Initializer merging behavior seems to have changed in 0.8.6 snapshots

[zml2008]

I'm afraid I've been a bit checked out of things for a while, but just poking around the the 0.8.6 snapshots on Sponge (see SpongePowered/Sponge#3937), I'm getting a new runtime failure based on the same Mixins and target classes. Looking at the exported transformed class file, the initializer merging isn't matching what I'd expect.

The Mixin in question

Stacktrace

[19:50:01] [main/ERROR]: Failed to invoke main class org.spongepowered.vanilla.applaunch.Main due to an error: 
Caused by: java.lang.ExceptionInInitializerError
	at net.minecraft.server.Bootstrap.bootStrap(Bootstrap.java:50)
	at org.spongepowered.vanilla.launch.IntegrationTestLaunch.performIntegrationTests(IntegrationTestLaunch.java:70)
	at org.spongepowered.vanilla.launch.VanillaBootstrap.perform(VanillaBootstrap.java:64)
	at org.spongepowered.vanilla.launch.IntegrationTestLaunch.performBootstrap(IntegrationTestLaunch.java:63)
	at org.spongepowered.vanilla.launch.VanillaLaunch.launchPlatform(VanillaLaunch.java:118)
	at org.spongepowered.vanilla.launch.IntegrationTestLaunch.launch(IntegrationTestLaunch.java:49)
	... 23 more
Caused by: java.lang.NullPointerException: Cannot invoke "java.util.Map.put(Object, Object)" because "this.impl$entries" is null
	at net.minecraft.core.MappedRegistry.bridge$register(MappedRegistry.java:1100)
	at net.minecraft.core.MappedRegistry.handler$zef000$impl$cacheRegistryEntry(MappedRegistry.java:1089)
	at net.minecraft.core.MappedRegistry.registerMapping(MappedRegistry.java:180)
	at net.minecraft.core.MappedRegistry.register(MappedRegistry.java:185)
	at net.minecraft.core.registries.BuiltInRegistries.internalRegister(BuiltInRegistries.java:221)
	at net.minecraft.core.registries.BuiltInRegistries.registerDefaultedWithIntrusiveHolders(BuiltInRegistries.java:213)
	at net.minecraft.core.registries.BuiltInRegistries.registerDefaultedWithIntrusiveHolders(BuiltInRegistries.java:201)
	at net.minecraft.core.registries.BuiltInRegistries.<clinit>(BuiltInRegistries.java:119)
	... 29 more

impl$entries is a field added by the Mixin with an initializer, so I wouldn't expect it to be null here. Looking at the dump, I get the following decompile output (comments mine) -- it seems the jump checking $$2 is jumping to right after those initializers:

  public MappedRegistry(ResourceKey<? extends net.minecraft.core.Registry<T>> $$0, Lifecycle $$1, boolean $$2) {
   /* [.. trimmed original target class initializer */
    if ($$2) {
      this.unregisteredIntrusiveHolders = new IdentityHashMap();
      /* our transformations start here */
      this.impl$entries = new LinkedHashMap();
      this.impl$isDynamic = true;
    }

    this.handler$zef000$impl$setType($$0, $$1, $$2, new CallbackInfo("<init>", false));
  }

