Context lost when awaiting on custom thenable

[Valeri Karpov]

Not sure if this is an issue or expected behavior, but figured I'd ask. I've been tracking down a bug with mongoose and express-http-context, and found that if you use await with a custom then() function, context goes away. Here's an example:

const clsHooked = require('cls-hooked');

const session = clsHooked.createNamespace('mytest');

session.run(() => {
  run().catch(err => console.log(err));
});

async function run() {
  session.set('test', '42');

  const thenable = {
    then: (resolve, reject) => {
      console.log('Session', session.get('test')); // undefined
      return Promise.resolve('').then(resolve, reject);
    }
  };

  await thenable; // This is the problem, context gets lost in `then()`

  // This works fine
  await new Promise((resolve, reject) => {
    console.log('Session2', session.get('test')); // 42
  });
}

Any ideas as to what might be going on here?

Re: Automattic/mongoose#7292

[Jeff Lewis]

@vkarpov15 ! Sorry I haven't gotten to this. I'm a little fuzzy on what mongoose's exec() does under the hood but my first thought is that a custom then() would need to be wrapped similar to how cls-bluebird does it.

[Roch Devost]

This is caused by an issue in Node where the context is restored by the Promise class and not the await language construct which is handled within v8.

[Valeri Karpov]

@Jeff-Lewis I've been looking more into this issue and found something you might find interesting: this issue only happens with run(). It works fine as long as you wrap your async function with runPromise() as shown below.

const clsHooked = require('cls-hooked');

const session = clsHooked.createNamespace('mytest');

session.runPromise(run);

async function run() {
  session.set('test', '42');

  const thenable = {
    then: (resolve, reject) => {
      console.log('Session', session.get('test')); // undefined
      return Promise.resolve('').then(resolve, reject);
    }
  };

  await thenable; // This is the problem, context gets lost in `then()`

  // This works fine
  await new Promise((resolve, reject) => {
    console.log('Session2', session.get('test')); // 42
  });
}

It seems like this only happens if you start using async functions inside of run() rather than runPromise().

[Roch Devost]

@vkarpov15 If your issue is caused by the bug in Node, it should be fixed by this PR in ^12.19.0 || >=14.5.0.