First executions of delayed and repeating tasks on `FoliaAsyncScheduler` run in a single timer thread, making tasks block each other

[metabrixkt]

Expected behavior

1. Bukkit.getAsyncScheduler().runDelayed(...) and Bukkit.getAsyncScheduler().runAtFixedRate(...) schedule the task using FoliaAsyncScheduler#timerThread: it doesn't matter if it's a delayed or a repeating task
2. FoliaAsyncScheduler.AsyncScheduledTask#run() gets run for the task
3. All runs of the task are executed on FoliaAsyncScheduler#executors

Observed/Actual behavior

1. Bukkit.getAsyncScheduler().runDelayed(...) and Bukkit.getAsyncScheduler().runAtFixedRate(...) schedule the task using FoliaAsyncScheduler#timerThread: it doesn't matter if it's a delayed or a repeating task
2. FoliaAsyncScheduler.AsyncScheduledTask#run() gets run for the task
3. Each run is meant to be dispatched into the pool (FoliaAsyncScheduler#executors) but the first one isn't. The work is dispatched into the pool only when AsyncScheduledTask#state is STATE_ON_TIMER, which is instead always STATE_SCHEDULED_EXECUTOR for first runs of all task:
   1. AsyncScheduledTask gets initialized in FoliaAsyncScheduler#scheduleTimerTask(...) by its constructor: the delay argument is always null, so this.state = delay == null ? STATE_SCHEDULED_EXECUTOR : STATE_ON_TIMER becomes this.state = STATE_SCHEDULED_EXECUTOR

      Paper/paper-server/src/main/java/io/papermc/paper/threadedregions/scheduler/FoliaAsyncScheduler.java

      Lines 125 to 128 in a392d47

      	final AsyncScheduledTask ret = new AsyncScheduledTask(
      	plugin, period <= 0 ? period : unit.toNanos(period), task, null,
      	System.nanoTime() + unit.toNanos(initialDelay)
      	);

      Paper/paper-server/src/main/java/io/papermc/paper/threadedregions/scheduler/FoliaAsyncScheduler.java

      Lines 172 to 180 in a392d47

      	public AsyncScheduledTask(final Plugin plugin, final long repeatDelay, final Consumer<ScheduledTask> run,
      	final ScheduledFuture<?> delay, final long firstTarget) {
      	this.plugin = plugin;
      	this.repeatDelay = repeatDelay;
      	this.run = run;
      	this.delay = delay;
      	this.state = delay == null ? STATE_SCHEDULED_EXECUTOR : STATE_ON_TIMER;
      	this.scheduleTarget = firstTarget;
      	}

   2. Then, AsyncScheduledTask#setDelay(ScheduledFuture<?>) gets called in the same #scheduleTimerTask(...) method, once again explicitly setting the state: this.state = STATE_SCHEDULED_EXECUTOR

      Paper/paper-server/src/main/java/io/papermc/paper/threadedregions/scheduler/FoliaAsyncScheduler.java

      Lines 130 to 134 in a392d47

      	synchronized (ret) {
      	// even though ret is not published, we need to synchronise while scheduling to avoid a race condition
      	// for when a scheduled task immediately executes before we update the delay field and state field
      	ret.setDelay(this.timerThread.schedule(ret, initialDelay, unit));
      	this.tasks.add(ret);

      Paper/paper-server/src/main/java/io/papermc/paper/threadedregions/scheduler/FoliaAsyncScheduler.java

      Lines 182 to 185 in a392d47

      	private void setDelay(final ScheduledFuture<?> delay) {
      	this.delay = delay;
      	this.state = STATE_SCHEDULED_EXECUTOR;
      	}

   3. When the AsyncScheduledTask#run() is called, the local timer variable responsible for dispatching the task into the pool or running it in-place is false, because of the state being STATE_SCHEDULED_EXECUTOR

      Paper/paper-server/src/main/java/io/papermc/paper/threadedregions/scheduler/FoliaAsyncScheduler.java

      Lines 193 to 206 in a392d47

      	if (this.state == STATE_ON_TIMER) {
      	timer = true;
      	this.delay = null;
      	this.state = STATE_SCHEDULED_EXECUTOR;
      	} else if (this.state != STATE_SCHEDULED_EXECUTOR) {
      	// cancelled
      	if (this.state != STATE_CANCELLED) {
      	throw new IllegalStateException("Wrong state: " + this.state);
      	}
      	return;
      	} else {
      	timer = false;
      	this.state = STATE_EXECUTING;
      	}

   4. Since timer is false, the pool part is skipped and the task is run in-place

      Paper/paper-server/src/main/java/io/papermc/paper/threadedregions/scheduler/FoliaAsyncScheduler.java

      Lines 209 to 217 in a392d47

      	if (timer) {
      	// the scheduled executor is single thread, and unfortunately not expandable with threads
      	// so we just schedule onto the executor
      	FoliaAsyncScheduler.this.executors.execute(this);
      	return;
      	}
      	try {
      	this.run.accept(this);

   5. If the task is a single-run delayed task, its state becomes STATE_FINISHED, but if it's a repeating task its state finally becomes STATE_ON_TIMER, so going back to the 3rd point: during subsequent runs timer is true and the task is properly dispatched into the thread pool

Steps/models to reproduce

public void onEnable() {
    Bukkit.getAsyncScheduler().runDelayed(this, task -> {
        System.out.println("Starting task 1");
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            System.out.println("Task 1 interrupted");
            throw new RuntimeException(e);
        }
        System.out.println("Finishing task 1");
    }, 5, TimeUnit.SECONDS);
    Bukkit.getAsyncScheduler().runDelayed(this, task -> {
        System.out.println("Starting task 2");
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            System.out.println("Task 2 interrupted");
            throw new RuntimeException(e);
        }
        System.out.println("Finishing task 2");
    }, 7, TimeUnit.SECONDS);
}

Expected output:

...
[00:00:00] Done (...s)! For help, type "help"
[00:00:05] [TestPaperPlugin] [STDOUT] Starting task 1
[00:00:07] [TestPaperPlugin] [STDOUT] Starting task 2
[00:00:15] [TestPaperPlugin] [STDOUT] Finishing task 1
[00:00:17] [TestPaperPlugin] [STDOUT] Finishing task 2
...

Observed output:

...
[00:00:00] Done (...s)! For help, type "help"
[00:00:05] [TestPaperPlugin] [STDOUT] Starting task 1
[00:00:15] [TestPaperPlugin] [STDOUT] Finishing task 1
[00:00:15] [TestPaperPlugin] [STDOUT] Starting task 2
[00:00:25] [TestPaperPlugin] [STDOUT] Finishing task 2
...

Plugin and Datapack List

Only the test plugin

Paper version

> ver
[16:12:37 INFO]: Checking version, please wait...
[16:12:37 INFO]: This server is running Paper version 1.21.4-130-main@a392d47 (2025-01-28T00:39:56Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)
You are running the latest version

Other

sorry for way too many letters, hopefully that helps 🎄