[dart2wasm] Async/microtask ordering is incorrect.

[eyebrowsoffire]

When compiling with dart2wasm, the order in which microtasks resolve is not consistent with dart2wasm. This is exhibited in some of the tests in the flutter/engine repo under flutter/lib/web_ui/test/channel_buffers_test.dart.

[askeksa-google]

Is the issue that microtasks are expected to execute before other tasks in the event queue? If so, we might be able to work around it by executing all pending microtasks in these cases:

• Before executing a callback that is not a microtask (e.g. Timer callbacks)
• When we come out of an await (i.e. after calling _futurePromise in _awaitHelper)

This will naturally mess up the stack traces, so we'll need to fix those up somehow.

[eyebrowsoffire]

I found this issue cropping up in a few other engine tests. I produced a pretty minimal test case here that fails in dart2wasm but passes in dart2js:

import 'dart:async';

import 'package:async_helper/async_minitest.dart';

main() {
  test('interleaved', async () {
    final controller1 = StreamController<int>();
    final controller2 = StreamController<int>();
    final stream1 = controller1.stream.asBroadcastStream();
    final stream2 = controller2.stream.asBroadcastStream();
    
    controller1.add(1);
    controller2.add(2);

    expect(await stream1.first, equals(1));
    expect(await stream2.first, equals(2));
    
    controller1.add(3);
    controller2.add(4);
    
    expect(await stream1.first, equals(3));
    expect(await stream2.first, equals(4));
  });
}

[osa1]

@eyebrowsoffire is there a syntax error in your example? I think first line in main should be () async { instead of async () {, otherwise I'm getting this error:

test.dart:6:23: Error: A function expression can't have a name.
  test('interleaved', async () {
                      ^^^^^

With the change, I see this output in JIT:

unittest-suite-wait-for-done
unittest-suite-success

With dart2wasm the second line is not printed, but I don't see any exception failures.

#51599 is another and smaller task scheduling issue.

[lrhn]

Be careful with assuming something about "coming out of an await". Futures don't always use the microtask queue. A completing future can propagate its result to a number of transitive listeners without going back to the event loop.

Microtasks must run before any other event. The design, taken from the browser, is that every top-level event can trigger further microtasks, which then run in the context of that top-level event. When the microtask queue is empty, control goes back to the main event loop.
"In the context of" is significant on the web, where, e.g., a database transaction is a single event and the transaction gets committed when the event ends. You can still run more microtasks in the same context, before the transaction ends.

That said, we don't actually promise the precise timing of microtasks.

I'm actually surprised that expect(await stream2.first, equals(4)); succeeds. I'd have expected that event to have been lost while the previous line is awaiting. Not code I'd ever promise a specific behavior of.
I'm guessing that's what happens in dart2wasm, the event was delivered before stream2.first was called the second time, so it hangs forever waiting for an event that never comes.

[osa1]

Micro-task scheduling should be fixed with 87a3de4 but async* compilation is still not quite right.

[mkustermann]

The minimal reproduction from #50778 (comment) (when fixing the syntax error) succeeds in both dart2wasm and the VM.

The linked #51599 is also fixed.

Assumed stale.