From a8fee247c04a539c268716e2907eae22405c9a41 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 25 Apr 2020 06:46:35 -0400
Subject: [PATCH] Fix numerous item duplication issues and teleport issues

This notably fixes the newest "Donkey Dupe", but also fixes a lot
of dupe bugs in general around nether portals and entity world transfer

We also fix item duplication generically by anytime we clone an item
to drop it on the ground, destroy the source item.

This avoid an itemstack ever existing twice in the world state pre
clean up stage.

So even if something NEW comes up, it would be impossible to drop the
same item twice because the source was destroyed.

diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index f973466ba5..a69969f46b 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -1966,11 +1966,12 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
         } else {
             // CraftBukkit start - Capture drops for death event
             if (this instanceof EntityLiving && !((EntityLiving) this).forceDrops) {
-                ((EntityLiving) this).drops.add(org.bukkit.craftbukkit.inventory.CraftItemStack.asBukkitCopy(itemstack));
+                ((EntityLiving) this).drops.add(org.bukkit.craftbukkit.inventory.CraftItemStack.asCraftMirror(itemstack)); // Paper - mirror so we can destroy it later
                 return null;
             }
             // CraftBukkit end
-            EntityItem entityitem = new EntityItem(this.world, this.locX(), this.locY() + (double) f, this.locZ(), itemstack);
+            EntityItem entityitem = new EntityItem(this.world, this.locX(), this.locY() + (double) f, this.locZ(), itemstack.cloneItemStack()); // Paper - clone so we can destroy original
+            itemstack.setCount(0); // Paper - destroy this item - if this ever leaks due to game bugs, ensure it doesn't dupe
 
             entityitem.defaultPickupDelay();
             // CraftBukkit start
@@ -2632,6 +2633,12 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
     @Nullable
     public Entity teleportTo(DimensionManager dimensionmanager, BlockPosition location) {
         // CraftBukkit end
+        // Paper start - fix bad state entities causing dupes
+        if (!isAlive() || !valid) {
+            LOGGER.warn("Illegal Entity Teleport " + this + " to " + dimensionmanager + ":" + location, new Throwable());
+            return null;
+        }
+        // Paper end
         if (!this.world.isClientSide && !this.dead) {
             this.world.getMethodProfiler().enter("changeDimension");
             MinecraftServer minecraftserver = this.getMinecraftServer();
@@ -2755,7 +2762,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
     }
 
     public boolean canPortal() {
-        return true;
+        return isAlive() && valid; // Paper
     }
 
     public float a(Explosion explosion, IBlockAccess iblockaccess, BlockPosition blockposition, IBlockData iblockdata, Fluid fluid, float f) {
diff --git a/src/main/java/net/minecraft/server/EntityArmorStand.java b/src/main/java/net/minecraft/server/EntityArmorStand.java
index 8ad131e4fc..d35a0b2d94 100644
--- a/src/main/java/net/minecraft/server/EntityArmorStand.java
+++ b/src/main/java/net/minecraft/server/EntityArmorStand.java
@@ -557,7 +557,7 @@ public class EntityArmorStand extends EntityLiving {
         for (i = 0; i < this.handItems.size(); ++i) {
             itemstack = (ItemStack) this.handItems.get(i);
             if (!itemstack.isEmpty()) {
-                drops.add(org.bukkit.craftbukkit.inventory.CraftItemStack.asBukkitCopy(itemstack)); // CraftBukkit - add to drops
+                drops.add(org.bukkit.craftbukkit.inventory.CraftItemStack.asCraftMirror(itemstack)); // CraftBukkit - add to drops // Paper - mirror so we can destroy it later - though this call site was safe
                 this.handItems.set(i, ItemStack.a);
             }
         }
@@ -565,7 +565,7 @@ public class EntityArmorStand extends EntityLiving {
         for (i = 0; i < this.armorItems.size(); ++i) {
             itemstack = (ItemStack) this.armorItems.get(i);
             if (!itemstack.isEmpty()) {
-                drops.add(org.bukkit.craftbukkit.inventory.CraftItemStack.asBukkitCopy(itemstack)); // CraftBukkit - add to drops
+                drops.add(org.bukkit.craftbukkit.inventory.CraftItemStack.asCraftMirror(itemstack)); // CraftBukkit - add to drops // Paper - mirror so we can destroy it later - though this call site was safe
                 this.armorItems.set(i, ItemStack.a);
             }
         }
diff --git a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
index 1f5d15bb49..75d38bc5d1 100644
--- a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+++ b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
@@ -769,7 +769,8 @@ public class CraftEventFactory {
         for (org.bukkit.inventory.ItemStack stack : event.getDrops()) {
             if (stack == null || stack.getType() == Material.AIR || stack.getAmount() == 0) continue;
 
-            world.dropItem(entity.getLocation(), stack);
+            world.dropItem(entity.getLocation(), stack); // Paper - note: dropItem already clones due to this being bukkit -> NMS
+            stack.setAmount(0); // Paper - destroy this item - if this ever leaks due to game bugs, ensure it doesn't dupe
         }
 
         return event;
-- 
2.25.1