Fix issues with Recipe API

paper-server/patches/sources/net/minecraft/world/item/crafting/ShapedRecipe.java.patch

@@ -70,7 +70,7 @@
 +        char c = 'a';
 +        for (Optional<Ingredient> list : this.pattern.ingredients()) {
 +            RecipeChoice choice = CraftRecipe.toBukkit(list);
-+            if (choice != null) {
++            if (choice != RecipeChoice.empty()) { // Paper
 +                recipe.setIngredient(c, choice);
 +            }
 +

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftRecipe.java

@@ -19,7 +19,7 @@ public interface CraftRecipe extends Recipe {
     void addToCraftingManager();
 
     default Optional<Ingredient> toNMSOptional(RecipeChoice bukkit, boolean requireNotEmpty) {
-        return (bukkit == null) ? Optional.empty() : Optional.of(this.toNMS(bukkit, requireNotEmpty));
+        return (bukkit == null || bukkit == RecipeChoice.empty()) ? Optional.empty() : Optional.of(this.toNMS(bukkit, requireNotEmpty)); // Paper - support "empty" choices
     }
 
     default Ingredient toNMS(RecipeChoice bukkit, boolean requireNotEmpty) {
@@ -36,6 +36,13 @@ public interface CraftRecipe extends Recipe {
             stack = Ingredient.of(((RecipeChoice.MaterialChoice) bukkit).getChoices().stream().map((mat) -> CraftItemType.bukkitToMinecraft(mat)));
         } else if (bukkit instanceof RecipeChoice.ExactChoice) {
             stack = Ingredient.ofStacks(((RecipeChoice.ExactChoice) bukkit).getChoices().stream().map((mat) -> CraftItemStack.asNMSCopy(mat)).toList());
+            // Paper start - support "empty" choices - legacy method that spigot might incorrectly call
+            // Their impl of Ingredient.of() will error, ingredients need at least one entry.
+            // Callers running into this exception may have passed an incorrect empty() recipe choice to a non-empty slot or
+            // spigot calls this method in a wrong place.
+        } else if (bukkit == RecipeChoice.empty()) {
+            throw new IllegalArgumentException("This ingredient cannot be empty");
+            // Paper end - support "empty" choices
         } else {
             throw new IllegalArgumentException("Unknown recipe stack instance " + bukkit);
         }
@@ -48,12 +55,12 @@ public interface CraftRecipe extends Recipe {
     }
 
     public static RecipeChoice toBukkit(Optional<Ingredient> list) {
-        return list.map(CraftRecipe::toBukkit).orElse(null);
+        return list.map(CraftRecipe::toBukkit).orElse(RecipeChoice.empty()); // Paper - fix issue with recipe API
     }
 
     public static RecipeChoice toBukkit(Ingredient list) {
         if (list.isEmpty()) {
-            return null;
+            return RecipeChoice.empty(); // Paper - null breaks API contracts
         }
 
         if (list.isExact()) {

paper-server/src/test/java/io/papermc/paper/inventory/recipe/TestRecipeChoice.java

@@ -0,0 +1,25 @@
+package io.papermc.paper.inventory.recipe;
+
+import java.util.Iterator;
+import org.bukkit.Bukkit;
+import org.bukkit.inventory.Recipe;
+import org.bukkit.support.environment.AllFeatures;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+@AllFeatures
+class TestRecipeChoice {
+
+    @Test
+    void testRecipeChoices() {
+        final Iterator<Recipe> iter = Bukkit.recipeIterator();
+        boolean foundRecipes = false;
+        while (iter.hasNext()) {
+            foundRecipes = true;
+            assertDoesNotThrow(iter::next, "Failed to convert a recipe to Bukkit recipe!");
+        }
+        assertTrue(foundRecipes, "No recipes found!");
+    }
+}

paper-server/src/test/java/org/bukkit/support/DummyServerHelper.java

@@ -92,6 +92,15 @@ public final class DummyServerHelper {
         // Paper end - testing additions
 
         io.papermc.paper.configuration.GlobalConfigTestingBase.setupGlobalConfigForTest(); // Paper - configuration files - setup global configuration test base
+
+        // Paper start - add test for recipe conversion
+        when(instance.recipeIterator()).thenAnswer(ignored ->
+            com.google.common.collect.Iterators.transform(
+                RegistryHelper.getDataPack().getRecipeManager().recipes.byType.entries().iterator(),
+                input -> input.getValue().toBukkitRecipe()
+            )
+        );
+        // Paper end - add test for recipe conversion
         return instance;
     }
 }