Add Channel initialization listeners

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
+}