How to handle linked data with 'fake user' set #105786

133dde41bb changed how 'fake user' flag is handled with linked data.

Previous behavior was a bug/inconsistency, by definition you cannot edit linked data, which means that its 'fake user' status is irrelevant (since it cannot be defined from the user side currently, only in the library). Therefore, it should be ignored, and the only way to decide whether to keep or not a linked ID is if it's actually used by some local data.

However, #103867 and #105687 show that this is causing issues in some cases, where users rely on the linked data's pre-defined 'Fake user' flag to keep their linked data in their production files, even if said data has no real user.

While not ideal, I think that for now we should start again considering 'fake user' flag for linked data as a real usage case.

Note: The 'fix' is trivial and safe for 3.5, but we need a quick decision on this:

diff --git a/source/blender/blenloader/intern/writefile.cc b/source/blender/blenloader/intern/writefile.cc
index 299735ccab0..eb4975bebfe 100644
--- a/source/blender/blenloader/intern/writefile.cc
+++ b/source/blender/blenloader/intern/writefile.cc
@@ -1141,6 +1141,16 @@ static bool write_file_handle(Main *mainvar,
            * asap afterward. */
           id_lib_extern(id_iter);
         }
+        else if (ID_FAKE_USERS(id_iter) > 0) {
+          /* Even though fake user is not directly editable by the user on linked data, it is a
+           * common 'work-around' to set it in library files on data-blocks that need to be linked
+           * but typically do not have an actual real user (e.g. texts, etc.).
+           * See e.g. #105687 and #103867.
+           *
+           * Would be good to find a better solution, but for now consider these as directly linked
+           * as well. */
+          id_lib_extern(id_iter);
+        }
         else {
           id_iter->tag |= LIB_TAG_INDIRECT;
           id_iter->tag &= ~LIB_TAG_EXTERN;

133dde41bb changed how 'fake user' flag is handled with linked data. Previous behavior was a bug/inconsistency, by definition you cannot edit linked data, which means that its 'fake user' status is irrelevant (since it cannot be defined from the user side currently, only in the library). Therefore, it should be ignored, and the only way to decide whether to keep or not a linked ID is if it's actually used by some local data. However, #103867 and #105687 show that this is causing issues in some cases, where users rely on the linked data's pre-defined 'Fake user' flag to keep their linked data in their production files, even if said data has no real user. While not ideal, I think that for now we should start again considering 'fake user' flag for linked data as a real usage case. ------------------- Note: The 'fix' is trivial and safe for 3.5, but we need a quick decision on this: ``` diff --git a/source/blender/blenloader/intern/writefile.cc b/source/blender/blenloader/intern/writefile.cc index 299735ccab0..eb4975bebfe 100644 --- a/source/blender/blenloader/intern/writefile.cc +++ b/source/blender/blenloader/intern/writefile.cc @@ -1141,6 +1141,16 @@ static bool write_file_handle(Main *mainvar, * asap afterward. */ id_lib_extern(id_iter); } + else if (ID_FAKE_USERS(id_iter) > 0) { + /* Even though fake user is not directly editable by the user on linked data, it is a + * common 'work-around' to set it in library files on data-blocks that need to be linked + * but typically do not have an actual real user (e.g. texts, etc.). + * See e.g. #105687 and #103867. + * + * Would be good to find a better solution, but for now consider these as directly linked + * as well. */ + id_lib_extern(id_iter); + } else { id_iter->tag |= LIB_TAG_INDIRECT; id_iter->tag &= ~LIB_TAG_EXTERN; ```

@brecht @dfelinto any opinion here?

For inspiration and consideration.

A detailed use-case for node groups

Add-ons (like animation nodes) uses node trees to store logic.
Logic is run based on a handler.
In motion graphics it is common to have a single node tree library that is linked in the scene using it. (Template)
When sending to a render farm this connection is being lost.
Work around is to store a reference to the node tree in the scene data. Although this is easy to implement, adding a automatic migration requires opening in Blender 3.4 and save, before opening in Blender 3.5.

For inspiration and consideration. *A detailed use-case for node groups* - Add-ons (like animation nodes) uses node trees to store logic. - Logic is run based on a handler. - In motion graphics it is common to have a single node tree library that is linked in the scene using it. (Template) - When sending to a render farm this connection is being lost. - Work around is to store a reference to the node tree in the scene data. Although this is easy to implement, adding a automatic migration requires opening in Blender 3.4 and save, before opening in Blender 3.5.

