Fix T45471: Blend file: Bad old_addr handling in mesh's customdata writing.

Issue is rather well explained in T45471: our current customdata writing code easily generates several different blocks in blend file with same 'old' address. This is bad, because those addresses are used as 'uid' during reading process (it kind of work in Blender's own reading process, by mere luck mostly, but breaks the file specs).

Solution (suggested by Campbell, thanks) implemented by this patch is to avoid duplicating everything, and instead just overwrite what we needs to skip some cdlayers on write:
* the CustomData's `totlayer` number;
* the CustomData's `layers` array of CustomDataLayer (keeping its original address using the `writestruct_at_address` helper).

New design allows us to get completely rid of the no_free flag stuff in `write_customdata()`.

Note that this implies written data is **not** directly valid from Blend PoV, since its written typemap does not match written layers (this is not an issue because typemap is rebuilt on read anyway - and it's easy to fix this if really needed).

Also, the backward compatibility saving of mface data remains an issue here, see comment in code.

Reviewers: sergey, campbellbarton

Projects: #bf_blender

Maniphest Tasks: T45471

Differential Revision: https://developer.blender.org/D1425

1 files changed, 3 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_customdata.h b/source/blender/blenkernel/BKE_customdata.h
index ab49270ca64..1c7fb79856a 100644
--- a/source/blender/blenkernel/BKE_customdata.h
+++ b/source/blender/blenkernel/BKE_customdata.h
@@ -343,6 +343,9 @@ void CustomData_to_bmesh_block(const struct CustomData *source,
 void CustomData_from_bmesh_block(const struct CustomData *source, 
                                  struct CustomData *dest, void *src_block, int dest_index);
 
+void CustomData_file_write_prepare(
+        struct CustomData *data,
+        struct CustomDataLayer **r_write_layers, struct CustomDataLayer *write_layers_buff, size_t write_layers_size);
 
 /* query info over types */
 void CustomData_file_write_info(int type, const char **structname, int *structnum);