Fix chunk future chain not being bypassed during entity load

Backport of neoforged/NeoForge#99

Author: embeddedt

forge/src/main/java/org/embeddedt/modernfix/forge/mixin/bugfix/chunk_deadlock/ChunkMapLoadMixin.java

@@ -0,0 +1,56 @@
+package org.embeddedt.modernfix.forge.mixin.bugfix.chunk_deadlock;
+
+import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
+import com.llamalad7.mixinextras.injector.wrapoperation.WrapOperation;
+import net.minecraft.server.level.ChunkHolder;
+import net.minecraft.server.level.ChunkMap;
+import net.minecraft.world.level.chunk.LevelChunk;
+import net.minecraftforge.fml.util.ObfuscationReflectionHelper;
+import org.embeddedt.modernfix.ModernFix;
+import org.jetbrains.annotations.Nullable;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+
+import java.lang.reflect.Field;
+
+@Mixin(ChunkMap.class)
+public abstract class ChunkMapLoadMixin {
+    @Shadow @Nullable protected abstract ChunkHolder getVisibleChunkIfPresent(long l);
+
+    private static final Field currentlyLoadingField = ObfuscationReflectionHelper.findField(ChunkHolder.class, "currentlyLoading");
+
+    private static void setCurrentlyLoading(ChunkHolder holder, LevelChunk value) {
+        try {
+            currentlyLoadingField.set(holder, value);
+        } catch(ReflectiveOperationException e) {
+            e.printStackTrace();
+        }
+    }
+
+    /**
+     * Set currentlyLoading before calling runPostLoad and restore its old value afterwards. We track the old value
+     * to avoid conflicting with Forge if/when this feature is added.
+     */
+    @WrapOperation(method = "*", at = @At(value = "INVOKE", target = "Lnet/minecraft/world/level/chunk/LevelChunk;runPostLoad()V"))
+    private void setCurrentLoadingThenPostLoad(LevelChunk chunk, Operation<Void> operation) {
+        ChunkHolder holder = this.getVisibleChunkIfPresent(chunk.getPos().toLong());
+        if(holder != null) {
+            LevelChunk prevLoading = null;
+            try {
+                prevLoading = (LevelChunk)currentlyLoadingField.get(holder);
+            } catch(ReflectiveOperationException e) {
+                e.printStackTrace();
+            }
+            try {
+                setCurrentlyLoading(holder, chunk);
+                operation.call(chunk);
+            } finally {
+                setCurrentlyLoading(holder, prevLoading);
+            }
+        } else {
+            ModernFix.LOGGER.warn("Unable to find chunk holder for loading chunk");
+            operation.call(chunk);
+        }
+    }
+}