SPIGOT-1927: Don't allow circular entity riding

nms-patches/Entity.patch

@@ -372,11 +372,13 @@
              EntityItem entityitem = new EntityItem(this.world, this.locX, this.locY + (double) f, this.locZ, itemstack);
  
              entityitem.q();
-@@ -1422,6 +1648,22 @@
+@@ -1422,6 +1648,24 @@
          if (entity.by() != this) {
              throw new IllegalStateException("Use x.startRiding(y), not y.addPassenger(x)");
          } else {
 +            // CraftBukkit start
++            com.google.common.base.Preconditions.checkState(!entity.passengers.contains(this), "Circular entity riding! %s %s", this, entity);
++
 +            CraftEntity craft = (CraftEntity) entity.getBukkitEntity().getVehicle();
 +            Entity orig = craft == null ? null : craft.getHandle();
 +            if (getBukkitEntity() instanceof Vehicle && entity.getBukkitEntity() instanceof LivingEntity) {
@@ -395,7 +397,7 @@
              if (!this.world.isClientSide && entity instanceof EntityHuman && !(this.bt() instanceof EntityHuman)) {
                  this.passengers.add(0, entity);
              } else {
-@@ -1435,6 +1677,22 @@
+@@ -1435,6 +1679,22 @@
          if (entity.by() == this) {
              throw new IllegalStateException("Use x.stopRiding(y), not y.removePassenger(x)");
          } else {
@@ -418,7 +420,7 @@
              this.passengers.remove(entity);
              entity.j = 60;
          }
-@@ -1577,10 +1835,38 @@
+@@ -1577,10 +1837,38 @@
      }
  
      public void onLightningStrike(EntityLightning entitylightning) {
@@ -459,7 +461,7 @@
          }
  
      }
-@@ -1715,19 +2001,67 @@
+@@ -1715,19 +2003,67 @@
          if (!this.world.isClientSide && !this.dead) {
              this.world.methodProfiler.a("changeDimension");
              MinecraftServer minecraftserver = this.h();
@@ -530,7 +532,7 @@
              BlockPosition blockposition;
  
              if (i == 1) {
-@@ -1756,12 +2090,18 @@
+@@ -1756,12 +2092,18 @@
                  blockposition = new BlockPosition(this);
              }
  
@@ -550,7 +552,7 @@
                  if (j == 1 && i == 1) {
                      BlockPosition blockposition1 = worldserver1.q(worldserver1.getSpawn());
  
-@@ -1769,6 +2109,7 @@
+@@ -1769,6 +2111,7 @@
                  } else {
                      entity.setPositionRotation(blockposition, entity.yaw, entity.pitch);
                  }
@@ -558,7 +560,7 @@
  
                  boolean flag = entity.attachedToPlayer;
  
-@@ -1776,6 +2117,14 @@
+@@ -1776,6 +2119,14 @@
                  worldserver1.addEntity(entity);
                  entity.attachedToPlayer = flag;
                  worldserver1.entityJoinedWorld(entity, false);
@@ -573,7 +575,7 @@
              }
  
              this.dead = true;
-@@ -1881,6 +2230,11 @@
+@@ -1881,6 +2232,11 @@
      }
  
      public void setCustomName(String s) {
@@ -585,7 +587,7 @@
          this.datawatcher.set(Entity.az, s);
      }
  
-@@ -1938,7 +2292,26 @@
+@@ -1938,7 +2294,26 @@
      }
  
      public void a(AxisAlignedBB axisalignedbb) {
@@ -613,7 +615,7 @@
      }
  
      public float getHeadHeight() {
-@@ -2110,7 +2483,7 @@
+@@ -2110,7 +2485,7 @@
          for (Iterator iterator = this.bu().iterator(); iterator.hasNext(); entity.a(oclass, set)) {
              entity = (Entity) iterator.next();
              if (oclass.isAssignableFrom(entity.getClass())) {