crash on latest paper

[ikuria]

Stack trace

https://mclo.gs/wJI0Zfx

Plugin and Datapack List

Paper Plugins (2):

• HuskHomes, HuskSync
  Bukkit Plugins (45):
• Bolt, Chunky, Citizens, CMI, CMILib, CoreProtect, CoreProtectFilter,
  dDiscordBot, Denizen, Depenizen
  dynmap, dynmapHider, EffectLib, FastAsyncWorldEdit,
  *ForcedGrammar, GuestMode, HeadDatabase, ImageDisplay,
  LibsDisguises, LiteBans
  LuckPerms, MapModCompanion, MapSync, MCCinema, Multiverse-Core,
  MyCommand, NoteBlockAPI, packetevents, PlaceholderAPI, ProtocolLib
  PvPToggle, RC_BetterSync, RC_BetterTreasure, RC_EntityCounter,
  Shopkeepers, SystemChat, Vault, ViaBackwards, ViaVersion, VoidGen
  Votifier, WorldBorder, WorldEditDisplay, WorldGuard,
  WorldGuardExtraFlags

Actions to reproduce (if known)

Paper version

paper #48

Other

No response

[lynxplay]

Please provide the crash and the exact /version output.

[ikuria]

looks like ProtocolLib Problem

This server is running Paper version 26.2-48-main@87c4d42
(2026-07-04T14:09:05Z) (Implementing API version
26.2.build.48-alpha)
You are running the latest version
Previous version: 26.2-47-ad5d26a (MC: 26.2)

[ikuria]

https://mclo.gs/XfnbSk0
https://mclo.gs/NXd2uw5

[JavierFlores09]

I managed to reproduce this with the following code:

public final class TestPlugin extends JavaPlugin implements Listener {
    private Entity victim;

    @Override
    public void onEnable() {
        this.getServer().getPluginManager().registerEvents(this, this);
        this.getLifecycleManager().registerEventHandler(LifecycleEvents.COMMANDS, event -> event.registrar().register(
            Commands.literal("tracker-crash")
                .executes(ctx -> {
                    if (!(ctx.getSource().getSender() instanceof Player player))
                        return Command.SINGLE_SUCCESS;
                    var base = player.getLocation();
                    this.victim = player.getWorld().spawn(base.clone().add(0.0, 0.0, 2.0), ArmorStand.class);
                    player.getScheduler().run(this, _ ->
                        player.setVelocity(player.getVelocity().add(new Vector(0.0, 0.01, 0.0))), null);
                    player.sendPlainMessage("waiting for PlayerVelocityEvent.");
                    return Command.SINGLE_SUCCESS;
                })
                .build()
        ));

        // io.papermc.testplugin.brigtests.Registration.registerViaOnEnable(this);
    }

    @EventHandler
    public void onVelocity(PlayerVelocityEvent event) {
        if (this.victim == null) {
            return;
        }

        victim.remove();
        this.getLogger().warning("Removed repro entity from PlayerVelocityEvent during ServerEntity#sendChanges");
    }
}

The issue here is that ChunkMap#newTrackerTick caches len of the trackerEntities ReferenceList, so if anything ends up removing entities while ServerEntity#sendChanges (i.e. in the PlayerVelocityEvent) is happening, tail ends up being null and well, this happens. Easiest fix would be to just not use the raw array (and avoid caching the len):

         final ca.spottedleaf.moonrise.common.list.ReferenceList<net.minecraft.world.entity.Entity> trackerEntities = entityLookup.trackerEntities;
-        final Entity[] trackerEntitiesRaw = trackerEntities.getRawDataUnchecked();
-        for (int i = 0, len = trackerEntities.size(); i < len; ++i) {
-            final Entity entity = trackerEntitiesRaw[i];
+        for (int i = 0; i < trackerEntities.size(); ++i) {
+            final Entity entity = trackerEntities.getUnchecked(i);
             final ChunkMap.TrackedEntity tracker = ((ca.spottedleaf.moonrise.patches.entity_tracker.EntityTrackerEntity)entity).moonrise$getTrackedEntity();
             if (tracker == null) {
                 continue;

Though I assume it used the raw array because it is a hot loop, I can't imagine it would be much different performance-wise.

[kennytv]

That patch will skip entities if something behind the cursor is removed