Whenever a Baby Skeleton or Baby Wither Skeleton spawns, the game crashes. Using the Fabric modloader.

Crash logs : https://paste.ee/p/CPomi (regular skeleton) https://paste.ee/p/h6NF5 (wither skeleton)
Link to mod : https://modrinth.com/mod/tiny-skeletons
I also reported the bug to the mod's author.

I've managed to reproduce this crash on both Fabric and Forge. I was initially very confused about the cause, since this crash should not ordinarily be possible.

Normally, Patched tries to capture the FallbackResourceManager instance since its inject is in a static method. This is fine because no one calls wrapForDebug and all the vanilla ones that do are already handled. The instance is also stored in a ThreadLocal so there can't in theory be any concurrency issues. This of course means that under normal circumstances this crash is impossible.

However, these aren't normal circumstances, and the stacktrace is very misleading about this. I believe what's happened is the following:

1. A tiny skeleton tries to render itself
2. The skeleton then renders its held item
3. This flushes the render buffer, causing Minecraft to render the skeleton itself (so it can setup the state for the item)
4. To do this, it must grab the skeleton's texture
5. The MultiPackResourceManager invokes the corresponding FallbackResourceManager to grab the texture
6. Patched captures the FallbackResourceManager instance
7. The FallbackResourceManager asks the PackResources for the resource

And now this is where things get interesting:

8. The BabySkeletonPackResources asks the MultiPackResourceManager for the vanilla skeleton texture
9. The MultiPackResourceManager invokes the corresponding FallbackResourceManager to grab the texture
10. Patched captures the FallbackResourceManager instance
11. The FallbackResourceManager asks the PackResources for the resource
12. Patched releases the FallbackResourceManager instance (!!!)
13. The resource is returned to the BabySkeletonPackResources
14. The BabySkeletonPackResources returns the raw IoSupplier

And here's the actual crash part:

14. The FallbackResourceManager receives the IoSupplier and calls wrapForDebug on it
15. Patched panics because it no longer has a FallbackResourceManager instance to work with (since this was released in step 12)

In summary, the problem is that the MultiPackResourceManager is queried twice when retrieving a resource, which causes Patched to become confused and crash.

Right now, I can see two options for addressing this problem:

• Patched could capture the FallbackResourceManager right before the createResource() call, instead of at the top of getResource()
• Tiny Skeletons could query a specific PackResources rather than fishing it out of the MultiPackResourceManager, although this is complicated and comes with its own problems

I can attempt to fix this at some point soon for 1.21.1 (since I'm sure it's still an issue there), but I'm not sure when I'll end up backporting it to 1.20.1. I've had a backport planned for the forty some other features added since then but haven't gotten around to actually backporting them yet.

This has now been fixed in the most recent 1.20.1 release (3.3.0), which should be available on Modrinth and CurseForge shortly. I will leave this issue open until I've released a build for 1.21.1 with the fix.