fix: retain reference-counted packets forwarded via handleGeneric (#1856)

During configuration, a ServerboundCustomClickActionPacket arriving
when connectionInFlight is null falls through to handleGeneric, which
writes it to the connected backend without retaining. The encoder
releases the packet, then MinecraftConnection.channelRead's finally
block releases again - double-free.

Two fixes:
- handle() now uses getConnectionInFlightOrConnectedServer() so the
  packet is properly retained before being written
- handleGeneric() retains any ByteBufHolder packet before write, not
  just PluginMessagePacket

Closes #1841

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientConfigSessionHandler.java

@@ -47,6 +47,7 @@ import com.velocitypowered.proxy.protocol.packet.config.FinishedUpdatePacket;
 import com.velocitypowered.proxy.protocol.packet.config.KnownPacksPacket;
 import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.ByteBufHolder;
 import io.netty.buffer.ByteBufUtil;
 import io.netty.buffer.Unpooled;
 import java.util.concurrent.CompletableFuture;
@@ -212,8 +213,9 @@ public class ClientConfigSessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(ServerboundCustomClickActionPacket packet) {
-    if (player.getConnectionInFlight() != null) {
-      player.getConnectionInFlight().ensureConnected().write(packet.retain());
+    VelocityServerConnection serverConnection = player.getConnectionInFlightOrConnectedServer();
+    if (serverConnection != null) {
+      serverConnection.ensureConnected().write(packet.retain());
       return true;
     }
 
@@ -240,8 +242,8 @@ public class ClientConfigSessionHandler implements MinecraftSessionHandler {
 
     MinecraftConnection smc = serverConnection.getConnection();
     if (smc != null && serverConnection.getPhase().consideredComplete()) {
-      if (packet instanceof PluginMessagePacket) {
-        ((PluginMessagePacket) packet).retain();
+      if (packet instanceof ByteBufHolder bufHolder) {
+        bufHolder.retain();
       }
       smc.write(packet);
     }

proxy/src/test/java/com/velocitypowered/proxy/connection/client/ClientConfigSessionHandlerTest.java

@@ -0,0 +1,119 @@
+/*
+ * Copyright (C) 2018-2026 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.connection.client;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.connection.MinecraftConnection;
+import com.velocitypowered.proxy.connection.backend.BackendConnectionPhase;
+import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
+import com.velocitypowered.proxy.protocol.packet.ServerboundCustomClickActionPacket;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import io.netty.util.ReferenceCountUtil;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class ClientConfigSessionHandlerTest {
+
+  private VelocityServer server;
+  private ConnectedPlayer player;
+  private ClientConfigSessionHandler handler;
+
+  @BeforeEach
+  void setUp() {
+    server = mock(VelocityServer.class);
+    player = mock(ConnectedPlayer.class);
+    handler = new ClientConfigSessionHandler(server, player);
+  }
+
+  @AfterEach
+  void tearDown() {
+    // nothing to clean up; each test manages its own ByteBufs
+  }
+
+  private ServerboundCustomClickActionPacket makePacket() {
+    ByteBuf frame = Unpooled.buffer().writeByte(0);
+    ServerboundCustomClickActionPacket pkt = new ServerboundCustomClickActionPacket();
+    pkt.replace(frame.readRetainedSlice(frame.readableBytes()));
+    return pkt;
+  }
+
+  @Test
+  void handleForwardsToInFlightServer() {
+    VelocityServerConnection inFlight = mock(VelocityServerConnection.class);
+    MinecraftConnection backend = mock(MinecraftConnection.class);
+    when(player.getConnectionInFlightOrConnectedServer()).thenReturn(inFlight);
+    when(inFlight.ensureConnected()).thenReturn(backend);
+
+    ServerboundCustomClickActionPacket pkt = makePacket();
+    assertTrue(handler.handle(pkt));
+    verify(backend).write(pkt);
+    ReferenceCountUtil.release(pkt);
+  }
+
+  @Test
+  void handleForwardsToConnectedServerWhenInFlightIsNull() {
+    VelocityServerConnection connected = mock(VelocityServerConnection.class);
+    MinecraftConnection backend = mock(MinecraftConnection.class);
+    when(player.getConnectionInFlightOrConnectedServer()).thenReturn(connected);
+    when(connected.ensureConnected()).thenReturn(backend);
+
+    ServerboundCustomClickActionPacket pkt = makePacket();
+    assertTrue(handler.handle(pkt));
+    verify(backend).write(pkt);
+    ReferenceCountUtil.release(pkt);
+  }
+
+  @Test
+  void handleReturnsFalseWhenNoServer() {
+    when(player.getConnectionInFlightOrConnectedServer()).thenReturn(null);
+
+    ServerboundCustomClickActionPacket pkt = makePacket();
+    assertFalse(handler.handle(pkt));
+    ReferenceCountUtil.release(pkt);
+  }
+
+  @Test
+  void handleGenericRetainsAndForwards() {
+    VelocityServerConnection connected = mock(VelocityServerConnection.class);
+    MinecraftConnection backend = mock(MinecraftConnection.class);
+    BackendConnectionPhase phase = mock(BackendConnectionPhase.class);
+    when(player.getConnectedServer()).thenReturn(connected);
+    when(connected.getConnection()).thenReturn(backend);
+    when(connected.getPhase()).thenReturn(phase);
+    when(phase.consideredComplete()).thenReturn(true);
+
+    ServerboundCustomClickActionPacket pkt = makePacket();
+    int refBefore = pkt.refCnt();
+
+    handler.handleGeneric(pkt);
+
+    // retain() was called (+1) before write
+    assertEquals(refBefore + 1, pkt.refCnt());
+    verify(backend).write(pkt);
+    ReferenceCountUtil.release(pkt);
+  }
+}