PaperMC/paper-server/patches/sources/net/minecraft/world/item/LeadItem.java.patch

--- a/net/minecraft/world/item/LeadItem.java
+++ b/net/minecraft/world/item/LeadItem.java
@@ -18,6 +18,11 @@
 import net.minecraft.world.level.block.state.BlockState;
 import net.minecraft.world.level.gameevent.GameEvent;
 import net.minecraft.world.phys.AABB;
+// CraftBukkit start
+import org.bukkit.craftbukkit.CraftEquipmentSlot;
+import org.bukkit.craftbukkit.block.CraftBlock;
+import org.bukkit.event.hanging.HangingPlaceEvent;
+// CraftBukkit end
 
 public class LeadItem extends Item {
 
@@ -35,37 +40,70 @@
             Player entityhuman = context.getPlayer();
 
             if (!world.isClientSide && entityhuman != null) {
-                return LeadItem.bindPlayerMobs(entityhuman, world, blockposition);
+                return LeadItem.bindPlayerMobs(entityhuman, world, blockposition, context.getHand()); // CraftBukkit - Pass hand
             }
         }
 
         return InteractionResult.PASS;
     }
 
-    public static InteractionResult bindPlayerMobs(Player player, Level world, BlockPos pos) {
+    public static InteractionResult bindPlayerMobs(Player entityhuman, Level world, BlockPos blockposition, net.minecraft.world.InteractionHand enumhand) { // CraftBukkit - Add EnumHand
         LeashFenceKnotEntity entityleash = null;
-        List<Leashable> list = LeadItem.leashableInArea(world, pos, (leashable) -> {
-            return leashable.getLeashHolder() == player;
+        List<Leashable> list = LeadItem.leashableInArea(world, blockposition, (leashable) -> {
+            return leashable.getLeashHolder() == entityhuman;
         });
 
         Leashable leashable;
 
-        for (Iterator iterator = list.iterator(); iterator.hasNext(); leashable.setLeashedTo(entityleash, true)) {
+        for (Iterator iterator = list.iterator(); iterator.hasNext();) { // CraftBukkit - handle setLeashedTo at end of loop
             leashable = (Leashable) iterator.next();
             if (entityleash == null) {
-                entityleash = LeashFenceKnotEntity.getOrCreateKnot(world, pos);
+                entityleash = LeashFenceKnotEntity.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, CraftBlock.at(world, blockposition), org.bukkit.block.BlockFace.SELF, hand);
+                world.getCraftServer().getPluginManager().callEvent(event);
+
+                if (event.isCancelled()) {
+                    entityleash.discard(null); // CraftBukkit - add Bukkit remove cause
+                    return InteractionResult.PASS;
+                }
+                // CraftBukkit end
                 entityleash.playPlacementSound();
             }
+
+            // CraftBukkit start
+            if (leashable instanceof Entity leashed) {
+                if (org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerLeashEntityEvent(leashed, entityleash, entityhuman, enumhand).isCancelled()) {
+                    iterator.remove();
+                    continue;
+                }
+            }
+
+            leashable.setLeashedTo(entityleash, true);
+            // CraftBukkit end
         }
 
         if (!list.isEmpty()) {
-            world.gameEvent((Holder) GameEvent.BLOCK_ATTACH, pos, GameEvent.Context.of((Entity) player));
+            world.gameEvent((Holder) GameEvent.BLOCK_ATTACH, blockposition, GameEvent.Context.of((Entity) entityhuman));
             return InteractionResult.SUCCESS_SERVER;
         } 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 InteractionResult.PASS;
         }
     }
 
+    // CraftBukkit start
+    public static InteractionResult bindPlayerMobs(Player player, Level world, BlockPos pos) {
+        return LeadItem.bindPlayerMobs(player, world, pos, net.minecraft.world.InteractionHand.MAIN_HAND);
+    }
+    // CraftBukkit end
+
     public static List<Leashable> leashableInArea(Level world, BlockPos pos, Predicate<Leashable> predicate) {
         double d0 = 7.0D;
         int i = pos.getX();