From 0000000000000000000000000000000000000000 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 5bec3b9692f64a792e26f89b4147644eaaf4ab11..60dae06ff5c016aecba23afd7a4152fcd3234616 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -1880,11 +1880,12 @@ 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
@@ -2532,6 +2533,12 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
     @Nullable
     public Entity teleportTo(WorldServer worldserver, BlockPosition location) {
         // CraftBukkit end
+        // Paper start - fix bad state entities causing dupes
+        if (!isAlive() || !valid) {
+            LOGGER.warn("Illegal Entity Teleport " + this + " to " + worldserver + ":" + location, new Throwable());
+            return null;
+        }
+        // Paper end
         if (this.world instanceof WorldServer && !this.dead) {
             this.world.getMethodProfiler().enter("changeDimension");
             // CraftBukkit start
@@ -2567,7 +2574,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
                     entity.bukkitEntity = this.getBukkitEntity();
 
                     if (this instanceof EntityInsentient) {
-                        ((EntityInsentient) this).unleash(true, false); // Unleash to prevent duping of leads.
+                        ((EntityInsentient) this).unleash(true, true); // Paper drop lead
                     }
                     // CraftBukkit end
                 }
@@ -2687,7 +2694,7 @@ 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 01c0ec114cf913e54058a2f5298807dbb83d1cf4..66597c6e1add4df2f093480f507f03788a3eb30b 100644
--- a/src/main/java/net/minecraft/server/EntityArmorStand.java
+++ b/src/main/java/net/minecraft/server/EntityArmorStand.java
@@ -560,7 +560,7 @@ 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.b);
             }
         }
@@ -568,7 +568,7 @@ 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.b);
             }
         }
diff --git a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
index c86e012931e4112152fbf72dac15fb357ac7f6b0..2fe655e0b7655cc9b4ce1509c6d6cfd51860c006 100644
--- a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+++ b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
@@ -811,7 +811,8 @@ 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
+            if (stack instanceof CraftItemStack) stack.setAmount(0); // Paper - destroy this item - if this ever leaks due to game bugs, ensure it doesn't dupe, but don't nuke bukkit stacks of manually added items
         }
 
         return event;