Initial implementation of local ID re-use when appending.

This commit adds to ID struct a new optional 'weak reference' to a
linked ID (in the form of a blend file library path and full ID name).

This can then be used on next append to try to find a matching local ID
instead of re-making the linked data local again.

Ref. T90545

NOTE: ID re-use will be disabled for regular append for the time being
(3.0 release), and only used for assets. Therefore, this commit should
not change anything user-wise.

Differential Revision: https://developer.blender.org/D12545

1 files changed, 28 insertions, 1 deletions

diff --git a/source/blender/makesdna/DNA_ID.h b/source/blender/makesdna/DNA_ID.h
index 5b7c99f2545..d829d707a71 100644
--- a/source/blender/makesdna/DNA_ID.h
+++ b/source/blender/makesdna/DNA_ID.h
@@ -392,7 +392,14 @@ typedef struct ID {
    *   that references this ID (the bones of an armature or the modifiers of an object for e.g.).
    */
   void *py_instance;
-  void *_pad1;
+
+  /**
+   * Weak reference to an ID in a given library file, used to allow re-using already appended data
+   * in some cases, instead of appending it again.
+   *
+   * May be NULL.
+   */
+  struct LibraryWeakReference *library_weak_reference;
 } ID;
 
 /**
@@ -426,6 +433,26 @@ typedef struct Library {
   short versionfile, subversionfile;
 } Library;
 
+/**
+ * A weak library/ID reference for local data that has been appended, to allow re-using that local
+ * data instead of creating a new copy of it in future appends.
+ *
+ * NOTE: This is by design a week reference, in other words code should be totally fine and perform
+ * a regular append if it cannot find a valid matching local ID.
+ *
+ * NOTE: There should always be only one single ID in current Main matching a given linked
+ * reference.
+ */
+typedef struct LibraryWeakReference {
+  /**  Expected to match a `Library.filepath`. */
+  char library_filepath[1024];
+
+  /** MAX_ID_NAME. May be different from the current local ID name. */
+  char library_id_name[66];
+
+  char _pad[2];
+} LibraryWeakReference;
+
 /* for PreviewImage->flag */
 enum ePreviewImage_Flag {
   PRV_CHANGED = (1 << 0),