fix(core): module execution with top level await

[bartlomieju]

Fixes #7649

This commit fixes implementation of top level await in "deno_core".

Previously promise returned from module execution was ignored causing to execute
modules out-of-order.

With this commit promise returned from module execution is stored on "JsRuntime"
and event loop is polled until the promise resolves.

[ry]

Given that it fixes a bug, I think we should land this - awesome work.

[ry]

Is it odd that JsRuntime itself is a future, but it now has a method which also returns a future?

[bartlomieju]

@ry there is a precedent with JsRuntime::load_module which we've had for a couple months. I think having load_module and mod_evaluate be async really well represents what's going on in V8.

[ry]

What happens if you try to tokio::select! on both the result of mod_evaluate and the JsRuntime itself?

[bartlomieju]

JsRuntime would be polled twice (because it's polled once in mod_evaluate). I don't like JsRuntime being a future, there's only a single time that you want to run event loop to complication - in main.rs. I would prefer to have explicit JsRuntime::poll_event_loop() instead.

[piscisaureus]

What happens if you try to tokio::select! on both the result of mod_evaluate and the JsRuntime itself?

That either isn't or shouldn't be possible.
You need a &mut reference to a Future to poll it, so you can't poll X and a container with X in it at the same time.

[bartlomieju]

Actually @piscisaureus is right:

  let fut = self.isolate.mod_evaluate(id);
    tokio::select! {
      _ = fut => {
        // 
        Ok(())
      }
      _ = self.isolate => {
        //
        Ok(())
      }
    }

error[E0507]: cannot move out of `self.isolate` which is behind a mutable reference
   --> cli/worker.rs:198:11
    |
198 |       _ = self.isolate => {
    |           ^^^^^^^^^^^^ move occurs because `self.isolate` has type `deno_core::JsRuntime`, which does not implement the `Copy` trait

error[E0505]: cannot move out of `self.isolate` because it is borrowed
   --> cli/worker.rs:198:11
    |
192 |       let fut = self.isolate.mod_evaluate(id);
    |                 ------------ borrow of `self.isolate` occurs here
193 | /     tokio::select! {
194 | |       _ = fut => {
195 | |         //
196 | |         Ok(())
197 | |       }
198 | |       _ = self.isolate => {
    | |           ^^^^^^^^^^^^ move out of `self.isolate` occurs here
...   |
201 | |       }
202 | |     }
    | |_____- borrow later used here

[piscisaureus]

The strategy seems sound to me.
Also underlines the need for this monster to be refactored...

[piscisaureus]

LGTM

A skilful hack; also great to accumulate all these tests.