Certain mobs attack themselves when targeted by a mob of the same type

Obviously there's ten billion other issues at hand, and this is probably low on the priority list, but what is the likelyhood of this getting fixed relatively soon?

Less than 10%? I just miss my mob arena 😞

Confirmed in 1.16.1 and 20w29a. Steps to reproduce:

1. Summon two zombies that cannot move:

   /summon zombie ~ ~ ~ {UUID:[I;0,0,0,1],Attributes:[{Name:generic.movement_speed,Base:0}]}
   /summon zombie ~5 ~ ~ {UUID:[I;0,0,0,2],Attributes:[{Name:generic.movement_speed,Base:0}]}

2. Summon a snowball, belonging to the first zombie, above the second zombie:

   /execute at @e[nbt={UUID:[I;0,0,0,2]}] run summon snowball ~ ~3 ~ {Motion:[0.0,-0.5],Owner:[I;0,0,0,1]}

   → ❌ The first zombie begins to attack itself

This occurs because after the second zombie becomes hostile towards the first zombie, the first zombie "joins in the fight" against itself.

This happens with bees too.

Thanks, added.

This also affects blazes (MC-227157).

Can confirm this also affects silverfish and endermites.

If you hit a zombie with a snowball, the second mob hitting itself doesn't have to be a zombie either. I found that hitting a zombie with a snowball shot from a zombie villager, husk, or drowned yields the same results.

This is based on an observation by @unknown on this comment.

Code analysis (Mojang mappings, 21w11a): The issue seems to stem from implementations of HurtByTargetGoal#alertOther(Mob, LivingEntity). These mobs are programmed to alert other mobs of the same type when hurt (either with a custom hurt goal that extends HurtByTargetGoal, like aggressive pandas and bees, or by adding a HurtByTargetGoal(this, new Class[0]).setAlertOthers(class)) to its target selector). When this happens, the mob sets its attack target to the first mob's target, ignoring the fact that it might be itself.

net.minecraft.world.entity.ai.goal.target.HurtByTargetGoal

{
    ...
    protected void alertOthers() {
        double $$0 = this.getFollowDistance();
        AABB $$1 = AABB.unitCubeFromLowerCorner(this.mob.position()).inflate($$0, 10.0, $$0);
        List<Entity> $$2 = this.mob.level.getEntitiesOfClass(this.mob.getClass(), $$1, EntitySelector.NO_SPECTATORS);
        for (Mob ayr2 : $$2) {
            if (this.mob == ayr2 || ayr2.getTarget() != null || this.mob instanceof TamableAnimal && ((TamableAnimal)this.mob).getOwner() != ((TamableAnimal)ayr2).getOwner() || ayr2.isAlliedTo(this.mob.getLastHurtByMob())) continue;
            if (this.toIgnoreAlert != null) {
                boolean $$4 = false;
                for (Class<?> $$5 : this.toIgnoreAlert) {
                    if (ayr2.getClass() != $$5) continue;
                    $$4 = true;
                    break;
                }
                if ($$4) continue;
            }
            this.alertOther(ayr2, this.mob.getLastHurtByMob());
        }
    }

    protected void alertOther(Mob $$0, LivingEntity $$1) {
        $$0.setTarget($$1);
    }
}

There needs to be a check in alertOther(...) to verify if the mob is not setting the target to itself.

Can confirm in 1.19 and 22w24a.

Can confirm in 1.19.2.