While the evaluated result is not well defined, we expect Blender to not crash
when there are dependency cycles.
The evaluation of one object often takes the evaluated geometry of another
object into account. This works fine if the other object is already fully
evaluated. However, if there is a dependency cycle, the other object may not be
evaluated already. Currently, we have no way to check for this and were mostly
just relying on luck that the other objects geometry is in some valid state
(even if it's not the fully evaluated geometry).
This patch adds the ability to explicitly check if an objects geometry is fully
evaluated already, so that it can be accessed by other objects. If there are not
dependency cycles, this should always be true. If not, it may be false
sometimes, and in this case the other objects geometry should be ignored. The
same also applies to the object transforms and the geometry of a collection.
For that, new functions are added in `DEG_depsgraph_query.hh`. Those should be
used whenever accessing another objects or collections object during depsgraph
evaluation. More similar functions may be added in the future.
```
bool DEG_object_geometry_is_evaluated(const Object &object);
bool DEG_object_transform_is_evaluated(const Object &object);
bool DEG_collection_geometry_is_evaluated(const Collection &collection);
```
To determine if the these components are fully evaluated, a reference to the
corresponding depsgraph is needed. A possible solution to that is to pass the
depsgraph through the call stack to these functions. While possible, there are a
couple of annoyances. For one, the parameter would need to be added in many new
places. I don't have an exact number, but it's like 50 or so. Another
complication is that under some circumstances, multiple depsgraphs may have to
be passed around, for example when evaluating node tools (also see
`GeoNodesOperatorDepsgraphs`).
To simplify the patch and other code in the future, a different route is taken
where the depsgraph pointer is added to `ID_Runtime`, making it readily
accessible similar to the `ID.orig_id`. The depsgraph pointer is set in the same
place where the `orig_id` is set.
As a nice side benefit, this also improves the situation in simple cases like
having two cubes with a boolean modifier and they union each other.
Pull Request: https://projects.blender.org/blender/blender/pulls/123444
ada367a0e9