Warning ! VERY Dangerous operation with Modifiers !

I have already submitted this bug in the 2.4x section, as bug #25271, but as it concerns also 2.5x, I report it here as well.

I have lost many hours of work because of an error I did using Shrinkwrap Modifiers, and before I understood what was the matter, I had to redo many things from scratch !

I wanted to tell you this venture, because an apparently simple error can have heavy consequences and leads to a blend file that seems to work perfectly well but can't be opened any more once saved ! I think that something should be done to avoid Blender entering in such conditions and corrupting data.

I was building a setup with several mesh objects using Shrinkwrap modifiers to match the surface of the terrain, in the desert scene I'm working on for a while. If you know my "Tracks Creator" creating an animated texture to be used in a second pass for displacement mapping, the last method I'm working on uses several Shrinkwrap Modifiers and allows to generate detailed and realistic tires tracks in relief in only one pass, directly visible in the 3D viewport.

For that, I used several objects with shrinkwrap modifiers applied in cascade. I did an error in an object name in the Modifier target field and the last object of the chain pointed to the first one. So it made a loop.

When using constraints, loops are usually not a problem, and sometimes they can even be an advantage, allowing very interesting things like the self tracking constraint that I use for the rear wheels of cars and trucks in my cars setups !

So I have not suspected this at first glance, and before I found out what happened, I came with a serie of unusable blend files ! When working, I save often my work in new files with incremental versions names, but as the problem occurs only once the file is closed, and because I haven't closed Blender for almost two days, all saved files during this session have been corrupted !

Once closed, the blend file containing a Modifier loop can't be opened. If you look at the ressources usage in Windows task manager, you will see that memory amount used increases up to the level where the whole memory (8GB in my case) is used and the computer becomes unresponsive and has to be hard-rebooted ! As the increase of memory use is not very fast, with enough memory in the machine and if you act soon, it is possible to kill Blender before the complete crash. I mean if you are aware of what is happening and if you have a eye on the ressource monitor opened !

Before, when (rarely) I encountered a file refusing to open, I had created a new file and imported the whole content of the locked file or opened the last working file and imported the recently added objects of the locked file, and this solved the problem.

With Modifiers, before I understood the nature of the trouble, I tried to import the last added objects (included in a group), but as soon I imported the litigious objects, the computer entered in a loop !

It happens as well with Blender 2.49b 32 and 64 bits versions and 2.55.

As files containing this kind of "error" can't be opened and lead to a crash, I haven't built a simplified one for demo. If you want one, just ask me.

My system config :

Asus P5k Motherboard - Intel Quad Xtreme 9650 3GHz - 8GB RAM - GTX 260 - Windows 7 Pro x64.

Happy Blending !

Best regards,

Philippe.