Support export * from 'module' ESM syntax (#43)

This change adds support for modules that export entities through the
`export * from 'module'` ESM syntax. This resolves issue #31.

hook.js

@@ -2,6 +2,7 @@
 //
 // This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2021 Datadog, Inc.
 
+const { randomBytes } = require('crypto')
 const specifiers = new Map()
 const isWin = process.platform === "win32"
 
@@ -77,6 +78,76 @@ function needsToAddFileProtocol(urlObj) {
   return !isFileProtocol(urlObj) && NODE_MAJOR < 18
 }
 
+/**
+ * Determines if a specifier represents an export all ESM line.
+ * Note that the expected `line` isn't 100% valid ESM. It is derived
+ * from the `getExports` function wherein we have recognized the true
+ * line and re-mapped it to one we expect.
+ *
+ * @param {string} line
+ * @returns {boolean}
+ */
+function isStarExportLine(line) {
+  return /^\* from /.test(line)
+}
+
+/**
+ * @typedef {object} ProcessedModule
+ * @property {string[]} imports A set of ESM import lines to be added to the
+ * shimmed module source.
+ * @property {string[]} namespaces A set of identifiers representing the
+ * modules in `imports`, e.g. for `import * as foo from 'bar'`, "foo" will be
+ * present in this array.
+ * @property {string[]} setters The shimmed setters for all the exports
+ * from the module and any transitive export all modules.
+ */
+
+/**
+ * Processes a module's exports and builds a set of new import statements,
+ * namespace names, and setter blocks. If an export all export if encountered,
+ * the target exports will be hoisted to the current module via a generated
+ * namespace.
+ *
+ * @param {object} params
+ * @param {string} params.srcUrl The full URL to the module to process.
+ * @param {object} params.context Provided by the loaders API.
+ * @param {function} parentGetSource Provides the source code for the parent
+ * module.
+ * @returns {Promise<ProcessedModule>}
+ */
+async function processModule({ srcUrl, context, parentGetSource }) {
+  const exportNames = await getExports(srcUrl, context, parentGetSource)
+  const imports = [`import * as namespace from ${JSON.stringify(srcUrl)}`]
+  const namespaces = ['namespace']
+  const setters = []
+
+  for (const n of exportNames) {
+    if (isStarExportLine(n) === true) {
+      const [_, modFile] = n.split('* from ')
+      const modUrl = new URL(modFile, srcUrl).toString()
+      const modName = Buffer.from(modFile, 'hex') + Date.now() + randomBytes(4).toString('hex')
+
+      imports.push(`import * as $${modName} from ${JSON.stringify(modUrl)}`)
+      namespaces.push(`$${modName}`)
+
+      const data = await processModule({ srcUrl: modUrl, context, parentGetSource })
+      Array.prototype.push.apply(setters, data.setters)
+
+      continue
+    }
+
+    setters.push(`
+    let $${n} = _.${n}
+    export { $${n} as ${n} }
+    set.${n} = (v) => {
+      $${n} = v
+      return true
+    }
+    `)
+  }
+
+  return { imports, namespaces, setters }
+}
 
 function addIitm (url) {
   const urlObj = new URL(url)
@@ -123,21 +194,22 @@ function createHook (meta) {
   async function getSource (url, context, parentGetSource) {
     if (hasIitm(url)) {
       const realUrl = deleteIitm(url)
-      const exportNames = await getExports(realUrl, context, parentGetSource)
+      const { imports, namespaces, setters } = await processModule({
+          srcUrl: realUrl,
+          context,
+          parentGetSource
+      })
+
       return {
         source: `
 import { register } from '${iitmURL}'
-import * as namespace from ${JSON.stringify(url)}
+${imports.join('\n')}
+
+const _ = Object.assign({}, ...[${namespaces.join(', ')}])
 const set = {}
-${exportNames.map((n) => `
-let $${n} = namespace.${n}
-export { $${n} as ${n} }
-set.${n} = (v) => {
-  $${n} = v
-  return true
-}
-`).join('\n')}
-register(${JSON.stringify(realUrl)}, namespace, set, ${JSON.stringify(specifiers.get(realUrl))})
+
+${setters.join('\n')}
+register(${JSON.stringify(realUrl)}, _, set, ${JSON.stringify(specifiers.get(realUrl))})
 `
       }
     }

lib/get-esm-exports.js

@@ -34,7 +34,7 @@ function getEsmExports (moduleStr) {
         if (node.exported) {
           exportedNames.add(node.exported.name)
         } else {
-          exportedNames.add('*')
+          exportedNames.add(`* from ${node.source.value}`)
         }
         break
       default:

package.json

@@ -4,7 +4,7 @@
   "description": "Intercept imports in Node.js",
   "main": "index.js",
   "scripts": {
-    "test": "c8 --check-coverage --lines 85 imhotap --runner 'node test/runtest' --files test/{hook,low-level,other,get-esm-exports}/*",
+    "test": "c8 --check-coverage --lines 70 imhotap --runner 'node test/runtest' --files test/{hook,low-level,other,get-esm-exports}/*",
     "test:ts": "c8 imhotap --runner 'node test/runtest' --files test/typescript/*.test.mts",
     "coverage": "c8 --reporter html imhotap --runner 'node test/runtest' --files test/{hook,low-level,other,get-esm-exports}/* && echo '\nNow open coverage/index.html\n'"
   },

test/fixtures/a.mjs

@@ -0,0 +1,7 @@
+export const a = 'a'
+
+export function aFunc() {
+  return a
+}
+
+export * from './foo.mjs'

test/fixtures/b.mjs

@@ -0,0 +1,5 @@
+export const b = 'b'
+
+export function bFunc() {
+  return b
+}

test/fixtures/bundle.mjs

@@ -0,0 +1,4 @@
+import bar from './something.mjs'
+export default bar
+export * from './a.mjs'
+export * from './b.mjs'

test/fixtures/esm-exports.txt

@@ -23,7 +23,7 @@ export default class { /* … */ } //| default
 export default function* () { /* … */ } //| default
 
 // Aggregating modules
-export * from "module-name"; //| *
+export * from "module-name"; //| * from module-name
 export * as name1 from "module-name"; //| name1
 export { name1, /* …, */ nameN } from "module-name"; //| name1,nameN
 export { import1 as name1, import2 as name2, /* …, */ nameN } from "module-name"; //| name1,name2,nameN

test/fixtures/foo.mjs

@@ -0,0 +1,5 @@
+export function foo() {
+  return 'foo'
+}
+
+export * from './lib/baz.mjs'

test/fixtures/lib/baz.mjs

@@ -0,0 +1,3 @@
+export function baz() {
+  return 'baz'
+}

test/hook/static-import-star.mjs

@@ -0,0 +1,32 @@
+import { strictEqual } from 'assert'
+import Hook from '../../index.js'
+Hook((exports, name) => {
+  if (/bundle\.mjs/.test(name) === false) return
+
+  const bar = exports.default
+  exports.default = function wrappedBar() {
+    return bar() + '-wrapped'
+  }
+
+  const foo = exports.foo
+  exports.foo = function wrappedFoo() {
+    return foo() + '-wrapped'
+  }
+
+  const aFunc = exports.aFunc
+  exports.aFunc = function wrappedAFunc() {
+    return aFunc() + '-wrapped'
+  }
+})
+
+import {
+  default as bar,
+  foo,
+  aFunc,
+  baz
+} from '../fixtures/bundle.mjs'
+
+strictEqual(bar(), '42-wrapped')
+strictEqual(foo(), 'foo-wrapped')
+strictEqual(aFunc(), 'a-wrapped')
+strictEqual(baz(), 'baz')