500 Internal Server Error / on 304 response and empty ReadableStream

[lukaszczerpak]

It seems there is inconsistency between CF and Miniflare. PoC code:

addEventListener('fetch', function(event) {
  event.respondWith(handleRequest(event.request))
})
async function handleRequest(request) {
  let url = new URL(request.url)
  const res = await fetch(`http://origin.cloudtest.it${url.pathname}`, {headers: {"if-modified-since": "Wed, 12 Jan 2022 10:43:29 GMT"}})
  return new Response(res.body, {
    headers: {
      ...Object.fromEntries(res.headers),
      "x-workers-hello": "Hello from Cloudflare Workers"
    },
    status: res.status,
    statusText: res.statusText
  })
}

Throws the following error:

GET /bigfiles/256m 500 Internal Server Error (98.22ms)
[mf:err] GET /bigfiles/256m: TypeError: Response with null body status cannot have body
    at new Response (/Users/lukes/labs/cf-workers/sandbox-lukasz-testing/node_modules/undici/lib/fetch/response.js:160:15)
    at new Response (/Users/lukes/labs/cf-workers/sandbox-lukasz-testing/node_modules/@miniflare/core/src/standards/http.ts:556:13)
    at handleRequest (/Users/lukes/labs/cf-workers/sandbox-lukasz-testing/index.js:22:10)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at EventTarget.[kDispatchFetch] (/Users/lukes/labs/cf-workers/sandbox-lukasz-testing/node_modules/@miniflare/core/src/standards/event.ts:359:13)
    at Server.<anonymous> (/Users/lukes/labs/cf-workers/sandbox-lukasz-testing/node_modules/@miniflare/http-server/src/index.ts:167:20)

The origin server returns 304 response with no body (as expected). The reason of the error is that res.body is a ReadableStream which is empty but the object itself is not, therefore the undici library doesn't like it and throws an error (304 responses don't have response body).
I was sitting on the fence if this should be reported here or maybe to the dependency's maintainer. In fact, the object type could be detected to behave properly. WDYT?

[mrbbot]

Hey! 👋 Thanks for raising this. This is the right place to report the issue. Executing this code in Chrome/Firefox/undici gives me a ReadableStream body, whereas Cloudflare gives null:

const  res = await fetch("https://httpbin.org/cache", {
  headers: {
    "If-None-Match": "etag"
  }
});
console.log(res.body);

I guess the solution here is to just return null from the res.body getter whenever a null-body status is being used?

[lukaszczerpak]

imho the following change should do the job:

diff --git a/packages/core/src/standards/http.ts b/packages/core/src/standards/http.ts
index bd59d92..aef6da8 100644
--- a/packages/core/src/standards/http.ts
+++ b/packages/core/src/standards/http.ts
@@ -551,7 +551,7 @@ export class Response<
         // This zero-length body behavior is allowed because it was previously
         // the only way to construct a Response with a null body status. It may
         // change in the future.
-        if (nullBodyStatus.includes(init.status) && body === "") body = null;
+        if (nullBodyStatus.includes(init.status)) body = null;
       }
       super(new BaseResponse(body, init));
     }

[lukaszczerpak]

if you are happy with the proposal I can create a PR?

[mrbbot]

Hey! 👋 I've just released version 2.3.0 including a fix for this. You can find the full changelog here.