PaperMC/paper-server/nms-patches/net/minecraft/world/entity/animal/EntityCow.patch

--- a/net/minecraft/world/entity/animal/EntityCow.java
+++ b/net/minecraft/world/entity/animal/EntityCow.java
@@ -30,6 +30,11 @@
 import net.minecraft.world.level.World;
 import net.minecraft.world.level.block.state.IBlockData;
 
+// CraftBukkit start
+import org.bukkit.craftbukkit.event.CraftEventFactory;
+import org.bukkit.craftbukkit.inventory.CraftItemStack;
+// CraftBukkit end
+
 public class EntityCow extends EntityAnimal {
 
     public EntityCow(EntityTypes<? extends EntityCow> entitytypes, World world) {
@@ -82,8 +87,16 @@
         ItemStack itemstack = entityhuman.b(enumhand);
 
         if (itemstack.getItem() == Items.BUCKET && !this.isBaby()) {
+            // CraftBukkit start - Got milk?
+            org.bukkit.event.player.PlayerBucketFillEvent event = CraftEventFactory.callPlayerBucketFillEvent((WorldServer) entityhuman.world, entityhuman, this.getChunkCoordinates(), this.getChunkCoordinates(), null, itemstack, Items.MILK_BUCKET);
+
+            if (event.isCancelled()) {
+                return EnumInteractionResult.PASS;
+            }
+            // CraftBukkit end
+
             entityhuman.playSound(SoundEffects.ENTITY_COW_MILK, 1.0F, 1.0F);
-            ItemStack itemstack1 = ItemLiquidUtil.a(itemstack, entityhuman, Items.MILK_BUCKET.createItemStack());
+            ItemStack itemstack1 = ItemLiquidUtil.a(itemstack, entityhuman, CraftItemStack.asNMSCopy(event.getItemStack())); // CraftBukkit
 
             entityhuman.a(enumhand, itemstack1);
             return EnumInteractionResult.a(this.world.isClientSide);