Add Channel initialization listeners

paper-server/patches/sources/net/minecraft/network/Connection.java.patch

@@ -29,7 +29,7 @@
      @Nullable
      private volatile PacketListener disconnectListener;
      @Nullable
-@@ -114,7 +119,25 @@
+@@ -114,6 +119,24 @@
      private volatile DisconnectionDetails delayedDisconnect;
      @Nullable
      BandwidthDebugMonitor bandwidthDebugMonitor;
@@ -39,7 +39,7 @@
 +    public java.net.InetSocketAddress virtualHost;
 +    private static boolean enableExplicitFlush = Boolean.getBoolean("paper.explicit-flush"); // Paper - Disable explicit network manager flushing
 +    // Paper end
- 
++
 +    // Paper start - add utility methods
 +    public final net.minecraft.server.level.ServerPlayer getPlayer() {
 +        if (this.packetListener instanceof net.minecraft.server.network.ServerGamePacketListenerImpl impl) {
@@ -51,10 +51,9 @@
 +        return null;
 +    }
 +    // Paper end - add utility methods
-+
+ 
      public Connection(PacketFlow side) {
          this.receiving = side;
-     }
 @@ -123,6 +146,9 @@
          super.channelActive(channelhandlercontext);
          this.channel = channelhandlercontext.channel();
@@ -163,7 +162,23 @@
              public void write(ChannelHandlerContext channelhandlercontext, Object object, ChannelPromise channelpromise) throws Exception {
                  super.write(channelhandlercontext, object, channelpromise);
              }
-@@ -661,6 +708,27 @@
+@@ -633,6 +680,7 @@
+             } else {
+                 this.channel.pipeline().addAfter("prepender", "compress", new CompressionEncoder(compressionThreshold));
+             }
++            this.channel.pipeline().fireUserEventTriggered(io.papermc.paper.network.ConnectionEvent.COMPRESSION_THRESHOLD_SET); // Paper - Add Channel initialization listeners
+         } else {
+             if (this.channel.pipeline().get("decompress") instanceof CompressionDecoder) {
+                 this.channel.pipeline().remove("decompress");
+@@ -641,6 +689,7 @@
+             if (this.channel.pipeline().get("compress") instanceof CompressionEncoder) {
+                 this.channel.pipeline().remove("compress");
+             }
++            this.channel.pipeline().fireUserEventTriggered(io.papermc.paper.network.ConnectionEvent.COMPRESSION_DISABLED); // Paper - Add Channel initialization listeners
+         }
+ 
+     }
+@@ -661,6 +710,27 @@
  
                      packetlistener1.onDisconnect(disconnectiondetails);
                  }

paper-server/patches/sources/net/minecraft/server/network/ServerConnectionListener.java.patch

@@ -27,7 +27,7 @@
  
      public ServerConnectionListener(MinecraftServer server) {
          this.server = server;
-@@ -100,16 +109,27 @@
+@@ -100,16 +109,28 @@
  
                      Connection.configureSerialization(channelpipeline, PacketFlow.SERVERBOUND, false, (BandwidthDebugMonitor) null);
                      int j = ServerConnectionListener.this.server.getRateLimitPacketsPerSecond();
@@ -39,6 +39,7 @@
 +                    pending.add(object); // Paper - prevent blocking on adding a new connection while the server is ticking
                      ((Connection) object).configurePacketHandler(channelpipeline);
                      ((Connection) object).setListenerForServerboundHandshake(new ServerHandshakePacketListenerImpl(ServerConnectionListener.this.server, (Connection) object));
++                    io.papermc.paper.network.ChannelInitializeListenerHolder.callListeners(channel); // Paper - Add Channel initialization listeners
                  }
 -            }).group(eventloopgroup).localAddress(address, port)).bind().syncUninterruptibly());
 +            }).group(eventloopgroup).localAddress(address, port)).option(ChannelOption.AUTO_READ, false).bind().syncUninterruptibly()); // CraftBukkit
@@ -58,7 +59,7 @@
      public SocketAddress startMemoryChannel() {
          List list = this.channels;
          ChannelFuture channelfuture;
-@@ -153,6 +173,14 @@
+@@ -153,6 +174,14 @@
          List list = this.connections;
  
