craftbukkit/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
@@ -31,6 +31,12 @@
 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;
+import org.bukkit.event.player.PlayerBucketFillEvent;
+// CraftBukkit end
+
 public class EntityCow extends EntityAnimal {
 
     private static final EntitySize BABY_DIMENSIONS = EntityTypes.COW.getDimensions().scale(0.5F).withEyeHeight(0.665F);
@@ -92,8 +98,16 @@
         ItemStack itemstack = entityhuman.getItemInHand(enumhand);
 
         if (itemstack.is(Items.BUCKET) && !this.isBaby()) {
+            // CraftBukkit start - Got milk?
+            PlayerBucketFillEvent event = CraftEventFactory.callPlayerBucketFillEvent((WorldServer) entityhuman.level(), entityhuman, this.blockPosition(), this.blockPosition(), null, itemstack, Items.MILK_BUCKET, enumhand);
+
+            if (event.isCancelled()) {
+                return EnumInteractionResult.PASS;
+            }
+            // CraftBukkit end
+
             entityhuman.playSound(SoundEffects.COW_MILK, 1.0F, 1.0F);
-            ItemStack itemstack1 = ItemLiquidUtil.createFilledResult(itemstack, entityhuman, Items.MILK_BUCKET.getDefaultInstance());
+            ItemStack itemstack1 = ItemLiquidUtil.createFilledResult(itemstack, entityhuman, CraftItemStack.asNMSCopy(event.getItemStack())); // CraftBukkit
 
             entityhuman.setItemInHand(enumhand, itemstack1);
             return EnumInteractionResult.SUCCESS;