Introduce a new feature that lets mods know if optional mods and optional network channels are present in the remote.

Allow connections from forge versions that don't have the vanilla splitter.

src/main/java/net/minecraftforge/fml/network/FMLConnectionData.java

@@ -0,0 +1,69 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.fml.network;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import net.minecraft.util.ResourceLocation;
+
+import java.util.List;
+import java.util.Map;
+
+public class FMLConnectionData
+{
+    private ImmutableList<String> modList;
+    private ImmutableMap<ResourceLocation, String> channels;
+
+    /* package private */ FMLConnectionData(List<String> modList, Map<ResourceLocation, String> channels)
+    {
+        this.modList = ImmutableList.copyOf(modList);
+        this.channels = ImmutableMap.copyOf(channels);
+    }
+
+    /**
+     * Returns the list of mods present in the remote.
+     * <b>WARNING: This list is not authoritative.
+     *    A mod missing from the list does not mean the mod isn't there,
+     *    and similarly a mod present in the list does not mean it is there.
+     *    People using hacked clients WILL hack the mod lists to make them look correct.
+     *    Do not use this as an anti-cheat feature!
+     *  </b>
+     * @return An immutable list of MODIDs
+     */
+    public ImmutableList<String> getModList()
+    {
+        return modList;
+    }
+
+    /**
+     * Returns the list of network channels present in the remote.
+     * <b>WARNING: This list is not authoritative.
+     *    A channel missing from the list does not mean the remote won't accept packets with that channel ID,
+     *    and similarly a channel present in the list does not mean the remote won't ignore it.
+     *    People using hacked clients WILL hack the channel list to make it look correct.
+     *    Do not use this as an anti-cheat feature!
+     *  </b>
+     * @return An immutable map of channel IDs and their respective protocol versions.
+     */
+    public ImmutableMap<ResourceLocation, String> getChannels()
+    {
+        return channels;
+    }
+}

src/main/java/net/minecraftforge/fml/network/FMLHandshakeHandler.java

@@ -184,6 +184,8 @@ public class FMLHandshakeHandler {
         LOGGER.debug(FMLHSMARKER, "Accepted server connection");
         // Set the modded marker on the channel so we know we got packets
         c.get().getNetworkManager().channel().attr(FMLNetworkConstants.FML_NETVERSION).set(FMLNetworkConstants.NETVERSION);
+        c.get().getNetworkManager().channel().attr(FMLNetworkConstants.FML_CONNECTION_DATA)
+                .set(new FMLConnectionData(serverModList.getModList(), serverModList.getChannels()));
 
         this.registriesToReceive = new HashSet<>(serverModList.getRegistries());
         this.registrySnapshots = Maps.newHashMap();
@@ -203,6 +205,8 @@ public class FMLHandshakeHandler {
     {
         LOGGER.debug(FMLHSMARKER, "Received client connection with modlist [{}]",  String.join(", ", clientModList.getModList()));
         boolean accepted = NetworkRegistry.validateServerChannels(clientModList.getChannels());
+        c.get().getNetworkManager().channel().attr(FMLNetworkConstants.FML_CONNECTION_DATA)
+                .set(new FMLConnectionData(clientModList.getModList(), clientModList.getChannels()));
         c.get().setPacketHandled(true);
         if (!accepted) {
             LOGGER.error(FMLHSMARKER, "Terminating connection with client, mismatched mod list");

src/main/java/net/minecraftforge/fml/network/FMLNetworkConstants.java

@@ -41,6 +41,7 @@ public class FMLNetworkConstants
     static final AttributeKey<String> FML_NETVERSION = AttributeKey.valueOf("fml:netversion");
     static final AttributeKey<FMLHandshakeHandler> FML_HANDSHAKE_HANDLER = AttributeKey.valueOf("fml:handshake");
     static final AttributeKey<FMLMCRegisterPacketHandler.ChannelList> FML_MC_REGISTRY = AttributeKey.valueOf("minecraft:netregistry");
+    static final AttributeKey<FMLConnectionData> FML_CONNECTION_DATA = AttributeKey.valueOf("fml:conndata");
     static final ResourceLocation FML_HANDSHAKE_RESOURCE = new ResourceLocation("fml:handshake");
     static final ResourceLocation FML_PLAY_RESOURCE = new ResourceLocation("fml:play");
     static final ResourceLocation MC_REGISTER_RESOURCE = new ResourceLocation("minecraft:register");

