This adds a new `ModifierData.persistent_uid` integer property with the following properties:
* It's unique within the object.
* Match between the original and evaluated object.
* Stable across Blender sessions.
* Stable across renames and reorderings of modifiers.
Potential use-cases:
* Everywhere where we currently use the name as identifier. For example,
`ModifierComputeContext` and `ModifierViewerPathElem`.
* Can be used as part of a key in `IDCacheKey` to support caches that stay
in-tact across undo steps.
* Can be stored in the `SpaceNode` to identify the modifier whose geometry node
tree is currently pinned (this could use the name currently, but that hasn't been
implemented yet).
This new identifier has some overlap with `ModifierData.session_uid`, but there
are some differences:
* `session_uid` is unique within the entire Blender session (except for duplicates
between the original and evaluated data blocks).
* `session_uid` is not stable across Blender sessions.
Especially due to the first difference, it's not immediately obvious that the new
`persistent_uid` can fulfill all use-cases of the existing `session_uid`. Nevertheless,
this seems likely and will be cleaned up separately.
Unfortunately, there is not a single place where modifiers are added to objects currently.
Therefore, there are quite a few places that need to ensure valid identifiers. I tried to catch
all the places, but it's hard to be sure. Therefore, I added an assert in `object_copy_data`
that checks if all identifiers are valid. This way, we should be notified relatively quickly if
issues are caused by invalid identifiers.
Pull Request: https://projects.blender.org/blender/blender/pulls/117347
1497005054