(Implementation) Allow injecting into backend connection

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -44,6 +44,8 @@ import com.velocitypowered.proxy.util.bossbar.VelocityBossBar;
 import com.velocitypowered.proxy.util.ratelimit.Ratelimiter;
 import com.velocitypowered.proxy.util.ratelimit.Ratelimiters;
 import io.netty.bootstrap.Bootstrap;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelInitializer;
 import io.netty.channel.EventLoopGroup;
 import java.io.IOException;
 import java.net.InetSocketAddress;
@@ -268,6 +270,10 @@ public class VelocityServer implements ProxyServer {
     return this.cm.createWorker(group);
   }
 
+  public ChannelInitializer<Channel> getBackendChannelInitializer() {
+    return this.cm.backendChannelInitializerHolder.get();
+  }
+
   public boolean isShutdown() {
     return shutdown;
   }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/VelocityServerConnection.java

@@ -80,22 +80,7 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     // Note: we use the event loop for the connection the player is on. This reduces context
     // switches.
     server.createBootstrap(proxyPlayer.getConnection().eventLoop())
-        .handler(new ChannelInitializer<Channel>() {
-          @Override
-          protected void initChannel(Channel ch) throws Exception {
-            ch.pipeline()
-                .addLast(READ_TIMEOUT,
-                    new ReadTimeoutHandler(server.getConfiguration().getReadTimeout(),
-                        TimeUnit.MILLISECONDS))
-                .addLast(FRAME_DECODER, new MinecraftVarintFrameDecoder())
-                .addLast(FRAME_ENCODER, MinecraftVarintLengthEncoder.INSTANCE)
-                .addLast(FLOW_HANDLER, new FlowControlHandler())
-                .addLast(MINECRAFT_DECODER,
-                    new MinecraftDecoder(ProtocolUtils.Direction.CLIENTBOUND))
-                .addLast(MINECRAFT_ENCODER,
-                    new MinecraftEncoder(ProtocolUtils.Direction.SERVERBOUND));
-          }
-        })
+        .handler(server.getBackendChannelInitializer())
         .connect(registeredServer.getServerInfo().getAddress())
         .addListener((ChannelFutureListener) future -> {
           if (future.isSuccess()) {

proxy/src/main/java/com/velocitypowered/proxy/network/BackendChannelInitializer.java

@@ -0,0 +1,45 @@
+package com.velocitypowered.proxy.network;
+
+import static com.velocitypowered.proxy.network.Connections.FLOW_HANDLER;
+import static com.velocitypowered.proxy.network.Connections.FRAME_DECODER;
+import static com.velocitypowered.proxy.network.Connections.FRAME_ENCODER;
+import static com.velocitypowered.proxy.network.Connections.MINECRAFT_DECODER;
+import static com.velocitypowered.proxy.network.Connections.MINECRAFT_ENCODER;
+import static com.velocitypowered.proxy.network.Connections.READ_TIMEOUT;
+
+import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import com.velocitypowered.proxy.protocol.netty.MinecraftDecoder;
+import com.velocitypowered.proxy.protocol.netty.MinecraftEncoder;
+import com.velocitypowered.proxy.protocol.netty.MinecraftVarintFrameDecoder;
+import com.velocitypowered.proxy.protocol.netty.MinecraftVarintLengthEncoder;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelInitializer;
+import io.netty.handler.flow.FlowControlHandler;
+import io.netty.handler.timeout.ReadTimeoutHandler;
+import java.util.concurrent.TimeUnit;
+
+@SuppressWarnings("WeakerAccess")
+public class BackendChannelInitializer extends ChannelInitializer<Channel> {
+
+  private final VelocityServer server;
+
+  public BackendChannelInitializer(VelocityServer server) {
+    this.server = server;
+  }
+
+  @Override
+  protected void initChannel(Channel ch) throws Exception {
+    ch.pipeline()
+        .addLast(READ_TIMEOUT,
+            new ReadTimeoutHandler(server.getConfiguration().getReadTimeout(),
+                TimeUnit.MILLISECONDS))
+        .addLast(FRAME_DECODER, new MinecraftVarintFrameDecoder())
+        .addLast(FRAME_ENCODER, MinecraftVarintLengthEncoder.INSTANCE)
+        .addLast(FLOW_HANDLER, new FlowControlHandler())
+        .addLast(MINECRAFT_DECODER,
+            new MinecraftDecoder(ProtocolUtils.Direction.CLIENTBOUND))
+        .addLast(MINECRAFT_ENCODER,
+            new MinecraftEncoder(ProtocolUtils.Direction.SERVERBOUND));
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/network/BackendChannelInitializerHolder.java

@@ -0,0 +1,35 @@
+package com.velocitypowered.proxy.network;
+
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelInitializer;
+import java.util.function.Supplier;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+public class BackendChannelInitializerHolder implements Supplier<ChannelInitializer<Channel>> {
+
+  private static final Logger LOGGER = LogManager.getLogger(ConnectionManager.class);
+  private ChannelInitializer<Channel> initializer;
+
+  BackendChannelInitializerHolder(final ChannelInitializer<Channel> initializer) {
+    this.initializer = initializer;
+  }
+
+  @Override
+  public ChannelInitializer<Channel> get() {
+    return this.initializer;
+  }
+
+  /**
+   * Sets the channel initializer.
+   *
+   * @param initializer the new initializer to use
+   * @deprecated Internal implementation detail
+   */
+  @Deprecated
+  public void set(final ChannelInitializer<Channel> initializer) {
+    LOGGER.warn("The backend channel initializer has been replaced by {}",
+        Thread.currentThread().getStackTrace()[2]);
+    this.initializer = initializer;
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/network/ConnectionManager.java

@@ -42,10 +42,12 @@ public final class ConnectionManager {
   private final EventLoopGroup bossGroup;
   private final EventLoopGroup workerGroup;
   private final VelocityServer server;
-  // This is intentionally made public for plugins like ViaVersion, which inject their own
+  // These are intentionally made public for plugins like ViaVersion, which inject their own
   // protocol logic into the proxy.
   @SuppressWarnings("WeakerAccess")
   public final ServerChannelInitializerHolder serverChannelInitializer;
+  @SuppressWarnings("WeakerAccess")
+  public final BackendChannelInitializerHolder backendChannelInitializerHolder;
 
   private final DnsAddressResolverGroup resolverGroup;
   private final AsyncHttpClient httpClient;
@@ -62,6 +64,8 @@ public final class ConnectionManager {
     this.workerGroup = this.transportType.createEventLoopGroup(TransportType.Type.WORKER);
     this.serverChannelInitializer = new ServerChannelInitializerHolder(
         new ServerChannelInitializer(this.server));
+    this.backendChannelInitializerHolder = new BackendChannelInitializerHolder(
+        new BackendChannelInitializer(this.server));
     this.resolverGroup = new DnsAddressResolverGroup(new DnsNameResolverBuilder()
         .channelType(this.transportType.datagramChannelClass)
         .negativeTtl(15)
@@ -204,4 +208,8 @@ public final class ConnectionManager {
   public AsyncHttpClient getHttpClient() {
     return httpClient;
   }
+
+  public BackendChannelInitializerHolder getBackendChannelInitializerHolder() {
+    return this.backendChannelInitializerHolder;
+  }
 }