ConcurrentModificationException caused by async command sending patch

Expected behavior

No errors and a clean reload.

Observed/Actual behavior

A ConcurrentModificationException.

Steps/models to reproduce

Start server
Reload until you see the error. ( it's at the bottom kinda )

Plugin and Datapack List

Plugins (all Bukkit) : FacilisCommon, MultiWorld and SkinsRestorer
Datapacks: vanilla (built-in), file/bukkit (world), bundle (feature), paper (built-in)

Paper version

[19:56:32 INFO]: Checking version, please wait...
[19:56:32 INFO]: This server is running Paper version 1.21-83-master@967f98a (2024-07-17T14:55:35Z) (Implementing API version 1.21-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21-81-ceeb8c1 (MC: 1.21)

Other

I noticed that every time I start my server, reload it a couple of times ( because I am developing my own plugin ) and look at the console ( to check for bugs ), I see a ConcurrentModificationException with a number that increments every time I see the error ( not every time I reload ).

So, I remove my plugin from the list and I see that it's still happening every couple of reloads. So I test it a bit more and then it starts happening every 2 reloads.

Start by reviewing linked pull request #13626 and the issue's reload reproduction steps. Trace the reported ConcurrentModificationException in the server's reload and async command-sending paths; done means repeated reloads no longer produce the exception or an unclean reload.