Implement a single-threaded emulation of DeferredWorkQueue (#104)

* Implement a single-threaded emulation of DeferredWorkQueue

* Add main thread checks and docs

Author: cittyinthecloud

patchwork-fml/src/main/java/net/minecraftforge/fml/DeferredWorkQueue.java

@@ -0,0 +1,87 @@
+/*
+ * Minecraft Forge, Patchwork Project
+ * Copyright (c) 2016-2020, 2019-2020
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.fml;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.CompletableFuture;
+import java.util.function.Supplier;
+
+import net.minecraft.util.thread.ThreadExecutor;
+
+import net.fabricmc.loader.api.FabricLoader;
+
+/**
+ * A dummy version of DeferredWorkQueue which instantly completes futures on the same thread that calls it.
+ *
+ * <p>Forge mods use this during loading to defer calls back to the loading thread, but they are already loading
+ * on the main thread under Patchwork.
+ */
+public class DeferredWorkQueue {
+	private static void mainThreadCheck() {
+		final Object gameInstance = FabricLoader.getInstance().getGameInstance();
+
+		if (!((ThreadExecutor) gameInstance).isOnThread()) {
+			throw new IllegalStateException("Multiple threads during loading is not supported.");
+		}
+	}
+
+	public static CompletableFuture<Void> runLater(Runnable workToEnqueue) {
+		mainThreadCheck();
+		workToEnqueue.run();
+		return CompletableFuture.completedFuture(null);
+	}
+
+	public static CompletableFuture<Void> runLaterChecked(CheckedRunnable workToEnqueue) {
+		mainThreadCheck();
+		CompletableFuture<Void> future = new CompletableFuture<>();
+
+		try {
+			workToEnqueue.run();
+			future.complete(null);
+		} catch (Throwable t) {
+			future.completeExceptionally(t);
+		}
+
+		return future;
+	}
+
+	public static <T> CompletableFuture<T> getLater(Supplier<T> workToEnqueue) {
+		mainThreadCheck();
+		return CompletableFuture.completedFuture(workToEnqueue.get());
+	}
+
+	public static <T> CompletableFuture<T> getLaterChecked(Callable<T> workToEnqueue) {
+		mainThreadCheck();
+		CompletableFuture<T> future = new CompletableFuture<>();
+
+		try {
+			future.complete(workToEnqueue.call());
+		} catch (Throwable t) {
+			future.completeExceptionally(t);
+		}
+
+		return future;
+	}
+
+	@FunctionalInterface
+	public interface CheckedRunnable {
+		void run() throws Exception;
+	}
+}