From ddc2233a280e9680858047cc016df1baa5e7d2fa Mon Sep 17 00:00:00 2001
From: andriihorpenko <andrii.horpenko@gmail.com>
Date: Sat, 5 Sep 2026 15:04:32 +0300
Subject: [PATCH] Fix concurrent modification errors when serializing
 item/fluid storage off-thread

---
 .../lowdraglib2/misc/FluidStorage.java        |  5 +++--
 .../lowdraglib2/misc/ItemStackTransfer.java   | 20 +++++++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/main/java/com/lowdragmc/lowdraglib2/misc/FluidStorage.java b/src/main/java/com/lowdragmc/lowdraglib2/misc/FluidStorage.java
index aaa428b8e..f09a42192 100644
--- a/src/main/java/com/lowdragmc/lowdraglib2/misc/FluidStorage.java
+++ b/src/main/java/com/lowdragmc/lowdraglib2/misc/FluidStorage.java
@@ -53,8 +53,9 @@ public FluidStorage copy() {
     @Override
     public CompoundTag serializeNBT(@NotNull HolderLookup.Provider provider) {
         var tag = new CompoundTag();
-        if (!fluid.isEmpty()) {
-            tag.put("fluid", fluid.save(provider));
+        var snapshot = fluid.copy();
+        if (!snapshot.isEmpty()) {
+            tag.put("fluid", snapshot.save(provider));
         }
         tag.putInt("capacity", capacity);
         return tag;
diff --git a/src/main/java/com/lowdragmc/lowdraglib2/misc/ItemStackTransfer.java b/src/main/java/com/lowdragmc/lowdraglib2/misc/ItemStackTransfer.java
index 8bfade918..c0d210369 100644
--- a/src/main/java/com/lowdragmc/lowdraglib2/misc/ItemStackTransfer.java
+++ b/src/main/java/com/lowdragmc/lowdraglib2/misc/ItemStackTransfer.java
@@ -4,7 +4,10 @@
 import com.lowdragmc.lowdraglib2.syncdata.IContentChangeAware;
 import lombok.Getter;
 import lombok.Setter;
+import net.minecraft.core.HolderLookup;
 import net.minecraft.core.NonNullList;
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.nbt.ListTag;
 import net.minecraft.world.item.ItemStack;
 import net.neoforged.neoforge.items.ItemStackHandler;
 
@@ -53,6 +56,23 @@ protected void onContentsChanged(int slot) {
         onContentsChanged.run();
     }
 
+    @Override
+    public CompoundTag serializeNBT(HolderLookup.Provider provider) {
+        var snapshot = stacks;
+        var list = new ListTag();
+        for (int i = 0; i < snapshot.size(); i++) {
+            var stack = snapshot.get(i).copy();
+            if (stack.isEmpty()) continue;
+            var itemTag = new CompoundTag();
+            itemTag.putInt("Slot", i);
+            list.add(stack.save(provider, itemTag));
+        }
+        var tag = new CompoundTag();
+        tag.put("Items", list);
+        tag.putInt("Size", snapshot.size());
+        return tag;
+    }
+
     public ItemStackTransfer copy() {
         var copiedStack = NonNullList.withSize(stacks.size(), ItemStack.EMPTY);
         for (int i = 0; i < stacks.size(); i++) {