src/main/java/net/minecraftforge/fml/network/NetworkHooks.java

@@ -53,6 +53,8 @@ import net.minecraftforge.common.MinecraftForge;
 import net.minecraftforge.event.entity.player.PlayerContainerEvent;
 import net.minecraftforge.fml.config.ConfigTracker;
 
+import javax.annotation.Nullable;
+
 public class NetworkHooks
 {
     private static final Logger LOGGER = LogManager.getLogger();
@@ -255,4 +257,10 @@ public class NetworkHooks
             FMLNetworkConstants.playChannel.sendTo(new FMLPlayMessages.SyncCustomTagTypes(customTagTypes), player.connection.getConnection(), NetworkDirection.PLAY_TO_CLIENT);
         }
     }
+
+    @Nullable
+    public static FMLConnectionData getConnectionData(NetworkManager mgr)
+    {
+        return mgr.channel().attr(FMLNetworkConstants.FML_CONNECTION_DATA).get();
+    }
 }

src/main/java/net/minecraftforge/fml/network/NetworkInstance.java

@@ -19,6 +19,8 @@
 
 package net.minecraftforge.fml.network;
 
+import net.minecraft.entity.player.PlayerEntity;
+import net.minecraft.entity.player.ServerPlayerEntity;
 import net.minecraft.network.NetworkManager;
 import net.minecraft.util.ResourceLocation;
 import net.minecraftforge.eventbus.api.BusBuilder;
@@ -108,4 +110,9 @@ public class NetworkInstance
     void dispatchEvent(final NetworkEvent networkEvent) {
         this.networkEventBus.post(networkEvent);
     }
+
+    public boolean isRemotePresent(NetworkManager manager) {
+        FMLConnectionData connectionData = NetworkHooks.getConnectionData(manager);
+        return connectionData != null && connectionData.getChannels().containsKey(channelName);
+    }
 }

src/main/java/net/minecraftforge/fml/network/NetworkRegistry.java

@@ -63,6 +63,26 @@ public class NetworkRegistry
     @SuppressWarnings("RedundantStringConstructorCall")
     public static String ACCEPTVANILLA  = new String("ALLOWVANILLA \uD83D\uDC93\uD83D\uDC93\uD83D\uDC93");
 
+    /**
+     * Makes a version predicate that accepts connections to vanilla or without the channel.
+     * @param protocolVersion The protocol version, which will be matched exactly.
+     * @return A new predicate with the new conditions.
+     */
+    public static Predicate<String> acceptMissingOr(final String protocolVersion)
+    {
+        return acceptMissingOr(protocolVersion::equals);
+    }
+
+    /**
+     * Makes a version predicate that accepts connections to vanilla or without the channel.
+     * @param versionCheck The main version predicate, which should check the version number of the protocol.
+     * @return A new predicate with the new conditions.
+     */
+    public static Predicate<String> acceptMissingOr(Predicate<String> versionCheck)
+    {
+        return versionCheck.or(ABSENT::equals).or(ACCEPTVANILLA::equals);
+    }
+
     public static List<String> getServerNonVanillaNetworkMods()
     {
         return listRejectedVanillaMods(NetworkInstance::tryClientVersionOnServer);

src/main/java/net/minecraftforge/fml/network/event/EventNetworkChannel.java

@@ -19,7 +19,9 @@
 
 package net.minecraftforge.fml.network.event;
 
