Fix a race condition in @babel/core

[JLHwung]

Q	A
Fixed Issues?	Fixes #14806 (comment)
Patch: Bug Fix?	Y
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This PR fixes a race condition in createUncachedDescriptors: After we asynchronously initialize plugins/presets descriptors, we don't check again whether it has been initialized in another task. So we assign different descriptors to config item and the plugins/presets are loaded more than once.

I recommend to review this PR by commits.

This PR also reverts #14806 per Nicolò's comment on that PR.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/52713/

[nicolo-ribaudo]

Instead of just ignoring the result of the second createPluginDescriptors call, can we avoid calling it twice in the first place? Something like this:

  let pluginsGen;
  return {
    options: optionsWithResolvedBrowserslistConfigFile(options, dirname),
    *plugins() {
      pluginsGen ??= createPluginDescriptors(
        options.plugins || [],
        dirname,
        alias,
      );
      return yield* pluginsGen;
    }
  };

[JLHwung]

If we cache the iterator instead, test will fail because the finished iterator always return undefined: https://github.com/JLHwung/babel/runs/7819160996?check_suite_focus=true

[nicolo-ribaudo]

I pushed a commit to do the caching, so that now:

• The first call runs createPluginDescriptors, and after a while returns it
• The second call waits for the result of the first call, and returns it

Instead of

• The first call runs createPluginDescriptors, and returns the result of the createPluginDescriptors that finishes first
• The second call runs createPluginDescriptors, and returns the result of the createPluginDescriptors that finishes first

This avoids resolving the same plugins/presets multiple times, which is a costly operation because it involves the filesystem.

[liuxingbaoyu]

Unfortunately, this test does not reflect the real problem.
CI still has tens of thousands of lines of logs, even before @nicolo-ribaudo's commit.

https://github.com/babel/babel/runs/7775496327?check_suite_focus=true

.

[nicolo-ribaudo]

It's probably because we build Babel using an older version of itself 🤔 This PR still fixes the issue shown in the test, we can potentially wait to revert #14806 until after the new release.

[liuxingbaoyu]

Makes sense! Let's merge it first.

For #14806 I think it's good to keep it that way, only a few of us will see these logs anyway.😄