[1.20.2] Connection between Neoforge and Vanilla Server/Client broken

Digging into this, it turns out this is caused by a problematic extension of a vanilla packet, specifically the ClientIntentionPacket. The issue is that we cannot, ever, add additional elements to vanilla packets. Adding additional stuff to existing elements (i.e. what the 1.20.1 and earlier patch to this packet did with the FML version appended to the host name) is fine as long as vanilla can ignore it on receipt. Adding additional elements however will break network compatibility with vanilla completely for two reasons:

Vanilla checks that the packet buffer is empty after decoding and throws if it's not (causes the ping fail and immediate disconnect for NF client -> vanilla server)
Neo will read out of bounds of the buffer and blow up because vanilla doesn't provide that data (causes the ping fail and immediate disconnect for vanilla client -> NF server)

It is worth noting that there is an additional issue with NF client -> vanilla server connections: NetworkHooks.handleClientLoginSuccess() is currently never called, causing server configs to not be initialized to defaults and then subsequently blowing up with "config not loaded".

This could technically be resolved with a band-aid fix by basically going back to the original approach of appending the FML version to the host name (this is possible due to a small trick with records that allows re-assigning values) and then calling NetworkHooks.handleClientLoginSuccess() from ClientConfigurationPacketListenerImpl#handleConfigurationFinished(). This works in my cursory test in both directions. The proper way to fix this however is to go through with the planned networking rework to properly support the configuration phase (which should provide better support for proxies and lobby servers) and implement the handshake spec hashed out with and already implemented by Fabric in https://github.com/FabricMC/fabric/pull/3244.

For sake of completeness, this is what the relevant part of the ClientIntentionPacket patch would look like.

public ClientIntentionPacket {
   if (fmlVersion ==  null) {
      fmlVersion = net.neoforged.neoforge.network.NetworkHooks.getFMLVersion(hostName);
      hostName = hostName.split("\0")[0];
   }
}

@Deprecated
public ClientIntentionPacket(int protocolVersion, String hostName, int port, ClientIntent intention) {
   this(protocolVersion, hostName, port, intention, net.neoforged.neoforge.network.NetworkConstants.NETVERSION);
}

public ClientIntentionPacket(FriendlyByteBuf p_179801_) {
   this(p_179801_.readVarInt(), p_179801_.readUtf(255), p_179801_.readUnsignedShort(), ClientIntent.byId(p_179801_.readVarInt()), null);
}

@Override
public void write(FriendlyByteBuf p_134737_) {
   p_134737_.writeVarInt(this.protocolVersion);
   p_134737_.writeUtf(this.hostName + "\0" + this.fmlVersion + "\0");
   p_134737_.writeShort(this.port);
   p_134737_.writeVarInt(this.intention.id());
}

neoforged / NeoForge

[1.20.2] Connection between Neoforge and Vanilla Server/Client broken #205