SPIGOT-4587: Enforce at least one valid choice for FurnaceRecipe

src/main/java/org/bukkit/craftbukkit/inventory/CraftFurnaceRecipe.java

@@ -25,6 +25,6 @@ public class CraftFurnaceRecipe extends FurnaceRecipe implements CraftRecipe {
     public void addToCraftingManager() {
         ItemStack result = this.getResult();
 
-        MinecraftServer.getServer().getCraftingManager().a(new net.minecraft.server.FurnaceRecipe(CraftNamespacedKey.toMinecraft(this.getKey()), this.getGroup(), toNMS(this.getInputChoice()), CraftItemStack.asNMSCopy(result), getExperience(), getCookingTime()));
+        MinecraftServer.getServer().getCraftingManager().a(new net.minecraft.server.FurnaceRecipe(CraftNamespacedKey.toMinecraft(this.getKey()), this.getGroup(), toNMS(this.getInputChoice(), true), CraftItemStack.asNMSCopy(result), getExperience(), getCookingTime()));
     }
 }

src/main/java/org/bukkit/craftbukkit/inventory/CraftRecipe.java

@@ -14,19 +14,26 @@ public interface CraftRecipe extends Recipe {
 
     void addToCraftingManager();
 
-    default net.minecraft.server.RecipeItemStack toNMS(RecipeChoice bukkit) {
-        if (bukkit == null) {
-            return RecipeItemStack.a;
-        } else if (bukkit instanceof RecipeChoice.MaterialChoice) {
-            return new RecipeItemStack(((RecipeChoice.MaterialChoice) bukkit).getChoices().stream().map((mat) -> new net.minecraft.server.RecipeItemStack.StackProvider(CraftItemStack.asNMSCopy(new ItemStack(mat)))));
-        } else if (bukkit instanceof RecipeChoice.ExactChoice) {
-            RecipeItemStack stack = new RecipeItemStack(Stream.of(new net.minecraft.server.RecipeItemStack.StackProvider(CraftItemStack.asNMSCopy(((RecipeChoice.ExactChoice) bukkit).getItemStack()))));
-            stack.exact = true;
+    default RecipeItemStack toNMS(RecipeChoice bukkit, boolean requireNotEmpty) {
+       RecipeItemStack stack;
 
-            return stack;
+        if (bukkit == null) {
+            stack= RecipeItemStack.a;
+        } else if (bukkit instanceof RecipeChoice.MaterialChoice) {
+            stack= new RecipeItemStack(((RecipeChoice.MaterialChoice) bukkit).getChoices().stream().map((mat) -> new net.minecraft.server.RecipeItemStack.StackProvider(CraftItemStack.asNMSCopy(new ItemStack(mat)))));
+        } else if (bukkit instanceof RecipeChoice.ExactChoice) {
+            stack = new RecipeItemStack(Stream.of(new net.minecraft.server.RecipeItemStack.StackProvider(CraftItemStack.asNMSCopy(((RecipeChoice.ExactChoice) bukkit).getItemStack()))));
+            stack.exact = true;
         } else {
             throw new IllegalArgumentException("Unknown recipe stack instance " + bukkit);
         }
+
+        stack.buildChoices();
+        if (requireNotEmpty && stack.choices.length == 0) {
+            throw new IllegalArgumentException("Recipe requires at least one non-air choice!");
+        }
+
+        return stack;
     }
 
     public static RecipeChoice toBukkit(RecipeItemStack list) {

src/main/java/org/bukkit/craftbukkit/inventory/CraftShapedRecipe.java

@@ -53,7 +53,7 @@ public class CraftShapedRecipe extends ShapedRecipe implements CraftRecipe {
         for (int i = 0; i < shape.length; i++) {
             String row = shape[i];
             for (int j = 0; j < row.length(); j++) {
-                data.set(i * width + j, toNMS(ingred.get(row.charAt(j))));
+                data.set(i * width + j, toNMS(ingred.get(row.charAt(j)), false));
             }
         }
 

src/main/java/org/bukkit/craftbukkit/inventory/CraftShapelessRecipe.java

@@ -42,7 +42,7 @@ public class CraftShapelessRecipe extends ShapelessRecipe implements CraftRecipe
         List<org.bukkit.inventory.RecipeChoice> ingred = this.getChoiceList();
         NonNullList<RecipeItemStack> data = NonNullList.a(ingred.size(), RecipeItemStack.a);
         for (int i = 0; i < ingred.size(); i++) {
-            data.set(i, toNMS(ingred.get(i)));
+            data.set(i, toNMS(ingred.get(i), true));
         }
 
         MinecraftServer.getServer().getCraftingManager().a(new ShapelessRecipes(CraftNamespacedKey.toMinecraft(this.getKey()), this.getGroup(), CraftItemStack.asNMSCopy(this.getResult()), data));