From 75c388ae8efdabad3fb9a599baa74263f32113c4 Mon Sep 17 00:00:00 2001
From: Michael Himing <mhiming@gmail.com>
Date: Sun, 16 Dec 2018 13:07:33 +1100
Subject: [PATCH] Fix PlayerEditBookEvent

- Updating book writing (not signing) mutated the original item, making
it impossible to properly cancel the event or modify the book meta

- When the event was cancelled, the client's book would keep the
cancelled writing

diff --git a/src/main/java/net/minecraft/server/PlayerConnection.java b/src/main/java/net/minecraft/server/PlayerConnection.java
index baab1a88a4..f007290ff0 100644
--- a/src/main/java/net/minecraft/server/PlayerConnection.java
+++ b/src/main/java/net/minecraft/server/PlayerConnection.java
@@ -860,9 +860,11 @@ public class PlayerConnection implements PacketListenerPlayIn {
                         itemstack2.a("pages", (NBTBase) nbttaglist);
                         this.player.a(packetplayinbedit.d(), CraftEventFactory.handleEditBookEvent(player, enumitemslot, itemstack1, itemstack2)); // CraftBukkit
                     } else {
-                        ItemStack old = itemstack1.cloneItemStack(); // CraftBukkit
-                        itemstack1.a("pages", (NBTBase) itemstack.getTag().getList("pages", 8));
-                        CraftEventFactory.handleEditBookEvent(player, enumitemslot, old, itemstack1); // CraftBukkit
+                        // Paper start - dont mutate players current item, set it from the event
+                        ItemStack newBook = itemstack1.cloneItemStack();
+                        newBook.getOrCreateTagAndSet("pages", (NBTBase)itemstack.getTag().getList("pages", 8));
+                        this.player.setSlot(enumitemslot, CraftEventFactory.handleEditBookEvent(player, enumitemslot, itemstack1, newBook));
+                        // Paper end
                     }
                 }
 
-- 
2.21.0