Bug

I currently have an animated gif that uses the flags +ANIM,+GLOW. Sometimes when players enter or leave the world. The gif suddenly stops.

Minecraft Paper version 1.19.2
Newest Version of the yamipa plugin

Why

After investigating your code a bit, I noticed that the FakeImage#nextStep method that should be called by the scheduler at line 376 FakeImage.java suddenly stops working. After investigating a bit further, I noticed that the for loop that runs through all the watching players generates a ConcurrentModificationException that stops the scheduler and thus the animation.

This exception is caused because your listeners in ImageRenderer.java modify the observingPlayers list with the FakeImage#destroy and FakeImage#spawn methods during the processing of the ImageRenderer#onPlayerLocationChange function.

Although the problem only occurred while players were joining and leaving, the code indicates that this problem can also occur when players respawn, teleport, or change to a different WorldAreaId.

Idea how to prevent

The easiest way around this problem is probably to wrap the entire FakeImage#nextStep method in a try-and-catch that catches the ConcurrentModificationException. This will cause the gif to play even if players exit and re-enter the game. It might be that a frame is skipped for some players, but that should be fine.

Good catch! Thanks, @Hasenzahn1!

Indeed, to me this looks like a thread synchronization issue when iterating over the observingPlayers property from FakeImage.

All modifications of the observingPlayers hashset occur on the same thread, and the exception is only thrown on the FakeImage#nextStep due to the fail-fast mechanism built into HashMap. So I think it's safe to just catch and ignore the exception.

The other option I'm considering is synchronizing all code blocks that read/write to observingPlayers, but that will probably affect performance and will be noticeable in large servers or with many animated images. I'll think about it and push a fix.