Fix a concurrent bug due to async execution

[shrinktofit]

Let me illustrate this BUG.

Here's 3 modules:

// main
export { stamp } from './dep.js';

// dep
import './async-dep.js';
export const stamp = Date.now();

// async-dep
await Promise.resolve();

And the top level execution flow:

const thread1 = import('main'); // #1
const thread2 = import('dep'); // #2
await thread1; // #3
await thread2; // #4

For module dep:

At the end of line #1, load.E was created. But load.e kept non-nil, since dep has async dependencies - the load.e only reset when load.E resolved.

Now enter line #2, because !!load.e, another load.E would be created.

Starting from line #3, the two load.E raced - both call and reset load.e.

[github-actions]

File size impact

Merging concurrent into main will not impact files in any group.

browser (0/2)

No impact on files in browser group.

node (0/1)

No impact on files in node group.

extras (0/8)

No impact on files in extras group.

_{Generated by file size impact during size-impact#2726803254}

[shrinktofit]

@guybedford I don't know why CI failed, network problem?

[guybedford]

@shrinktofit I was able to update the CI in #2407 so we're no longer on Travis.

This PR feels something closer to the first option I offered before, in us iterating on cases as we go while accepting that the solution is still not yet comprehensive. I'm totally fine with that and trust your judgement of the problem space.

I'm going to merge this and will aim to release in the next couple of days.

If you are interested in some more advanced test cases, it get's pretty wild in https://github.com/evanw/tla-fuzzer.

[Fu-Shengsheng]

Let me illustrate this BUG.

Here's 3 modules:

// main
export { stamp } from './dep.js';

// dep
import './async-dep.js';
export const stamp = Date.now();

// async-dep
await Promise.resolve();

And the top level execution flow:

const thread1 = import('main'); // #1
const thread2 = import('dep'); // #2
await thread1; // #3
await thread2; // #4

For module dep:

At the end of line #1, load.E was created. But load.e kept non-nil, since dep has async dependencies - the load.e only reset when load.E resolved.

Now enter line #2, because !!load.e, another load.E would be created.

Starting from line #3, the two load.E raced - both call and reset load.e.

The same problem.
When I found it and want issue, it fixed lol.
Anyway, thanks a lot!