+import net.minecraft.network.NetworkManager;
 import net.minecraftforge.fml.network.NetworkEvent;
+import net.minecraftforge.fml.network.NetworkHooks;
 import net.minecraftforge.fml.network.NetworkInstance;
 
 import java.util.function.Consumer;
@@ -47,4 +49,8 @@ public class EventNetworkChannel
     {
         instance.unregisterObject(object);
     }
+
+    public boolean isRemotePresent(NetworkManager manager) {
+        return instance.isRemotePresent(manager);
+    }
 }

src/main/java/net/minecraftforge/fml/network/simple/SimpleChannel.java

@@ -134,6 +134,13 @@ public class SimpleChannel
         context.getPacketDispatcher().sendPacket(instance.getChannelName(), toBuffer(msgToReply).getLeft());
     }
 
+    /**
+     * Returns true if the channel is present in the given connection.
+     */
+    public boolean isRemotePresent(NetworkManager manager) {
+        return instance.isRemotePresent(manager);
+    }
+
     /**
      * Build a new MessageBuilder. The type should implement {@link java.util.function.IntSupplier} if it is a login
      * packet.

src/main/java/net/minecraftforge/network/ForgeConnectionNetworkFilter.java

@@ -1,3 +1,22 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
 package net.minecraftforge.network;
 
 import java.util.List;
@@ -34,7 +53,7 @@ public class ForgeConnectionNetworkFilter extends VanillaPacketFilter
     protected boolean isNecessary(NetworkManager manager)
     {
         // not needed on local connections, because packets are not encoded to bytes there
-        return !manager.isMemoryConnection() && !NetworkHooks.isVanillaConnection(manager);
+        return !manager.isMemoryConnection() && !VanillaPacketSplitter.isRemoteCompatible(manager);
     }
 
     private static void splitPacket(IPacket<?> packet, List<? super IPacket<?>> out)

src/main/java/net/minecraftforge/network/VanillaPacketSplitter.java

@@ -1,3 +1,22 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
 package net.minecraftforge.network;
 
 import java.io.IOException;
@@ -5,21 +24,20 @@ import java.io.UncheckedIOException;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Set;
+import java.util.function.Predicate;
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.Unpooled;
 import net.minecraft.client.Minecraft;
 import net.minecraft.client.network.play.ClientPlayNetHandler;
-import net.minecraft.network.IPacket;
-import net.minecraft.network.PacketBuffer;
-import net.minecraft.network.PacketDirection;
-import net.minecraft.network.ProtocolType;
+import net.minecraft.network.*;
 import net.minecraft.network.play.server.SCustomPayloadPlayPacket;
 import net.minecraft.network.play.server.STagsListPacket;
 import net.minecraft.network.play.server.SUpdateRecipesPacket;
 import net.minecraft.util.ResourceLocation;
 import net.minecraftforge.fml.network.NetworkDirection;
 import net.minecraftforge.fml.network.NetworkEvent;
+import net.minecraftforge.fml.network.NetworkHooks;
 import net.minecraftforge.fml.network.NetworkRegistry;
 import net.minecraftforge.fml.network.event.EventNetworkChannel;
 
@@ -54,9 +72,12 @@ public class VanillaPacketSplitter
     private static final byte STATE_FIRST = 1;
     private static final byte STATE_LAST = 2;
 
+    private static EventNetworkChannel channel;
+
     public static void register()
     {
-        EventNetworkChannel channel = NetworkRegistry.newEventChannel(CHANNEL, () -> VERSION, VERSION::equals, VERSION::equals);
+        Predicate<String> versionCheck = NetworkRegistry.acceptMissingOr(VERSION);
+        channel = NetworkRegistry.newEventChannel(CHANNEL, () -> VERSION, versionCheck, versionCheck);
         channel.addListener(VanillaPacketSplitter::onClientPacket);
     }
 
@@ -185,4 +206,8 @@ public class VanillaPacketSplitter
         }
     }
 
+    public static boolean isRemoteCompatible(NetworkManager manager)
+    {
+        return !NetworkHooks.isVanillaConnection(manager) && channel.isRemotePresent(manager);
+    }
 }