fix: convert all modules running in loader thread to ESM

This is a second attempt at fixing the double-loader issue in #205. The
tests from that commit are included.

Author: bengl

hook.js

@@ -2,7 +2,7 @@
 //
 // This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2021 Datadog, Inc.
 
-import { createHook } from './hook.js'
+import { createHook } from './create-hook.mjs'
 
 const { initialize, load, resolve, getFormat, getSource } = createHook(import.meta)
 

hook.mjs

@@ -0,0 +1,9 @@
+import { readFile } from 'fs/promises'
+
+export async function load (url, context, nextLoad) {
+  const result = await nextLoad(url, context)
+  if (!result.source && url.startsWith('file:')) {
+    result.source = await readFile(new URL(url))
+  }
+  return result
+}

lib/get-esm-exports.js

@@ -1,3 +1,3 @@
 // v18.19.0 backported ESM hook execution to a separate thread,
 // thus being equivalent to >=v20.
-require('./v20-get-esm-exports')
+import './v20-get-esm-exports.mjs'

lib/get-exports.js

@@ -1,11 +1,14 @@
 'use strict'
 
-const getEsmExports = require('../../lib/get-esm-exports.js')
-const fs = require('fs')
-const assert = require('assert')
-const path = require('path')
+import getEsmExports from '../../lib/get-esm-exports.mjs'
+import fs from 'fs'
+import assert from 'assert'
+import path from 'path'
+import { fileURLToPath } from 'url'
 
-const fixturePath = path.join(__dirname, '../fixtures/esm-exports.txt')
+const dirname = path.dirname(fileURLToPath(import.meta.url))
+
+const fixturePath = path.join(dirname, '../fixtures/esm-exports.txt')
 const fixture = fs.readFileSync(fixturePath, 'utf8')
 
 fixture.split('\n').forEach(line => {

test/fixtures/double-loader.mjs

@@ -0,0 +1,15 @@
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2.0 License.
+//
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2021 Datadog, Inc.
+
+import { execSync } from 'child_process'
+import { doesNotThrow } from 'assert'
+
+const env = {
+  ...process.env,
+  NODE_OPTIONS: '--no-warnings --experimental-loader ./test/fixtures/double-loader.mjs --experimental-loader ./hook.mjs'
+}
+
+doesNotThrow(() => {
+  execSync('node -p 0', { env })
+})