cloud.Service causes thread to hang

[Chriscbr]

I tried this:

I wrote this Wing program:

bring cloud;
bring util;

let counter = new cloud.Counter();

let service = new cloud.Service(onStart: inflight () => {
  while true {
    if counter.peek() > 0 {
      counter.inc();
      return;
    }
    util.sleep(0.1s);
  }
});

test "counter increases by 2" {
  assert(counter.peek() == 0);
  counter.inc();
  util.sleep(1s);
  assert(counter.peek() == 2);
}

and I ran wing test main.w

This happened:

The test hangs indefinitely

I expected this:

I expected the test to pass. cloud.Service represents a long-running service, so it should be possible to have it run code continuously in the background.

Is there a workaround?

No response

Component

SDK

Wing Version

0.28.1

Node.js Version

18.14.1

Platform(s)

MacOS

Anything else?

Even though the simulator is single-threaded, this shouldn't cause an issue since the service's onStart code calls util.sleep() which gives the JavaScript runtime the chance switch to other tasks.

See this passing test with cloud.Function:

bring cloud;
bring util;

let queue = new cloud.Queue();
let counter = new cloud.Counter();

let fn = new cloud.Function(inflight () => {
  while true {
    if counter.peek() > 0 {
      counter.inc();
      return;
    }
    util.sleep(0.1s);
  }
});
queue.setConsumer(inflight () => {
  fn.invoke("");
});

test "counter increases by 2" {
  assert(counter.peek() == 0);
  queue.push("");
  counter.inc();
  util.sleep(1s);
  assert(counter.peek() == 2);
}

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[monadabot]

Congrats! 🚀 This was released in Wing 0.29.7.

[eladb]

@Chriscbr this feels wrong (at least in my mind). The intent was that onStart is asynchronous and never blocks.

Maybe we need to change its name or document it better but as I see it onStart should kick off the start of the server and yield asynchronously. A while(true) model is less common in Node runtimes.

[Chriscbr]

@eladb OK, got it. Definitely sounds worth documenting.

as I see it onStart should kick off the start of the server and yield asynchronously

right, if I understand your comment correctly, if I wanted to achieve the behavior in the example I gave, I'd have to move the while loop into an inflight function like startServer, and then call defer startServer(); inside of onStart so that it doesn't block things? Yes I think this should work 👍

[eladb]

I'd have to move the while loop

Not sure exactly what's the use case behind the example, but generally speaking, busy-waiting is not a common pattern in node.js based programs. Perhaps some kind of an async interval is the right fit for something like this (setInterval(), which we don't support in util yet I believe).

[Chriscbr]

Not sure exactly what's the use case behind the example

Say if you were trying to poll for tasks from an SQS queue from an ECS task container (you aren't a built-in AWS integration, so you are responsible for doing polling yourself). If I was implementing this, my initial hunch would be to write a busy loop of some kind, right?

1. Try to receive a batch of messages
2. If messages were received, process them, go back to step 1
3. If there were no messages, sleep for a bit, go back to step 1

But I think you're right that using setInterval() would be more idiomatic in the Node.js world, and it feels like a better way to solve this...