Race Condition in the Presence of Bigger Stacks, ModernFix, and probably any 6 random libraries or mods

[Caritas-Dei]

Bug Description

For Minecraft 1.20.1, Forge 47.4.16, Bigger Stacks 2026.02.04 and ModernFix 5.26.2 in the presence of 6 (but seemingly not 5) other random libraries or mods causes a java.util.ConcurrentModificationException with the following stacktrace:

[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: java.util.ConcurrentModificationException
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at java.base/java.util.HashMap$HashIterator.nextNode(Unknown Source)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at java.base/java.util.HashMap$ValueIterator.next(Unknown Source)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at MC-BOOTSTRAP/cpw.mods.modlauncher@10.0.9/cpw.mods.modlauncher.LaunchPluginHandler.computeLaunchPluginTransformerSet(LaunchPluginHandler.java:68)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at MC-BOOTSTRAP/cpw.mods.modlauncher@10.0.9/cpw.mods.modlauncher.ClassTransformer.transform(ClassTransformer.java:64)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at MC-BOOTSTRAP/cpw.mods.modlauncher@10.0.9/cpw.mods.modlauncher.TransformingClassLoader.maybeTransformClassBytes(TransformingClassLoader.java:50)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.readerToClass(ModuleClassLoader.java:113)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.lambda$findClass$15(ModuleClassLoader.java:219)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.loadFromModule(ModuleClassLoader.java:229)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.findClass(ModuleClassLoader.java:219)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.loadClass(ModuleClassLoader.java:135)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at TRANSFORMER/modernfix@5.26.2+mc1.20.1/org.embeddedt.modernfix.forge.config.NightConfigWatchThrottler$1.values(NightConfigWatchThrottler.java:37)
[22:17:02] [Thread-0] [STDERR/INFO]:
[java.lang.ThreadGroup:uncaughtException:-1]: at MC-BOOTSTRAP/com.electronwill.nightconfig.core@3.6.4/com.electronwill.nightconfig.core.file.FileWatcher$WatcherThread.run(FileWatcher.java:162)

Reproduction Steps

My CPU is a Ryzen 9 3950X, 16 cores, 32 threads; GPU is Radeon RX 5700 XT with driver version 26.1.1 .

1. Install Minecraft 1.20.1 with authenticated account, Forge 47.4.6, latest Bigger Stacks and ModernFix, followed by any 6 mods whether libraries or not. The additional mods I tested with, however (ignoring a few random enables), are: TelluriumForge, TenshiLib, Uranus, Voltaic, YetAnotherConfigLib, and Zeta.
2. Run the game
3. Check for the above type of stacktrace occurring as a ConcurrentModificationException
4. Try with 5 mods and repeat steps 2-3.
5. Try with 5 mods but no ModernFix enabled and repeat steps 2-3.
6. Try with at least one new mod replacing one of the 6 mods and repeat steps 2-3.
7. Try with at least one new mod replacing one of the 6 mods and repeat steps 2-3, but without ModernFix.

Note: setting mixin.feature.cause_lag_by_disabling_threads=true will not change these results.

Log File

launcher_log.txt is latest.log but with lines added before and after by Modrinth Launcher

[embeddedt]

I'm aware of this getting printed randomly but does it cause a crash or just complain in the log?

[Caritas-Dei]

I'm aware of this getting printed randomly but does it cause a crash or just complain in the log?

It only complains in the log. I'm not sure if it might actually cause problems, but if we're talking about other mods in the presence of ModernFix causing fundamental Minecraft system problems (such as two threads trying to write to the same config or data file, implying common use), then that necessarily will result in further conflicts elsewhere.

Even if it is just an error isolated to a new system within the system, it's obviously still worth fixing by adding the necessary sychronizations and conflict resolution. Y'never know.

[uhloin]

Here is additional crash for Neoforge 1.21.1 with possible race condition on game start

crash-2026-02-28_12.26.39-fml.txt

[P3rf3ctXZer0]

Too much log complaints does lag servers @embeddedt generally bad practice to ignore lots of log spam. Mind you some tools work... just not always well.

The only reason I care about this bug at all is always use bigger stacks when available and I did notice log spam even in neoforge 21.1.219

[embeddedt]

I think multiple issues are being discussed here. The original issue should only affect 1.20.1 (the NightConfig hacks are removed in 1.21.1+), and shouldn't cause log spam. Under certain conditions the classloading fails on the config watcher thread due to a race with the rest of modloading. I have been aware of that issue for a while but didn't prioritize fixing it as I understand the trigger and know it's relatively harmless.

If there is a different issue with Bigger Stacks that leads to actual log spam in 1.21.1, and not a single error at startup from the config watcher in 1.20.1, please open a separate issue with reproduction steps.

[P3rf3ctXZer0]

Understood. Next time I decide to launch Minecraft I will gather the data you requested. Also open a new issue.