Recompiling an ABP that another ABP has a Property Access dependency on without recompiling/linking the other causes a crash

Development Character & Animation
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1

The crash comes from the pointers in FAnimSubsystem_PropertyAccess::Library::Indirections in the layer ABP from pointing to old/invalid functions and properties from the trash ABP_Manny, as they have not been relinked after it was recompiled when manually recompiling the character BP it depends on.

The crash was found on 5.5 but is still present in 5.6.

I was able to prevent it by adding the following code to `UAnimBlueprintGeneratedClass::Link`, it’s rather hacky as I don’t have a great understanding of the compilation process:

// Avoid infinite recursion in case of circular dependencies
static TArray<const UAnimBlueprintGeneratedClass*, TInlineAllocator<4>> LinkStack;
for (const auto& Dependent : Cast<UBlueprint>(ClassGeneratedBy)->CachedDependents)
{
	LinkStack.Push(this);
	if (UBlueprint* BP = Dependent.Get())
	{
		if (UAnimBlueprintGeneratedClass* ABPC = Cast<UAnimBlueprintGeneratedClass>(BP->GeneratedClass); IsValid(ABPC))
		{
			if (!LinkStack.Contains(ABPC))
			{
				BP->GeneratedClass->StaticLink();
			}
		}
	}
	LinkStack.Pop();
}
2

Steps to Reproduce
Open the repro project (in UE 5.5 or 5.6), which includes

  • A base animation blueprint ABP_Manny, modified to
    • depend on BP_ThirdPersonCharacter
    • have a function GetMovementComponent which returns the character movement component
    • add a simple animation layer
  • An animation layer blueprint ABP_Layer which calls this GetMovementComponent in a property access node
  • A BP_ThirdPersonCharacter_WithRefToLayer which uses this layer and is placed in the level.

Open BP_ThirdPersonCharacter in the blueprint editor and press the Compile button. This will also cause its dependencies (including ABP_Manny) to recompile

Then press play in editor to trigger the crash.

3

Thanks for the repro! We have had this issue reported internally on our crash reporter, but were unable to synthesize a local repro. This will be very helpful.

FWIW, some of this code was hardened in 5.7 (such as guarding against null functions/return properties), but I can imagine that it will still crash given the repro steps you give here.

I’ve opened a ticket to take a look at this (Unreal Engine Issues and Bug Tracker (UE\-346992\)).

4

I just checked the repro project against 5.7 and it looks like the crash no longer occurs. What is the expected behavior of that project? The player character is in t-pose following the repro steps.

For reference the P4 CL in 5.7 this was addressed was 45959023 (Github).

5

Thank you, good to know.

I’m afraid the sample project isn’t supposed to do anything except trigger the crash, I’m not familiar with animation so I just took recreated the basic circumstances necessary to reproduce it.

The crash fix looks like it would leave the invalid property access unevaluated causing incorrect behavior, but that would require a better sample to test.