Needs to wait

- Commit to non-blocking waitgroup impl
- Debug log when await takes suspiciously long

src/main/java/com/jozufozu/flywheel/backend/task/ParallelTaskExecutor.java

@@ -8,6 +8,7 @@ import java.util.concurrent.ConcurrentLinkedDeque;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
 
@@ -99,10 +100,12 @@ public class ParallelTaskExecutor implements TaskExecutor {
 
 		threads.clear();
 		taskQueue.clear();
+		mainThreadQueue.clear();
+		waitGroup._reset();
 	}
 
 	@Override
-	public void execute(Runnable task) {
+	public void execute(@NotNull Runnable task) {
 		if (!running.get()) {
 			throw new IllegalStateException("Executor is stopped");
 		}
@@ -130,39 +133,35 @@ public class ParallelTaskExecutor implements TaskExecutor {
 	@Override
 	public void syncPoint() {
 		Runnable task;
-		// Finish everyone else's work...
 		while (true) {
 			if ((task = mainThreadQueue.poll()) != null) {
+				// Prioritize main thread tasks.
 				processMainThreadTask(task);
 			} else if ((task = taskQueue.pollLast()) != null) {
+				// then work on tasks from the queue.
 				processTask(task);
 			} else {
-				// and wait for any stragglers.
+				// then wait for the other threads to finish.
 				waitGroup.await();
+				// at this point there will be no more tasks in the queue, but
+				// one of the worker threads may have submitted a main thread task.
 				if (mainThreadQueue.isEmpty()) {
+					// if they didn't, we're done.
 					break;
 				}
 			}
 		}
 	}
 
-	@Nullable
-	private Runnable pollForSyncPoint() {
-		Runnable task = mainThreadQueue.poll();
-		if (task != null) {
-			return task;
-		}
-		return taskQueue.pollLast();
-	}
-
 	public void discardAndAwait() {
 		// Discard everyone else's work...
 		while (taskQueue.pollLast() != null) {
 			waitGroup.done();
 		}
 
-		// and wait for any stragglers.
+		// ...wait for any stragglers...
 		waitGroup.await();
+		// ...and clear the main thread queue.
 		mainThreadQueue.clear();
 	}
 
@@ -183,7 +182,6 @@ public class ParallelTaskExecutor implements TaskExecutor {
 		return task;
 	}
 
-	// TODO: task context
 	private void processTask(Runnable task) {
 		try {
 			task.run();

src/main/java/com/jozufozu/flywheel/lib/task/WaitGroup.java

@@ -2,8 +2,14 @@ package com.jozufozu.flywheel.lib.task;
 
 import java.util.concurrent.atomic.AtomicInteger;
 
-// https://stackoverflow.com/questions/29655531
+import org.slf4j.Logger;
+
+import com.jozufozu.flywheel.util.StringUtil;
+import com.mojang.logging.LogUtils;
+
 public class WaitGroup {
+	private static final Logger LOGGER = LogUtils.getLogger();
+
 	private final AtomicInteger counter = new AtomicInteger(0);
 
 	public void add() {
@@ -19,38 +25,25 @@ public class WaitGroup {
 	}
 
 	public void done() {
-		var result = counter.decrementAndGet();
+		if (counter.decrementAndGet() < 0) {
-		if (result == 0) {
-			synchronized (this) {
-				this.notifyAll();
-			}
-		} else if (result < 0) {
 			throw new IllegalStateException("WaitGroup counter is negative!");
 		}
 	}
 
 	public void await() {
-		try {
+		// TODO: comprehensive performance tracking for tasks
-			awaitInternal();
+		long start = System.nanoTime();
-		} catch (InterruptedException ignored) {
+		int count = 0;
-			// noop
-		}
-	}
-
-	private void awaitInternal() throws InterruptedException {
-		//		var start = System.nanoTime();
 		while (counter.get() > 0) {
 			// spin in place to avoid sleeping the main thread
-			//			synchronized (this) {
+			count++;
-			//				this.wait(timeoutMs);
+		}
-			//			}
+		long end = System.nanoTime();
+		long elapsed = end - start;
+
+		if (elapsed > 1000000) { // > 1ms
+			LOGGER.debug("Waited " + StringUtil.formatTime(elapsed) + ", looped " + count + " times");
 		}
-		//		var end = System.nanoTime();
-		//		var elapsed = end - start;
-		//
-		//		if (elapsed > 1000000) {
-		//			Flywheel.LOGGER.info("Waited " + StringUtil.formatTime(elapsed));
-		//		}
 	}
 
 	public void _reset() {