Imported modules are undefined if also used inside loader

[bartosz-m]

I'm writing custom loader for esm modules and encountered problem.

1. Inside loader I import module A
2. I use loader to load module B which also uses module A.
3. Path to module A is correctly resolved
4. In loader module A is correctly loaded but in B its value is undefined.
5. Removing module A from loader "fixes" problem.

I bypass problem by keeping track of what modules are used in loader and when one of them is requested I use dynamicInstantiate hook.

Minimal code for issue looks like this:

loader.mjs

import express from 'express'; // same module

export async function resolve(specifier, parentModuleURL, defaultResolver) {
  return defaultResolver(specifier, parentModuleURL);
}

issue.mjs

import express from 'express'; // same module
import assert from 'assert';

assert(express);

command

node --experimental-modules --loader ./loader.mjs ./issue.mjs

expected

(node:8534) ExperimentalWarning: The ESM module loader is experimental.

actual

(node:8534) ExperimentalWarning: The ESM module loader is experimental.
{ AssertionError [ERR_ASSERTION]: undefined == true
    at file:///home/bartek/Projekty/github/issues/issue.mjs:4:1
    at ModuleJob.run (internal/loader/ModuleJob.js:96:14)
    at <anonymous>
  generatedMessage: true,
  name: 'AssertionError [ERR_ASSERTION]',
  code: 'ERR_ASSERTION',
  actual: undefined,
  expected: true,
  operator: '==' }

• Version: v.9.2.0
• Platform: Linux 4.13.0-16-generic Ubuntu 17.10 x86_64

[guybedford]

Thanks for reporting, I've created a fix at #17131.

[guybedford]

The fix here has been merged.