The bug

Summoning any living entity on the wings of the ender dragon (same position as the middle of the wing) will give the entity infinite velocity and crash the server. This is due to a divide by zero case with the wings and a living entity.

How to reproduce

1. Create a new world
2. Go to the end
3. Give yourself a command block and set it to repeat
4. Enter this command in the command block and power the block:

   /tp @e[type=minecraft:ender_dragon] 0.0 70.0 0.0
   

5. Summon armor stands on the dragon:

   /execute at @e[type=minecraft:ender_dragon] run summon minecraft:armor_stand
   

   → The server crashing after a minute or two

Code analysis

Using 1.16.3 yarn mappings.

We will be looking at the EnderDragonEntity code today.

Let’s go to:

public void tickMovement()

Inside this function there is the launchLivingEntities function. Let’s explore this.

if (!this.world.isClient && this.hurtTime == 0) {

this.launchLivingEntities(this.world.getOtherEntities(this, this.partWingRight.getBoundingBox().expand(4.0, 2.0, 4.0).offset(0.0, -2.0, 0.0), EntityPredicates.EXCEPT_CREATIVE_OR_SPECTATOR));

this.launchLivingEntities(this.world.getOtherEntities(this, this.partWingLeft.getBoundingBox().expand(4.0, 2.0, 4.0).offset(0.0, -2.0, 0.0), EntityPredicates.EXCEPT_CREATIVE_OR_SPECTATOR));
//removed code
}

This function takes in the entities around the dragon wing (will later be limited to Living Entities )

private void launchLivingEntities(List<Entity> entities) {

    double d = (this.partBody.getBoundingBox().minX + this.partBody.getBoundingBox().maxX) / 2.0;
    double e = (this.partBody.getBoundingBox().minZ + this.partBody.getBoundingBox().maxZ) / 2.0;
    for (Entity lv : entities) {
        if (!(lv instanceof LivingEntity)) continue;
        double f = lv.getX() - d;
        double g = lv.getZ() - e;
        double h = f * f + g * g;
        lv.addVelocity(f / h * 4.0, 0.2f, g / h * 4.0);

//removed stuff below
    }
}

From this function call, it takes the wing and finds the middle of it. For the next part, let's say we have an entity at the same location as the middle of the wing and it is a living entity. As the x and z are the same, f and g will be zero. As f and g are zero, h will be zero. Now, as h is zero, the lv.addVelocity will make the x and z velocity infinite as in java a float divide by zero will make a value infinity.