Alembic: integrate cache file into the dependency graph

* The cache file datablock is now evaluated as part of the dependency graph,
  creating/freeing the Alembic file handle matching the current frame.
  Modifiers and constraints depend on this evaluation.
* Cache file handles and readers now only exist on COW datablocks, never the
  original ones.
* Object data paths are flushed back to the original for the user interface.
* The cache file keeps a list of all readers associated with its handle, and
  automatically frees them when the handle is freed. This kind of sharing of
  data across datablocks is weak but we have no better mechanism for it.

Fix T62720: Alembic sequences not working and crashing

Differential Revision: https://developer.blender.org/D4774

1 files changed, 4 insertions, 1 deletions

diff --git a/source/blender/makesdna/DNA_constraint_types.h b/source/blender/makesdna/DNA_constraint_types.h
index 3ca61400e2e..f839f22ec9f 100644
--- a/source/blender/makesdna/DNA_constraint_types.h
+++ b/source/blender/makesdna/DNA_constraint_types.h
@@ -593,9 +593,12 @@ typedef struct bObjectSolverConstraint {
 /* Transform matrix cache constraint */
 typedef struct bTransformCacheConstraint {
   struct CacheFile *cache_file;
-  struct CacheReader *reader;
   /** FILE_MAX. */
   char object_path[1024];
+
+  /* Runtime. */
+  struct CacheReader *reader;
+  char reader_object_path[1024];
 } bTransformCacheConstraint;
 
 /* ------------------------------------------ */