Fix T77774: New undo code broken by 'make local' behavior.

This is actually a nice issue due to too much optimization...
* Making an ID local just reuse the linked one whenever possible, instead of
  actually making a copy of it.
* Therefore, the collection containing that ID is seen as unchanged, since
  the pointer itself remained the same.
* But on undo step, there is no way to reuse that local object, which then
  gets deleted, and linked one gets re-created - at a different address.
* Collection, however, since unchanged, is not updated at all and thus keeps
  reference to the to-be-deleted local object, instead of the linked one.
* Issue gets even worse with viewlayers, this leads to the crash.

To address this, this patch adds a 'virtual' update flags that does nothing
in update case, but will ensure that the affected IDs using the one made local
are properly detected as changed across the relevant undo step.

Note that the recalc flags were chosen mostly for a logical reason, and also
because they are already properly dealt with and cleared by undo code,
so this looks like the optimal solution.

Reviewed By: brecht

Maniphest Tasks: T77774

Differential Revision: https://developer.blender.org/D8006

1 files changed, 9 insertions, 0 deletions

diff --git a/source/blender/makesdna/DNA_ID.h b/source/blender/makesdna/DNA_ID.h
index 7ce84e0092c..0cf45fa4bcc 100644
--- a/source/blender/makesdna/DNA_ID.h
+++ b/source/blender/makesdna/DNA_ID.h
@@ -680,6 +680,15 @@ typedef enum IDRecalcFlag {
    * input file or for color space changes. */
   ID_RECALC_SOURCE = (1 << 23),
 
+  /* Virtual recalc tag/marker required for undo in some cases, where actual data does not change
+   * and hence do not require an update, but conceptually we are dealing with something new.
+   *
+   * Current known case: linked IDs made local without requiring any copy. While their users do not
+   * require any update, they have actually been 'virtually' remapped from the linked ID to the
+   * local one.
+   */
+  ID_RECALC_TAG_FOR_UNDO = (1 << 24),
+
   /***************************************************************************
    * Pseudonyms, to have more semantic meaning in the actual code without
    * using too much low-level and implementation specific tags. */