Object *)id)->data == *id_p)) ||
 		    (skip_indirect && (is_proxy || is_indirect)))
 		{
-			if (!is_indirect && (is_never_null || is_proxy || is_obj_editmode)) {
+			if (is_indirect) {
+				id_remap_data->skipped_indirect++;
+			}
+			else if (is_never_null || is_proxy || is_obj_editmode) {
 				id_remap_data->skipped_direct++;
 			}
 			else {
-				id_remap_data->skipped_indirect++;
+				BLI_assert(0);
 			}
 			if (cb_flag & IDWALK_USER) {
 				id_remap_data->skipped_refcounted++;
@@ -461,9 +469,10 @@ void BKE_libblock_remap(Main *bmain, void *old_idv, void *new_idv, const short r
  * \param do_flag_never_null: If true, all IDs using \a idv in a 'non-NULL' way are flagged by \a LIB_TAG_DOIT flag
  * (quite obviously, 'non-NULL' usages can never be unlinked by this function...).
  */
-void BKE_libblock_unlink(Main *bmain, void *idv, const bool do_flag_never_null)
+void BKE_libblock_unlink(Main *bmain, void *idv, const bool do_flag_never_null, const bool do_skip_indirect)
 {
-	const short remap_flags = ID_REMAP_SKIP_INDIRECT_USAGE | (do_flag_never_null ? ID_REMAP_FLAG_NEVER_NULL_USAGE : 0);
+	const short remap_flags = (do_skip_indirect ? ID_REMAP_SKIP_INDIRECT_USAGE : 0) |
+	                          (do_flag_never_null ? ID_REMAP_FLAG_NEVER_NULL_USAGE : 0);
 
 	BKE_main_lock(bmain);
 
@@ -709,7 +718,7 @@ void BKE_libblock_free_us(Main *bmain, void *idv)      /* test users */
 	}
 
 	if (id->us == 0) {
-		BKE_libblock_unlink(bmain, id, false);
+		BKE_libblock_unlink(bmain, id, false, false);
 		
 		BKE_libblock_free(bmain, id);
 	}