fix: make Iterator polyfill cover jsdom vm contexts and spawned child processes

changelog.d/fixed-iterator-polyfill-followups.md

@@ -0,0 +1 @@
+Fixed the Node 20 `Iterator` polyfill so vitest passes again on the prow image: the polyfill is now mirrored into the vm contexts jsdom 30 creates per window, and `NODE_OPTIONS` is rewritten to the polyfill's absolute path so child `node` processes spawned by script tests from tmpdir cwds no longer die with MODULE_NOT_FOUND.

scripts/node-iterator-polyfill.cjs

@@ -13,4 +13,49 @@
 // prototype methods; on Node >= 22 the native implementation is kept.
 if (typeof globalThis.Iterator === 'undefined') {
   require('core-js/actual/iterator')
+
+  // jsdom >= 30 also reads every JS global out of the fresh vm context it
+  // creates per window (lib/jsdom/browser/Window.js: vm.runInContext(name,
+  // window) for each host global it found, which now includes our polyfilled
+  // `Iterator`). A fresh context on Node 20 has no native Iterator, so window
+  // creation throws "ReferenceError: Iterator is not defined" from inside
+  // runInContext. Mirror the polyfilled Iterator into every new context.
+  const vm = require('vm')
+  const originalCreateContext = vm.createContext
+  vm.createContext = function (...args) {
+    const context = originalCreateContext.apply(this, args)
+    // createContext returns the contextified object (or the context's global
+    // object with DONT_CONTEXTIFY); property sets on it reflect as globals
+    // inside the context.
+    const globalObject =
+      context !== null && typeof context === 'object' ? context : args[0]
+    try {
+      if (
+        globalObject &&
+        vm.runInContext('typeof Iterator', globalObject) === 'undefined'
+      ) {
+        globalObject.Iterator = globalThis.Iterator
+      }
+    } catch {
+      // Non-contextified or frozen targets: leave untouched.
+    }
+    return context
+  }
+}
+
+// The `test` script preloads this file with a cwd-relative path
+// (`--require ./scripts/node-iterator-polyfill.cjs`). Node resolves that
+// against the cwd of EACH process, so a child `node` process a test spawns
+// with a tmpdir cwd dies at startup with MODULE_NOT_FOUND before running
+// anything. Rewrite NODE_OPTIONS to this file's absolute path so inherited
+// environments keep working from any cwd.
+const RELATIVE_SPEC = './scripts/node-iterator-polyfill.cjs'
+if (
+  process.env.NODE_OPTIONS &&
+  process.env.NODE_OPTIONS.includes(RELATIVE_SPEC) &&
+  !__filename.includes(' ')
+) {
+  process.env.NODE_OPTIONS = process.env.NODE_OPTIONS
+    .split(RELATIVE_SPEC)
+    .join(__filename)
 }