Weather2 Concurrent Packet Fix

A lightweight Mixin patch that fixes a long-standing bug in Weather2, where the game client gets disconnected with the following error:

Failed to encode packet 'clientbound/minecraft:custom_payload'
Caused by: java.lang.RuntimeException: Failed encoding custom payload weather2:nbt_client
Caused by: java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextNode
    at net.minecraft.nbt.CompoundTag.write

This bug has existed for a long time and has not been fixed upstream. This mod serves as a drop-in fix until Weather2 is officially patched.

Root Cause

Weather2 encodes its nbt_client packet from Netty's I/O thread while the game thread simultaneously modifies the same CompoundTag's internal HashMap. Java's HashMap is not thread-safe — concurrent iteration and modification throws ConcurrentModificationException, which Minecraft catches and turns into a client disconnect.

Fix

This mod injects into CompoundTag.write() via Mixin. Before iterating the tag's entries for serialisation, it retries snapshotting the internal map until no concurrent modification occurs:

@Inject(method = "write", at = @At("HEAD"), cancellable = true)
private void safeWrite(DataOutput output, CallbackInfo ci) throws IOException {
    // Weather2 modifies tags without any lock, so synchronized(this) is insufficient.
    // Retry until a clean snapshot is obtained.
    List<Map.Entry<String, Tag>> entries = null;
    int attempts = 0;
    while (entries == null) {
        try {
            entries = new ArrayList<>(this.tags.entrySet());
        } catch (ConcurrentModificationException e) {
            if (++attempts >= MAX_RETRIES) {
                // Write empty tag to prevent disconnect as last resort
                output.writeByte(0);
                ci.cancel();
                return;
            }
        }
    }
    for (Map.Entry<String, Tag> e : entries) { ... }
    ci.cancel();
}

A retry cap of 100 is enforced — if exceeded, an empty tag is written, and a warning is logged to prevent an infinite loop. In practice, the snapshot succeeds within 1–3 attempts.

Compatibility

Does not modify Weather2 — may works with any version of Weather2
The patch applies to net.minecraft.nbt.CompoundTag, a vanilla class — any mod that writes a CompoundTag from a non-game thread benefits from this fix
No config required — install and forget

Supported Versions

File	Loader	Minecraft
`weather2patch-x.x.x-neoforge-1.21.jar`	NeoForge	1.21.1, 1.21
`weather2patch-x.x.x-neoforge-1.20.1.jar`	NeoForge	1.20.1
`weather2patch-x.x.x-forge-1.20.1.jar`	Forge	1.20.1

Requirements

Weather2 + CoroUtil (this mod is only useful alongside Weather2, but has no hard dependency)

Notes

This mod patches at the CompoundTag level, not Weather2 specifically. If Weather2 is ever fixed upstream, this mod becomes a no-op (the injection still runs, but the behaviour is identical to vanilla since the fix is purely additive thread-safety).
On first activation, the following message is printed to console: [weather2patch] CompoundTag thread-safety patch is active.
Tested on NeoForge 21.1.220 + Weather2 2.8.6 + CoroUtil 1.3.8 at accelerated tickrate (1000 ticks/s) for ~20 minutes with zero disconnects.
I failed to replicate the issue on Forge 47.4.20 - 1.20.1, but I'm going to release a version for it anyway, hope it works 😅 (Updated to the functional version weather2patch-1.1.2-forge-1.20.1.jar).
Please avoid using the alpha version, as it may not work properly.
I'm not planning to regularly maintain or add support for other versions of this mod, as it is only a temporary fix for a modpack I am working on. I might change my mind if enough people ask me, though. Also, I'll archive this mod once the Weather2 issue is resolved in the upstream version.

Weather, Storms & Tornadoes - Concurrent Packet Fix