Paper/paper-server/nms-patches/net/minecraft/world/item/ItemLeash.patch

--- a/net/minecraft/world/item/ItemLeash.java
+++ b/net/minecraft/world/item/ItemLeash.java
@@ -16,6 +16,11 @@
 import net.minecraft.world.level.gameevent.GameEvent;
 import net.minecraft.world.phys.AxisAlignedBB;
 
+// CraftBukkit start
+import org.bukkit.craftbukkit.CraftEquipmentSlot;
+import org.bukkit.event.hanging.HangingPlaceEvent;
+// CraftBukkit end
+
 public class ItemLeash extends Item {
 
     public ItemLeash(Item.Info item_info) {
@@ -32,7 +37,7 @@
             EntityHuman entityhuman = itemactioncontext.getPlayer();
 
             if (!world.isClientSide && entityhuman != null) {
-                bindPlayerMobs(entityhuman, world, blockposition);
+                bindPlayerMobs(entityhuman, world, blockposition, itemactioncontext.getHand()); // CraftBukkit - Pass hand
             }
 
             return EnumInteractionResult.sidedSuccess(world.isClientSide);
@@ -41,7 +46,7 @@
         }
     }
 
-    public static EnumInteractionResult bindPlayerMobs(EntityHuman entityhuman, World world, BlockPosition blockposition) {
+    public static EnumInteractionResult bindPlayerMobs(EntityHuman entityhuman, World world, BlockPosition blockposition, net.minecraft.world.EnumHand enumhand) { // CraftBukkit - Add EnumHand
         EntityLeash entityleash = null;
         double d0 = 7.0D;
         int i = blockposition.getX();
@@ -54,19 +59,50 @@
 
         EntityInsentient entityinsentient;
 
-        for (Iterator iterator = list.iterator(); iterator.hasNext(); entityinsentient.setLeashedTo(entityleash, true)) {
+        for (Iterator iterator = list.iterator(); iterator.hasNext();) { // CraftBukkit - handle setLeashedTo at end of loop
             entityinsentient = (EntityInsentient) iterator.next();
             if (entityleash == null) {
                 entityleash = EntityLeash.getOrCreateKnot(world, blockposition);
+
+                // CraftBukkit start - fire HangingPlaceEvent
+                org.bukkit.inventory.EquipmentSlot hand = CraftEquipmentSlot.getHand(enumhand);
+                HangingPlaceEvent event = new HangingPlaceEvent((org.bukkit.entity.Hanging) entityleash.getBukkitEntity(), entityhuman != null ? (org.bukkit.entity.Player) entityhuman.getBukkitEntity() : null, world.getWorld().getBlockAt(i, j, k), org.bukkit.block.BlockFace.SELF, hand);
+                world.getCraftServer().getPluginManager().callEvent(event);
+
+                if (event.isCancelled()) {
+                    entityleash.discard(null); // CraftBukkit - add Bukkit remove cause
+                    return EnumInteractionResult.PASS;
+                }
+                // CraftBukkit end
                 entityleash.playPlacementSound();
             }
+
+            // CraftBukkit start
+            if (org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerLeashEntityEvent(entityinsentient, entityleash, entityhuman, enumhand).isCancelled()) {
+                iterator.remove();
+                continue;
+            }
+
+            entityinsentient.setLeashedTo(entityleash, true);
+            // CraftBukkit end
         }
 
         if (!list.isEmpty()) {
             world.gameEvent((Holder) GameEvent.BLOCK_ATTACH, blockposition, GameEvent.a.of((Entity) entityhuman));
             return EnumInteractionResult.SUCCESS;
         } else {
+            // CraftBukkit start- remove leash if we do not leash any entity because of the cancelled event
+            if (entityleash != null) {
+                entityleash.discard(null);
+            }
+            // CraftBukkit end
             return EnumInteractionResult.PASS;
         }
     }
+
+    // CraftBukkit start
+    public static EnumInteractionResult bindPlayerMobs(EntityHuman entityhuman, World world, BlockPosition blockposition) {
+        return bindPlayerMobs(entityhuman, world, blockposition, net.minecraft.world.EnumHand.MAIN_HAND);
+    }
+    // CraftBukkit end
 }