lib: make fetch sync and return a Promise

lib/internal/process/pre_execution.js

@@ -292,7 +292,8 @@ function setupUndici() {
  }
 
  if (!getOptionValue('--no-experimental-fetch')) {
- async function fetch(input, init = undefined) {
+ // Fetch is meant to return a Promise, but not be async.
+ function fetch(input, init = undefined) {
  return lazyUndici().fetch(input, init);
  }
 

test/parallel/test-fetch.mjs

@@ -10,6 +10,14 @@ assert.strictEqual(typeof globalThis.Headers, 'function');
 assert.strictEqual(typeof globalThis.Request, 'function');
 assert.strictEqual(typeof globalThis.Response, 'function');
 
+{
+ const asyncFunction = async function() {}.constructor;
+
+ assert.ok(!(fetch instanceof asyncFunction));
+ assert.notStrictEqual(Reflect.getPrototypeOf(fetch), Reflect.getPrototypeOf(async function() {}));
+ assert.strictEqual(Reflect.getPrototypeOf(fetch), Reflect.getPrototypeOf(function() {}));
+}
+
 const server = http.createServer(common.mustCall((req, res) => {
  res.end('Hello world');
 }));