Fix book recipe type being broken.

In the original commit message, I claimed outputBook was always null, but that was a lie,
I just can't read.

Restore calling getResultItem and add comments/safety checks about why what we're doing is safe.

Back out "Refine commit 8224fcf by dropping the method entirely"
Original commit changeset: f994182

Xplat/src/main/java/vazkii/patchouli/common/recipe/BookRecipeSerializer.java

@@ -13,10 +13,11 @@
 import vazkii.patchouli.common.item.PatchouliItems;
 
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.function.BiFunction;
 
-public record BookRecipeSerializer<T extends Recipe<?>, U extends T> (RecipeSerializer<T> compose, BiFunction<T, ResourceLocation, U> converter) implements RecipeSerializer<U> {
+public record BookRecipeSerializer<T extends Recipe<?>, U extends T> (RecipeSerializer<T> compose, BiFunction<T, @Nullable ResourceLocation, U> converter) implements RecipeSerializer<U> {
 	@Override
 	@NotNull
 	public U fromJson(@NotNull ResourceLocation id, @NotNull JsonObject json) {

Xplat/src/main/java/vazkii/patchouli/common/recipe/ShapedBookRecipe.java

@@ -1,12 +1,18 @@
 package vazkii.patchouli.common.recipe;
 
+import com.google.common.base.Preconditions;
+
+import net.minecraft.core.RegistryAccess;
 import net.minecraft.resources.ResourceLocation;
+import net.minecraft.world.item.ItemStack;
 import net.minecraft.world.item.crafting.CraftingBookCategory;
 import net.minecraft.world.item.crafting.RecipeSerializer;
 import net.minecraft.world.item.crafting.ShapedRecipe;
 
 import vazkii.patchouli.api.PatchouliAPI;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * Recipe type for shaped book recipes.
  * The format is the same as vanilla shaped recipes, but the
@@ -15,10 +21,17 @@
 public class ShapedBookRecipe extends ShapedRecipe {
 	public static final RecipeSerializer<ShapedBookRecipe> SERIALIZER = new BookRecipeSerializer<>(RecipeSerializer.SHAPED_RECIPE, ShapedBookRecipe::new);
 
-	public ShapedBookRecipe(ShapedRecipe compose, ResourceLocation outputBook) {
-		super(compose.getId(), compose.getGroup(), CraftingBookCategory.MISC,
-				compose.getWidth(), compose.getHeight(), compose.getIngredients(),
-				PatchouliAPI.get().getBookStack(outputBook));
+	public ShapedBookRecipe(ShapedRecipe compose, @Nullable ResourceLocation outputBook) {
+		super(compose.getId(), compose.getGroup(), CraftingBookCategory.MISC, compose.getWidth(), compose.getHeight(), compose.getIngredients(), getOutputBook(compose, outputBook));
+	}
+
+	private static ItemStack getOutputBook(ShapedRecipe compose, @Nullable ResourceLocation outputBook) {
+		Preconditions.checkArgument(compose.getClass() == ShapedRecipe.class, "Must be exactly ShapedRecipe");
+		if (outputBook != null) {
+			return PatchouliAPI.get().getBookStack(outputBook);
+		}
+		// The vanilla ShapedRecipe implementation never uses the passed RegistryAccess, so this is ok.
+		return compose.getResultItem(RegistryAccess.EMPTY);
 	}
 
 	@Override

Xplat/src/main/java/vazkii/patchouli/common/recipe/ShapelessBookRecipe.java

@@ -1,12 +1,18 @@
 package vazkii.patchouli.common.recipe;
 
+import com.google.common.base.Preconditions;
+
+import net.minecraft.core.RegistryAccess;
 import net.minecraft.resources.ResourceLocation;
+import net.minecraft.world.item.ItemStack;
 import net.minecraft.world.item.crafting.CraftingBookCategory;
 import net.minecraft.world.item.crafting.RecipeSerializer;
 import net.minecraft.world.item.crafting.ShapelessRecipe;
 
 import vazkii.patchouli.api.PatchouliAPI;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * Recipe type for shapeless book recipes.
  * The format is the same as vanilla shapeless recipes, but the
@@ -15,10 +21,17 @@
 public class ShapelessBookRecipe extends ShapelessRecipe {
 	public static final RecipeSerializer<ShapelessBookRecipe> SERIALIZER = new BookRecipeSerializer<>(RecipeSerializer.SHAPELESS_RECIPE, ShapelessBookRecipe::new);
 
-	public ShapelessBookRecipe(ShapelessRecipe compose, ResourceLocation outputBook) {
-		super(compose.getId(), compose.getGroup(), CraftingBookCategory.MISC,
-				PatchouliAPI.get().getBookStack(outputBook),
-				compose.getIngredients());
+	public ShapelessBookRecipe(ShapelessRecipe compose, @Nullable ResourceLocation outputBook) {
+		super(compose.getId(), compose.getGroup(), CraftingBookCategory.MISC, getOutputBook(compose, outputBook), compose.getIngredients());
+	}
+
+	private static ItemStack getOutputBook(ShapelessRecipe compose, @Nullable ResourceLocation outputBook) {
+		Preconditions.checkArgument(compose.getClass() == ShapelessRecipe.class, "Must be exactly ShapelessRecipe");
+		if (outputBook != null) {
+			return PatchouliAPI.get().getBookStack(outputBook);
+		}
+		// The vanilla ShapelessRecipe implementation never uses the passed RegistryAccess, so this is ok.
+		return compose.getResultItem(RegistryAccess.EMPTY);
 	}
 
 	@Override