Addon development - Blender crashes when Undoing, how to find the problem?

Question

I’m currently developing an addon for Blender. The addon (simply put) is just a cool material, but I’m creating some custom UI elements for changing the material nodes, in the sidebar (n-panel). I’ve already spent about 1 month on this (part time) and am close to releasing it. Though I’ve just found some big problems with it and am not sure what the reason is.

When changing some node settings with the UI elements, and then using ctrl+Z, blender crashes. Sometimes 1 undo works and it crashes on the second one, sometimes on the third one. But nearly always it crashes. It just closes. I tried to look for some crash reports to find out the reason. Even though, in the temp folder, there is a blender folder with the correct creation time the crash occurred, the folder is empty. I tried to narrow it down myself, by activating the system memory and video memory statistics, by right clicking in the bottom right, where the version number is. Even on the crash, it uses just about 30% of the memory. Another thing I tried was to have the task manager run on the side. Also here, everything stayed below 30% except the GPU. 1 second after the crash, it spiked for a few milliseconds to 100% I’m not sure if that is the reason of the crash, or just a reaction to the crash. Nevertheless, I tried updating my GTX 1070 Drivers, though I could not install the latest version. It just says an error occurred, and to try again. Another thing I tried was to do the same thing I did before (changing the same nodes and undoing), but in the shader editor, without loading the script for the UI elements. This did not crash Blender, and everything worked fine.

So the Problem seems to come from the script I’m developing. But since there is no Error message or crash report, I have no Idea what is causing it. Like I said, I’ve already spent quite some time on the script, and am quite proud of it. Question: How can I find the cause of the problem?

Answer 1

bpy_struct references cannot be safely used outside of the continuous python execution they were created in. The Undo/Redo rebuilds the references of the python exposed bpy.data and the dereferencing may access the memory which is not longer the original objects.

The Gotchas about it: https://docs.blender.org/api/current/info_gotcha.html#help-my-script-crashes-blender

For your case.

When you assign context.scene.planet_list.add() to bpy.app.driver_namespace["current_item"] in planet_list_01.LIST_OT_NewItem.execute and do the Undo/Redo the reference to the bpy_struct ListItem is invalid and using item = bpy.app.driver_namespace["current_item"] in Easy PLANETS_13.CUSTOMPLANET_PT_main.draw may lead to a crash.

You can replace the use of bpy.app.driver_namespace["current_item"] with bpy.context.scene.planet_list[bpy.context.scene.list_index] if bpy.context.scene.planet_list else None and remove the use of bpy.app.driver_namespace["current_item"] completely.

Answer 2

Be sure to read Gotchas section in the documentation for Blender's Python API.

You could reduce your script's functionality by removing code bit by bit while testing. When crashing stops - it means the thing you removed last is the cause.

Sharing the code or part of it here if possible might also help as there are many experienced Python coders around and they might find the issue, or at least come up with more specific suggestions how to approach this.