          synchronized (this.connections) {
@@ -73,7 +74,7 @@
              Iterator<Connection> iterator = this.connections.iterator();
  
              while (iterator.hasNext()) {
-@@ -176,6 +204,10 @@
+@@ -176,6 +205,10 @@
                              networkmanager.setReadOnly();
                          }
                      } else {

paper-server/src/main/java/io/papermc/paper/network/ChannelInitializeListener.java

@@ -0,0 +1,15 @@
+package io.papermc.paper.network;
+
+import io.netty.channel.Channel;
+import org.checkerframework.checker.nullness.qual.NonNull;
+
+/**
+ * Internal API to register channel initialization listeners.
+ * <p>
+ * This is not officially supported API and we make no guarantees to the existence or state of this interface.
+ */
+@FunctionalInterface
+public interface ChannelInitializeListener {
+
+    void afterInitChannel(@NonNull Channel channel);
+}

paper-server/src/main/java/io/papermc/paper/network/ChannelInitializeListenerHolder.java

@@ -0,0 +1,74 @@
+package io.papermc.paper.network;
+
+import io.netty.channel.Channel;
+import net.kyori.adventure.key.Key;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Internal API to register channel initialization listeners.
+ * <p>
+ * This is not officially supported API and we make no guarantees to the existence or state of this class.
+ */
+public final class ChannelInitializeListenerHolder {
+
+    private static final Map<Key, ChannelInitializeListener> LISTENERS = new HashMap<>();
+    private static final Map<Key, ChannelInitializeListener> IMMUTABLE_VIEW = Collections.unmodifiableMap(LISTENERS);
+
+    private ChannelInitializeListenerHolder() {
+    }
+
+    /**
+     * Registers whether an initialization listener is registered under the given key.
+     *
+     * @param key key
+     * @return whether an initialization listener is registered under the given key
+     */
+    public static boolean hasListener(@NonNull Key key) {
+        return LISTENERS.containsKey(key);
+    }
+
+    /**
+     * Registers a channel initialization listener called after ServerConnection is initialized.
+     *
+     * @param key      key
+     * @param listener initialization listeners
+     */
+    public static void addListener(@NonNull Key key, @NonNull ChannelInitializeListener listener) {
+        LISTENERS.put(key, listener);
+    }
+
+    /**
+     * Removes and returns an initialization listener registered by the given key if present.
+     *
+     * @param key key
+     * @return removed initialization listener if present
+     */
+    public static @Nullable ChannelInitializeListener removeListener(@NonNull Key key) {
+        return LISTENERS.remove(key);
+    }
+
+    /**
+     * Returns an immutable map of registered initialization listeners.
+     *
+     * @return immutable map of registered initialization listeners
+     */
+    public static @NonNull Map<Key, ChannelInitializeListener> getListeners() {
+        return IMMUTABLE_VIEW;
+    }
+
+    /**
+     * Calls the registered listeners with the given channel.
+     *
+     * @param channel channel
+     */
+    public static void callListeners(@NonNull Channel channel) {
+        for (ChannelInitializeListener listener : LISTENERS.values()) {
+            listener.afterInitChannel(channel);
+        }
+    }
+}

paper-server/src/main/java/io/papermc/paper/network/ConnectionEvent.java

@@ -0,0 +1,10 @@
+package io.papermc.paper.network;
+
+/**
+ * Internal connection pipeline events.
+ */
+public enum ConnectionEvent {
+
+    COMPRESSION_THRESHOLD_SET,
+    COMPRESSION_DISABLED
+}