Use signal handler for checkpointing

SpigotCore_Main/src/com/comphenix/tinyprotocol/TinyProtocol.java

@@ -34,7 +34,7 @@ public class TinyProtocol implements Listener {
 		return getServerConnection.get(getMinecraftServer.get(getPlayerList.get(plugin.getServer())));
 	}
 	private static final Class<?> networkManager = Reflection.getClass("{nms.network}.NetworkManager");
-	private static final FieldAccessor<List> getConnections = Reflection.getField(serverConnection, List.class, 0, networkManager);
+	public static final FieldAccessor<List> networkManagers = Reflection.getField(serverConnection, List.class, 0, networkManager);
 
 	public static final TinyProtocol instance = new TinyProtocol(Core.getInstance());
 	private static int id = 0;
@@ -54,7 +54,7 @@ public class TinyProtocol implements Listener {
 	private TinyProtocol(final Plugin plugin) {
 		this.plugin = plugin;
 		this.handlerName = "tiny-" + plugin.getName() + "-" + ++id;
-		this.connections = getConnections.get(getServerConnection(plugin));
+		this.connections = networkManagers.get(getServerConnection(plugin));
 
 		plugin.getServer().getPluginManager().registerEvents(this, plugin);
 

SpigotCore_Main/src/de/steamwar/core/CheckpointUtils.java

@@ -25,8 +25,10 @@ import com.viaversion.viaversion.api.Via;
 import de.steamwar.sql.internal.Statement;
 import io.netty.channel.ChannelFuture;
 import org.bukkit.Bukkit;
+import org.bukkit.Chunk;
 import org.eclipse.openj9.criu.CRIUSupport;
 import org.eclipse.openj9.criu.JVMCRIUException;
+import sun.misc.Signal;
 
 import java.io.DataInputStream;
 import java.io.File;
@@ -35,6 +37,7 @@ import java.net.InetAddress;
 import java.nio.file.FileSystems;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Arrays;
 import java.util.Comparator;
 import java.util.List;
 import java.util.logging.Level;
@@ -43,11 +46,22 @@ import java.util.stream.Stream;
 public class CheckpointUtils {
     private CheckpointUtils() {}
 
-    private static final Reflection.FieldAccessor<List> channelFutures = Reflection.getField(TinyProtocol.serverConnection, List.class, 0, ChannelFuture.class);
-    private static final Reflection.MethodInvoker bind = Reflection.getMethod(TinyProtocol.serverConnection, null, InetAddress.class, int.class);
+    public static void signalHandler() {
+        Signal.handle(new Signal("USR1"), signal -> Bukkit.getScheduler().runTask(Core.getInstance(), CheckpointUtils::freeze));
+    }
 
     public static void freeze() {
-        Path path = FileSystems.getDefault().getPath(System.getProperty("checkpoint"));
+        String checkpointFile = System.getProperty("checkpoint");
+        if(!CRIUSupport.isCheckpointAllowed() || checkpointFile == null) {
+            Bukkit.shutdown();
+            return;
+        }
+
+        Bukkit.getOnlinePlayers().forEach(player -> player.kickPlayer(null));
+
+        List<?> networkManagers = TinyProtocol.networkManagers.get(TinyProtocol.getServerConnection(Core.getInstance()));
+        if(networkManagers.isEmpty()) {
+            Path path = FileSystems.getDefault().getPath(checkpointFile);
 
             try {
                 freezeInternal(path);
@@ -62,9 +76,17 @@ public class CheckpointUtils {
                     //ignore
                 }
             }
+            return;
         }
 
+        Core.getInstance().getLogger().log(Level.INFO, "Waiting for players to disconnect for checkpointing");
+        Bukkit.getScheduler().runTaskLater(Core.getInstance(), CheckpointUtils::freeze, 1);
+    }
+
+    private static final Reflection.FieldAccessor<List> channelFutures = Reflection.getField(TinyProtocol.serverConnection, List.class, 0, ChannelFuture.class);
+    private static final Reflection.MethodInvoker bind = Reflection.getMethod(TinyProtocol.serverConnection, null, InetAddress.class, int.class);
     private static void freezeInternal(Path path) throws Exception {
+        Bukkit.getWorlds().forEach(world -> Arrays.stream(world.getLoadedChunks()).forEach(Chunk::unload));
         Statement.closeAll();
 
         // Close socket

SpigotCore_Main/src/de/steamwar/core/Core.java

@@ -97,6 +97,7 @@ public class Core extends JavaPlugin{
 			AuthlibInjector.inject();
 
 		TinyProtocol.init();
+		CheckpointUtils.signalHandler();
 		new AntiNocom();
 
 		if(Core.getVersion() < 17 && Bukkit.getPluginManager().getPlugin("ViaVersion") != null)