Fix #129207: GPv3: Linked GPv2 objects disappear when reopening file.

This happens because blender 4.3 only knows about GPv3 IDs, and
therefore stores a reference to a directly linked GPv2 obdata as a GPv3
one, which cannot be found again on next file load in the GPv2 library.

The idea of this fix is, if the looup of a idcode+idname fails, to also
search for the same name but with an older, deprecated ID code.

Pull Request: https://projects.blender.org/blender/blender/pulls/129220

source/blender/blenloader/intern/readfile.cc

@@ -108,6 +108,7 @@
 #include "SEQ_utils.hh"
 
 #include "readfile.hh"
+#include "versioning_common.hh"
 
 /* Make preferences read-only. */
 #define U (*((const UserDef *)&U))
@@ -3932,9 +3933,27 @@ static BHead *find_bhead_from_code_name(FileData *fd, const short idcode, const
 static BHead *find_bhead_from_idname(FileData *fd, const char *idname)
 {
 #ifdef USE_GHASH_BHEAD
-  return static_cast<BHead *>(BLI_ghash_lookup(fd->bhead_idname_hash, idname));
+  BHead *bhead = static_cast<BHead *>(BLI_ghash_lookup(fd->bhead_idname_hash, idname));
 #else
-  return find_bhead_from_code_name(fd, GS(idname), idname + 2);
+  BHead *bhead = find_bhead_from_code_name(fd, GS(idname), idname + 2);
+#endif
+  if (LIKELY(bhead)) {
+    return bhead;
+  }
+
+  /* Expected ID was not found, attempt to load the same name, but for an older, deprecated and
+   * converted ID type. */
+  const short id_code_old = do_versions_new_to_old_idcode_get(GS(idname));
+  if (id_code_old == ID_LINK_PLACEHOLDER) {
+    return bhead;
+  }
+#ifdef USE_GHASH_BHEAD
+  char id_name_old[MAX_ID_NAME];
+  BLI_strncpy(id_name_old, idname, sizeof(id_name_old));
+  *reinterpret_cast<short *>(id_name_old) = id_code_old;
+  return static_cast<BHead *>(BLI_ghash_lookup(fd->bhead_idname_hash, id_name_old));
+#else
+  return find_bhead_from_code_name(fd, id_code_old, idname + 2);
 #endif
 }
 

source/blender/blenloader/intern/versioning_common.cc

@@ -47,6 +47,17 @@
 using blender::Map;
 using blender::StringRef;
 
+short do_versions_new_to_old_idcode_get(const short id_code_new)
+{
+  switch (id_code_new) {
+    case ID_GP:
+      /* ID_GD_LEGACY (Grease Pencil v2) is now converted to ID_GP (Grease Pencil v3). */
+      return ID_GD_LEGACY;
+    default:
+      return ID_LINK_PLACEHOLDER;
+  }
+}
+
 ARegion *do_versions_add_region_if_not_found(ListBase *regionbase,
                                              int region_type,
                                              const char *allocname,

source/blender/blenloader/intern/versioning_common.hh

@@ -24,6 +24,22 @@ struct SceneRenderLayer;
 
 using blender::FunctionRef;
 
+/**
+ * Mapping from new ID types to old converted ID types.
+ *
+ * This is used by linking code, when a directly linked data comes from a library where it exists
+ * as an older, different type of ID.
+ *
+ * Since the current blendfile will convert that older ID type to the new one as part of its
+ * versioning process, when saved, it will store a reference to that linked ID using the _new_ ID
+ * type, which will not be found on next blendfile opening/reloading when searching for that ID in
+ * the library blendfile.
+ *
+ * \returns The old, deprecated ID type code if any matches the given `id_code_new` one, otherwise
+ * `ID_LINK_PLACEHOLDER`.
+ */
+short do_versions_new_to_old_idcode_get(short id_code_new);
+
 /**
  * Check if a region of type \a region_type exists in \a regionbase. Otherwise add it after the
  * first region of type \a link_after_region_type.