Method bytecode

  // access flags 0x1
  // signature (Lnet/minecraft/resources/ResourceKey<+Lnet/minecraft/core/Registry<TT;>;>;Lcom/mojang/serialization/Lifecycle;Z)V
  // declaration: void <init>(net.minecraft.resources.ResourceKey<? extends net.minecraft.core.Registry<T>>, com.mojang.serialization.Lifecycle, boolean)
  public <init>(Lnet/minecraft/resources/ResourceKey;Lcom/mojang/serialization/Lifecycle;Z)V
   L0
    LINENUMBER 90 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
   L1
    LINENUMBER 40 L1
    ALOAD 0
    NEW it/unimi/dsi/fastutil/objects/ObjectArrayList
    DUP
    SIPUSH 256
    INVOKESPECIAL it/unimi/dsi/fastutil/objects/ObjectArrayList.<init> (I)V
    PUTFIELD net/minecraft/core/MappedRegistry.byId : Lit/unimi/dsi/fastutil/objects/ObjectList;
   L2
    LINENUMBER 40 L2
    ALOAD 0
    NEW it/unimi/dsi/fastutil/objects/Object2IntOpenCustomHashMap
    DUP
    INVOKESTATIC net/minecraft/Util.identityStrategy ()Lit/unimi/dsi/fastutil/Hash$Strategy;
    INVOKESPECIAL it/unimi/dsi/fastutil/objects/Object2IntOpenCustomHashMap.<init> (Lit/unimi/dsi/fastutil/Hash$Strategy;)V
    INVOKEDYNAMIC accept()Ljava/util/function/Consumer; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.metafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
      // arguments:
      (Ljava/lang/Object;)V, 
      // handle kind 0x6 : INVOKESTATIC
      net/minecraft/core/MappedRegistry.lambda$new$0(Lit/unimi/dsi/fastutil/objects/Object2IntOpenCustomHashMap;)V, 
      (Lit/unimi/dsi/fastutil/objects/Object2IntOpenCustomHashMap;)V
    ]
    INVOKESTATIC net/minecraft/Util.make (Ljava/lang/Object;Ljava/util/function/Consumer;)Ljava/lang/Object;
    CHECKCAST it/unimi/dsi/fastutil/objects/Object2IntMap
    PUTFIELD net/minecraft/core/MappedRegistry.toId : Lit/unimi/dsi/fastutil/objects/Object2IntMap;
   L3
    LINENUMBER 56 L3
    ALOAD 0
    NEW java/util/HashMap
    DUP
    INVOKESPECIAL java/util/HashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.byLocation : Ljava/util/Map;
   L4
    LINENUMBER 56 L4
    ALOAD 0
    NEW java/util/HashMap
    DUP
    INVOKESPECIAL java/util/HashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.byKey : Ljava/util/Map;
   L5
    LINENUMBER 56 L5
    ALOAD 0
    NEW java/util/IdentityHashMap
    DUP
    INVOKESPECIAL java/util/IdentityHashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.byValue : Ljava/util/Map;
   L6
    LINENUMBER 56 L6
    ALOAD 0
    NEW java/util/IdentityHashMap
    DUP
    INVOKESPECIAL java/util/IdentityHashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.lifecycles : Ljava/util/Map;
   L7
    LINENUMBER 56 L7
    ALOAD 0
    NEW java/util/IdentityHashMap
    DUP
    INVOKESPECIAL java/util/IdentityHashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.tags : Ljava/util/Map;
   L8
    LINENUMBER 56 L8
    ALOAD 0
    NEW net/minecraft/core/MappedRegistry$1
    DUP
    ALOAD 0
    INVOKESPECIAL net/minecraft/core/MappedRegistry$1.<init> (Lnet/minecraft/core/MappedRegistry;)V
    PUTFIELD net/minecraft/core/MappedRegistry.lookup : Lnet/minecraft/core/HolderLookup$RegistryLookup;
   L9
    LINENUMBER 90 L9
    ALOAD 1
    INVOKEDYNAMIC get(Lnet/minecraft/resources/ResourceKey;)Ljava/util/function/Supplier; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.metafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
      // arguments:
      ()Ljava/lang/Object;, 
      // handle kind 0x6 : INVOKESTATIC
      net/minecraft/core/MappedRegistry.lambda$new$1(Lnet/minecraft/resources/ResourceKey;)Ljava/lang/String;, 
      ()Ljava/lang/String;
    ]
    INVOKESTATIC net/minecraft/server/Bootstrap.checkBootstrapCalled (Ljava/util/function/Supplier;)V
   L10
    LINENUMBER 91 L10
    ALOAD 0
    ALOAD 1
    PUTFIELD net/minecraft/core/MappedRegistry.key : Lnet/minecraft/resources/ResourceKey;
   L11
    LINENUMBER 92 L11
    ALOAD 0
    ALOAD 2
    PUTFIELD net/minecraft/core/MappedRegistry.registryLifecycle : Lcom/mojang/serialization/Lifecycle;
   L12
    LINENUMBER 93 L12
    ILOAD 3
    IFEQ L13
   L14
    LINENUMBER 94 L14
    ALOAD 0
    NEW java/util/IdentityHashMap
    DUP
    INVOKESPECIAL java/util/IdentityHashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.unregisteredIntrusiveHolders : Ljava/util/Map;
    ALOAD 0
    NEW java/util/LinkedHashMap
    DUP
    INVOKESPECIAL java/util/LinkedHashMap.<init> ()V
    PUTFIELD net/minecraft/core/MappedRegistry.impl$entries : Ljava/util/Map;
    ALOAD 0
    ICONST_1
    PUTFIELD net/minecraft/core/MappedRegistry.impl$isDynamic : Z
   L13
    LINENUMBER 96 L13
   FRAME FULL [net/minecraft/core/MappedRegistry net/minecraft/resources/ResourceKey com/mojang/serialization/Lifecycle I] []
    ALOAD 0
    ALOAD 1
    ALOAD 2
    ILOAD 3
    NEW org/spongepowered/asm/mixin/injection/callback/CallbackInfo
    DUP
    LDC "<init>"
    ICONST_0
    INVOKESPECIAL org/spongepowered/asm/mixin/injection/callback/CallbackInfo.<init> (Ljava/lang/String;Z)V
    INVOKESPECIAL net/minecraft/core/MappedRegistry.handler$zef000$impl$setType (Lnet/minecraft/resources/ResourceKey;Lcom/mojang/serialization/Lifecycle;ZLorg/spongepowered/asm/mixin/injection/callback/CallbackInfo;)V
    RETURN
   L15
    LOCALVARIABLE this Lnet/minecraft/core/MappedRegistry; L0 L15 0
    // signature Lnet/minecraft/core/MappedRegistry<TT;>;
    // declaration: this extends net.minecraft.core.MappedRegistry<T>
    LOCALVARIABLE $$0 Lnet/minecraft/resources/ResourceKey; L0 L15 1
    // signature Lnet/minecraft/resources/ResourceKey<+Lnet/minecraft/core/Registry<TT;>;>;
    // declaration: $$0 extends net.minecraft.resources.ResourceKey<? extends net.minecraft.core.Registry<T>>
    LOCALVARIABLE $$1 Lcom/mojang/serialization/Lifecycle; L0 L15 2
    LOCALVARIABLE $$2 Z L0 L15 3
    MAXSTACK = 8
    MAXLOCALS = 4

[Mumfrey]

Thanks for this, I have changed the structure of how initialisers are handled to pull functionality out of the applicator and into the target so that information can be shared with CTOR_HEAD, but it's likely this has introduced this issue. I'll have a look into it. This was the entire point of releasing a snaphot first.