Memory leak when using Bukkit#unloadWorld with save=false

[janrabek]

Timings or Profile link

You don't see any info abouit that leak in any timings.

Description of issue

When unloading worlds using Bukkit#unloadWorld with save=false they get removed from CraftServer#worlds but they still retain in the memory (especially ChunkTaskSchedulers):

In this screenshot you see there are officially only 6 worlds loaded:

This is a massive issue, as you don't know this through timings but only through testing.

Also one guy in the Spigot forum had the same issue:
https://www.spigotmc.org/threads/how-to-fix-memory-leak-unload-world.588677/

Plugin and Datapack List

Doesn't matter

Server config files

Doesn't matter

Paper version

Other

No response

[janrabek]

We observed that this issue only happened after updating to 1.19.*

[olijeffers0n]

1.19.3 isn’t latest, is this the same on .4

[lynxplay]

I am having trouble replicating this.
The test code

@EventHandler
public void on(final PlayerDropItemEvent event) {
    event.setCancelled(true);
    if (test == null) {
        test = Bukkit.createWorld(WorldCreator.name("test").keepSpawnLoaded(TriState.TRUE));
        event.getPlayer().sendMessage(Component.text("Loaded custom world `test`"));
        event.getPlayer().teleport(test.getSpawnLocation());
    } else {
        event.getPlayer().teleport(Bukkit.getWorld(NamespacedKey.minecraft("overworld")).getSpawnLocation());
        Bukkit.unloadWorld(test, false);
        event.getPlayer().sendMessage(Component.text("Unloaded custom world `test`"));
    }
}

was used to load and unload the world named "test" numerous times, while

jmap -dump:live,file=./server.hprof "$(jcmd | grep 'craftbukkit.Main' | cut -d' ' -f1)"

was used to create the heap dump. Notably this command only collects live objects, e.g. those that are still referenced.
I end up with exactly three ChunkTaskScheduler instances as expected.

I might be missing something tho, so this issue needs further triage.
It would be great if you can replicate this a) on latest and b) ensure that only live objects are captured as the GC may not have collected the now unloaded world at your point of capture.

Beyond that, spigot references for this issue are rather useless, paper rewrites pretty much the entire system there 😅

[janrabek]

Hey,
I'm very sorry, it was a memory leak of FastAsyncWorldEdit. The save parameter didn't fix it, I only believed this was the issue because of the Spigot thread.

Best regards
Jan

[Kryniowesegryderiusz]

Hey, I'm very sorry, it was a memory leak of FastAsyncWorldEdit. The save parameter didn't fix it, I only believed this was the issue because of the Spigot thread.

Best regards Jan

Im sorry to dig up this thread, but could you maybe do some follow up? I have similar issue, but updating FAWE doesnt seem to help :c

[OneLiteFeather]

@jan can you provide more information about the plugins you were using with fawe on IntellectualSites/FastAsyncWorldEdit#2278 ? That might help as well.

[jan]

@OneLiteFeather wrong @jan.

[OneLiteFeather]

@OneLiteFeather wrong @jan.

My bad, it doesn't really differenciate between on the phone.