Prevent mods from causing deadlocks in BlockState.getOffset

If this method is called with a ServerLevel, we switch the BlockGetter
for a safe wrapper that will only work on loaded chunks

Related: https://github.com/N1nn1/twigs/issues/6
Related: https://github.com/N1nn1/etcetera/issues/28

common/src/main/java/org/embeddedt/modernfix/chunk/SafeBlockGetter.java

@@ -0,0 +1,68 @@
+package org.embeddedt.modernfix.chunk;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.BlockGetter;
+import net.minecraft.world.level.block.Blocks;
+import net.minecraft.world.level.block.entity.BlockEntity;
+import net.minecraft.world.level.block.state.BlockState;
+import net.minecraft.world.level.chunk.ChunkAccess;
+import net.minecraft.world.level.chunk.ChunkStatus;
+import net.minecraft.world.level.material.FluidState;
+import net.minecraft.world.level.material.Fluids;
+import org.jetbrains.annotations.Nullable;
+
+public class SafeBlockGetter implements BlockGetter {
+    private final ServerLevel wrapped;
+    private final Thread mainThread;
+
+    public SafeBlockGetter(ServerLevel wrapped) {
+        this.wrapped = wrapped;
+        this.mainThread = Thread.currentThread();
+    }
+
+    public boolean shouldUse() {
+        return Thread.currentThread() != this.mainThread;
+    }
+
+    @Nullable
+    private BlockGetter getChunkSafe(BlockPos pos) {
+        // can safely call getChunkForLighting off-thread
+        BlockGetter access = this.wrapped.getChunkSource().getChunkForLighting(pos.getX() >> 4, pos.getZ() >> 4);
+        if(!(access instanceof ChunkAccess))
+            return null;
+        ChunkAccess chunk = (ChunkAccess)access;
+        if(!chunk.getStatus().isOrAfter(ChunkStatus.FULL))
+            return null;
+        return chunk;
+    }
+
+    @Override
+    public int getMaxBuildHeight() {
+        return this.wrapped.getMaxBuildHeight();
+    }
+
+    @Override
+    public int getMaxLightLevel() {
+        return this.wrapped.getMaxLightLevel();
+    }
+
+    @Nullable
+    @Override
+    public BlockEntity getBlockEntity(BlockPos pos) {
+        BlockGetter g = getChunkSafe(pos);
+        return g == null ? null : g.getBlockEntity(pos);
+    }
+
+    @Override
+    public BlockState getBlockState(BlockPos pos) {
+        BlockGetter g = getChunkSafe(pos);
+        return g == null ? Blocks.AIR.defaultBlockState() : g.getBlockState(pos);
+    }
+
+    @Override
+    public FluidState getFluidState(BlockPos pos) {
+        BlockGetter g = getChunkSafe(pos);
+        return g == null ? Fluids.EMPTY.defaultFluidState() : g.getFluidState(pos);
+    }
+}

common/src/main/java/org/embeddedt/modernfix/common/mixin/bugfix/chunk_deadlock/BlockStateBaseMixin.java

@@ -0,0 +1,22 @@
+package org.embeddedt.modernfix.common.mixin.bugfix.chunk_deadlock;
+
+import net.minecraft.world.level.BlockGetter;
+import net.minecraft.world.level.block.state.BlockBehaviour;
+import org.embeddedt.modernfix.chunk.SafeBlockGetter;
+import org.embeddedt.modernfix.duck.ISafeBlockGetter;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.ModifyVariable;
+
+@Mixin(value = BlockBehaviour.BlockStateBase.class, priority = 100)
+public class BlockStateBaseMixin {
+    @ModifyVariable(method = "getOffset", at = @At("HEAD"), argsOnly = true, index = 1)
+    private BlockGetter useSafeGetter(BlockGetter g) {
+        if(g instanceof ISafeBlockGetter) {
+            SafeBlockGetter replacement = ((ISafeBlockGetter) g).mfix$getSafeBlockGetter();
+            if(replacement.shouldUse())
+                return replacement;
+        }
+        return g;
+    }
+}

common/src/main/java/org/embeddedt/modernfix/common/mixin/bugfix/chunk_deadlock/ServerLevelMixin.java

@@ -0,0 +1,18 @@
+package org.embeddedt.modernfix.common.mixin.bugfix.chunk_deadlock;
+
+import net.minecraft.server.level.ServerLevel;
+import org.embeddedt.modernfix.chunk.SafeBlockGetter;
+import org.embeddedt.modernfix.duck.ISafeBlockGetter;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Unique;
+
+@Mixin(ServerLevel.class)
+public class ServerLevelMixin implements ISafeBlockGetter {
+    @Unique
+    private final SafeBlockGetter mfix$safeBlockGetter = new SafeBlockGetter((ServerLevel)(Object)this);
+
+    @Override
+    public SafeBlockGetter mfix$getSafeBlockGetter() {
+        return mfix$safeBlockGetter;
+    }
+}

common/src/main/java/org/embeddedt/modernfix/duck/ISafeBlockGetter.java

@@ -0,0 +1,7 @@
+package org.embeddedt.modernfix.duck;
+
+import org.embeddedt.modernfix.chunk.SafeBlockGetter;
+
+public interface ISafeBlockGetter {
+    SafeBlockGetter mfix$getSafeBlockGetter();
+}