Android `fetch` hangs indefinitely with IPv6 hosts on some devices (Happy Eyeballs)

Description

This is a very bizarre issue that has been previously reported a bunch of times, and this is basically a continuation of:

https://github.com/facebook/react-native/issues/29608

I initially started running into this on RN 0.66 with AWS Cognito. Bumping to 0.66.3 didn't help.

I'm also pretty sure this used to work before and I'm not sure when it broke. It's on an app that has been shelved for a while.

The problem is very strange because the network request does not seem to be issued, but simply hitting CMD+S to save any file so that a hot-reload is issued will immediately dispatch the network request.

I discovered the promise hanging issue by adding some logs to the fetch calls the cognito library was doing:

Notice how the .then is not executed.

While troubleshooting I came across a mention here of a workaround: https://github.com/facebook/react-native/issues/29608#issuecomment-884521699 (courtesy of @danmaas) which seems to completely resolve the issue.

Here's the same .then correctly being executed after applying that patch:

Version

0.66.3

Output of react-native info

System:
    OS: macOS 12.0.1
    CPU: (8) arm64 Apple M1
    Memory: 142.27 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 16.13.0 - /private/var/folders/9p/k1yqxx0d7rn1nlztg_wm7sbw0000gn/T/xfs-2dcae145/node
    Yarn: 2.4.0-git.20210330.hash-ebcd71d5 - /private/var/folders/9p/k1yqxx0d7rn1nlztg_wm7sbw0000gn/T/xfs-2dcae145/yarn
    npm: 7.20.1 - ~/.nvm/versions/node/v16.13.0/bin/npm
    Watchman: 2021.11.01.00 - /opt/homebrew/bin/watchman
  Managers:
    CocoaPods: 1.11.0 - /Users/andreialecu/.rbenv/shims/pod
  SDKs:
    iOS SDK:
      Platforms: DriverKit 21.0.1, iOS 15.0, macOS 12.0, tvOS 15.0, watchOS 8.0
    Android SDK: Not Found
  IDEs:
    Android Studio: 4.2 AI-202.7660.26.42.7351085
    Xcode: 13.1/13A1030d - /usr/bin/xcodebuild
  Languages:
    Java: 11.0.8 - /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/javac
  npmPackages:
    @react-native-community/cli: Not Found
    react: 17.0.2 => 17.0.2 
    react-native: 0.66.3 => 0.66.3 
    react-native-macos: Not Found
  npmGlobalPackages:
    *react-native*: Not Found

Steps to reproduce

I'm able to reproduce it with this:

  React.useEffect(() => {
    console.log('confirm start', new Date());
    fetch('https://cognito-idp.eu-west-1.amazonaws.com/', {
      method: 'POST',
      mode: 'cors',
    })
      .then(() => console.log('then', new Date()))
      .catch(() => console.log('catch', new Date()));
    setTimeout(() => {
      console.log('5 seconds passed');
    }, 5000);
  }, []);

Output:

After applying https://github.com/facebook/react-native/issues/29608#issuecomment-884521699:

Snack, code example, screenshot, or link to a repository

No response

Skip to this comment for the actual cause: https://github.com/facebook/react-native/issues/32730#issuecomment-990764376

Comienza con la reproducción de fetch en TypeScript contra la URL de Cognito y lee el comentario enlazado que identifica la causa real y el workaround. En el issue no se menciona ningún archivo fuente ni ninguna prueba; se considera terminado cuando se confirma que fetch se resuelve o falla, en lugar de quedar bloqueado indefinidamente en los dispositivos Android afectados.