I think for at least 3.5 and 3.6 the right thing is to do is restore previous behavior to avoid breaking compatibility.

Longer term, I don't think it's correct for the node group use case to expect that link to remain. If it needs to retain the link there should be a user somewhere using a PointerProperty wherever is appropriate. To me that is not a workaround but the right way for a script to reference a datablock.

For the text datablocks, I think that should be part of a set of datablock types that are never auto deleted. But that's part of a bigger picture as discussed in #61209.

I think for at least 3.5 and 3.6 the right thing is to do is restore previous behavior to avoid breaking compatibility. Longer term, I don't think it's correct for the node group use case to expect that link to remain. If it needs to retain the link there should be a user somewhere using a `PointerProperty` wherever is appropriate. To me that is not a workaround but the right way for a script to reference a datablock. For the text datablocks, I think that should be part of a set of datablock types that are never auto deleted. But that's part of a bigger picture as discussed in #61209.

The proposed patch looks ok to me in isolation. Is that restoring exactly what it did before? Or are there differences still where it would now keep links to less or more data than before?

Current proposed patch only affects linked data with 'Fake user' flag set. In that case, it may save more references to linked data than before:

before 133dde41bb (3.4 etc.), IDs marked as indirectly linked were not saved, regardless of their 'Fake User' flag values.
after 133dde41bb (current 3.5), only really used linked IDs are saved, regardless of their 'Fake User' flag values.
with proposed patch, all linked IDs with 'Fake User' set are saved, regardless of whether they are really used or not.

I think that the only way to restore exactly the same behavior as before would be to revert 133dde41bb. We would then get again into the initial issue that this commit tried to address: the fact that the direct vs. indirect linked status tags are more or less unreliable.

Current proposed patch only affects linked data with 'Fake user' flag set. In that case, it may save more references to linked data than before: * before 133dde41bb (3.4 etc.), IDs marked as indirectly linked were not saved, regardless of their 'Fake User' flag values. * after 133dde41bb (current 3.5), only really used linked IDs are saved, regardless of their 'Fake User' flag values. * with proposed patch, all linked IDs with 'Fake User' set are saved, regardless of whether they are really used or not. I think that the only way to restore exactly the same behavior as before would be to revert 133dde41bb. We would then get again into the initial issue that this commit tried to address: the fact that the direct vs. indirect linked status tags are more or less unreliable.

Discussed this with @mont29 offline, and we couldn't really come up with a safe solution for 3.5 better than the proposed patch.

It does have some possibility of ending up with indirectly linked fake user datablocks after deleting directly linked datablocks, that would have to be manually cleaned up by the user. But a proper solution is probably best left for a bigger change to fake user, or at least for after 3.5.

Discussed this with @mont29 offline, and we couldn't really come up with a safe solution for 3.5 better than the proposed patch. It does have some possibility of ending up with indirectly linked fake user datablocks after deleting directly linked datablocks, that would have to be manually cleaned up by the user. But a proper solution is probably best left for a bigger change to fake user, or at least for after 3.5.

There is no ideal solution (other than reverting the behavour back to 3.4 days). So I'm okay with the proposed patch.

Committed as a60f651502

Committed as a60f651502

I wonder, could we get an option added to the purge operator (bpy.ops.outliner.orphans_purge) that ignores the fake user setting on linked data? Then we could have both behaviours sortof existing alongside each other.

I think we're gonna need something like that at the end of the current open movie production, when we try to make nice and tidy packaged files.

This option would probably be False by default, so add-ons that want the "old" behaviour would need to be updated, but that's still better than the old behaviour not being available at all.

Would be much appreciated!

I wonder, could we get an option added to the purge operator (bpy.ops.outliner.orphans_purge) that ignores the fake user setting on linked data? Then we could have both behaviours sortof existing alongside each other. I think we're gonna need something like that at the end of the current open movie production, when we try to make nice and tidy packaged files. This option would probably be False by default, so add-ons that want the "old" behaviour would need to be updated, but that's still better than the old behaviour not being available at all. Would be much appreciated!

@mets need to create a task about proper fix for this topic actually, don't want to ship 3.6 with same half-broken situation. :(

Done as #106321.

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How to handle linked data with 'fake user' set #105786