Depsgraph: Initial groundwork for copy-on-write support

< Dependency graph Copy-on-Write >
 --------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

This is an initial commit of Copy-on-write support added to dependency graph.
Main priority for now: get playback (Alt-A) and all operators (selection,
transform etc) to work with the new concept of clear separation between
evaluated data coming from dependency graph and original data coming from
.blend file (and stored in bmain).

= How does this work? =

The idea is to support Copy-on-Write on the ID level. This means, we duplicate
the whole ID before we cann it's evaluaiton function. This is currently done
in the following way:

- At the depsgraph construction time we create "shallow" copy of the ID
  datablock, just so we know it's pointer in memory and can use for function
  bindings.

- At the evaluaiton time, the copy of ID get's "expanded" (needs a better
  name internally, so it does not conflict with expanding datablocks during
  library linking), which means the content of the datablock is being
  copied over and all IDs are getting remapped to the copied ones.

  Currently we do the whole copy, in the future we will support some tricks
  here to prevent duplicating geometry arrays (verts, edges, loops, faces
  and polys) when we don't need that.

- Evaluation functions are operating on copied datablocks and never touching
  original datablock.

- There are some cases when we need to know non-ID pointers for function
  bindings. This mainly applies to scene collections and armatures. The
  idea of dealing with this is to "expand" copy-on-write datablock at
  the dependency graph build time. This might introduce some slowdown to the
  dependency graph construction time, but allows us to have minimal changes
  in the code and avoid any hash look-up from evaluation function (one of
  the ideas to avoid using pointers as function bindings is to pass name
  of layer or a bone to the evaluation function and look up actual data based
  on that name).

  Currently there is a special function in depsgraph which does such a
  synchronization, in the future we might want to make it more generic.

At some point we need to synchronize copy-on-write version of datablock with
the original version. This happens, i.e., when we change active object or
change selection. We don't want any actual evaluation of update flush happening
for such thins, so now we have a special update tag:

  DEG_id_tag_update((id, DEG_TAG_COPY_ON_WRITE)

- For the render engines we now have special call for the dependency graph to
  give evaluated datablock for the given original one. This isn't fully ideal
  but allows to have Cycles viewport render.

  This is definitely a subject for further investigation / improvement.

This call will tag copy-on-write component tagged for update without causing
updates to be flushed to any other objects, causing chain reaction of updates.
This tag is handy when selection in the scene changes.

This basically summarizes ideas underneath this commit. The code should be
reasonably documented.

Here is a demo of dependency graph with all copy-on-write stuff in it:

  https://developer.blender.org/F635468

= What to expect to (not) work? =

- Only meshes are properly-ish aware of copy-on-write currently, Non-mesh
  geometry will probably crash or will not work at all.

- Armatures will need similar depsgraph built-time expansion of the copied
  datablock.

- There are some extra tags / relations added, to keep things demo-able but
  which are slowing things down for evaluation.

- Edit mode works for until click selection is used (due to the selection
  code using EditDerivedMesh created ad-hoc).

- Lots of tools will lack tagging synchronization of copied datablock for
  sync with original ID.

= How to move forward? =

There is some tedious work related on going over all the tools, checking
whether they need to work with original or final evaluated object and make
the required changes.

Additionally, there need synchronization tag done in fair amount of tools
and operators as well. For example, currently it's not possible to change
render engine without re-opening the file or forcing dependency graph for
re-build via python console.

There is also now some thoughts required about copying evaluated properties
between objects or from collection to a new object. Perhaps easiest way
would be to move base flag flush to Object ID node and tag new objects for
update instead of doing manual copy.

here is some WIP patch which moves such evaluaiton / flush:

  https://developer.blender.org/F635479

Lots of TODOs in the code, with possible optimization.

= How to test? =

This is a feature under heavy development, so obviously it is disabled by
default. The only reason it goes to 2.8 branch is to avoid possible merge
hell.

In order to enable this feature use WITH_DEPSGRAPH_COPY_ON_WRITE CMake
configuration option.

1 files changed, 3 insertions, 0 deletions

diff --git a/source/blender/depsgraph/intern/nodes/deg_node_operation.cc b/source/blender/depsgraph/intern/nodes/deg_node_operation.cc
index 7467264f612..84b3d33f494 100644
--- a/source/blender/depsgraph/intern/nodes/deg_node_operation.cc
+++ b/source/blender/depsgraph/intern/nodes/deg_node_operation.cc
@@ -76,6 +76,9 @@ string OperationDepsNode::full_identifier() const
 
 void OperationDepsNode::tag_update(Depsgraph *graph)
 {
+	if (flag & DEPSOP_FLAG_SKIP_FLUSH) {
+		flag &= ~DEPSOP_FLAG_SKIP_FLUSH;
+	}
 	if (flag & DEPSOP_FLAG_NEEDS_UPDATE) {
 		return;
 	}