🐛 BUG: Pages `next()` headers mutable in local dev, but not in production

What version of Wrangler are you using?

2.0.9

What operating system are you using?

Windows 10

Describe the Bug

The Pages Functions docs has this example:

const hello = async ({ next }) => {
  const response = await next();
  response.headers.set('X-Hello', 'Hello from functions Middleware!');
  return response;
};

This works perfectly fine in local dev with pages dev, but fails in production due to the next() response having immutable headers there. This leads to frequent frustration for folks since there’s no way to get function error logs to debug this inconsistency.

To make this work in prod, you have to do something like this:

const hello = async ({ next }) => {
  const rawResponse = await next();
  const response = new Response(rawResponse.body, rawResponse); // clone response to create mutable headers
  response.headers.set('X-Hello', 'Hello from functions Middleware!');
  return response;
};

Related: https://github.com/cloudflare/cloudflare-docs/pull/4773

[Cherry]

The change in #1325 appears to work great, thanks very much! I ran the following:

export async function onRequestGet({next}){
	const response = await next();
	response.headers.set('x-test', 'hello')
	return response;
}

and was able to see the x-test header when using the new wrangler version both in prod and dev, vs an exception in prod when using the current release version, as expected. Woot! 🎉

[sidharthachatterjee]

@Cherry It’s now out in wrangler@2.0.15 🎉