Changing post effects can disconnect the player with ConcurrentModificationException

Changing a player's post effects can disconnect the player while clientbound/minecraft:post_effects is being encoded.

Steps to reproduce

1. Add the attached reproduction data pack to a vanilla singleplayer world.

2. Run /reload.

3. Run /function postfx_repro:start.

4. Wait a few seconds.

The data pack initially adds 4096 fixed post-effect identifiers and one toggle identifier.

After setup, it only performs the following once per game tick:

posteffect remove @s postfx_repro:toggle

posteffect add @s postfx_repro:toggle

4096 entries are used because this reproduces the issue consistently on my system. I do not know whether 4096 represents an actual limit.

Expected result

The post effects are updated without disconnecting the player.

Actual result

The player is disconnected while clientbound/minecraft:post_effects is being encoded.

The server log shows:

Error sending packet clientbound/minecraft:post_effects

Caused by:

java.util.ConcurrentModificationException

with:

java.util.ArrayList$Itr.checkForComodification

java.util.ArrayList$Itr.next

net.minecraft.network.codec.ByteBufCodecs$26.encode