fix: handle reentrance in ServerEntityManager#unloadChunks

Cherry-picked from 4feda31

Author: ishland

c2me-rewrites-chunk-system/src/main/java/com/ishland/c2me/rewrites/chunksystem/mixin/fixes/MixinServerEntityManager.java

@@ -0,0 +1,50 @@
+package com.ishland.c2me.rewrites.chunksystem.mixin.fixes;
+
+import it.unimi.dsi.fastutil.longs.LongLinkedOpenHashSet;
+import it.unimi.dsi.fastutil.longs.LongSet;
+import net.minecraft.server.world.ServerEntityManager;
+import org.spongepowered.asm.mixin.Final;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Mutable;
+import org.spongepowered.asm.mixin.Overwrite;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(ServerEntityManager.class)
+public abstract class MixinServerEntityManager {
+
+    @Mutable
+    @Shadow @Final private LongSet pendingUnloads;
+
+    @Shadow protected abstract boolean unload(long chunkPos);
+
+    @Inject(method = "<init>", at = @At("RETURN"))
+    private void replacePendingUnloads(CallbackInfo ci) {
+        this.pendingUnloads = new LongLinkedOpenHashSet(this.pendingUnloads);
+    }
+
+    /**
+     * @author ishland
+     * @reason use alternative method for unloading
+     */
+    @Overwrite
+    private void unloadChunks() {
+        LongSet pendingUnloads = this.pendingUnloads;
+        if (!(pendingUnloads instanceof LongLinkedOpenHashSet)) {
+            // set is replaced by someone else, replace it again
+            pendingUnloads = this.pendingUnloads = new LongLinkedOpenHashSet(pendingUnloads);
+        }
+
+        // apparently vanilla also have a `this.trackingStatuses.get(pos) != EntityTrackingStatus.HIDDEN` check?????
+        // removed here because chunks in pendingUnloads should always satisfy that constraint
+        // if it does not, you have other serious problems
+        LongLinkedOpenHashSet linkedOpenHashSet = (LongLinkedOpenHashSet) pendingUnloads;
+        while (!linkedOpenHashSet.isEmpty()) {
+            long pos = linkedOpenHashSet.removeFirstLong();
+            this.unload(pos);
+        }
+    }
+
+}

c2me-rewrites-chunk-system/src/main/resources/c2me-rewrites-chunk-system.mixins.json

@@ -21,6 +21,7 @@
     "async_serialization.MixinProtoChunk",
     "async_serialization.MixinStorageIoWorker",
     "async_serialization.MixinThreadedAnvilChunkStorage",
-    "fluid_postprocessing.MixinWorldChunk"
+    "fluid_postprocessing.MixinWorldChunk",
+    "fixes.MixinServerEntityManager"
   ]
 }