Mirage-overridden fetch cannot download an image binary blob

[skarger]

Mirage has a bug when using the fetch API to download an image Blob.

Here is a repo that reproduces the problem: https://github.com/skarger/mirage-blob-fetch.
It's a simple Ember app that can be run with yarn start.

The Mirage-overridden fetch function seems to return a text version of the image's bytes, instead of the desired Blob. Same problem if trying to access the image as an ArrayBuffer.

The fetched data is corrupt, and also larger. I believe it's larger because the text representation of the image's bytes is UTF-8 encoded, and therefore each of its "characters" may expand in width from 1 byte to up to 4 bytes.

There are a few players in the mix:

• Mirage

• Pretender, which Mirage uses to mock the fetch function.

• whatwg-fetch, a polyfill for window.fetch, which Pretender uses to
  overwrite the browser's native fetch function.

• I believe the bug only happens when using Mirage.

• I've confirmed it does not happen when only using whatwg-fetch.

The example app's index route provides a demo that reproduces the problem.

Clicking the buttons will show the returned blob type and byte sizes when using
the native fetch function and when using the Mirage-overridden fetch.

You can see they are different. Clicking these buttons prompts console logging as well.

Demo app screenshot:

In order to make this demo, I used an Ember application initializer to capture
the native fetch function as window.nativeFetch. Otherwise I would have no way to access
native fetch because Mirage / Pretender overrides it.

[samselikoff]

You know I think these issues might be related...

• Passthrough changes Response to be not compliant with WebAssembly.instantiateStreaming miragejs#339
• passthroughed requests are altered by mirage miragejs#343

Haven't had time to dig yet but it's possible they all have the same root cause. Would be great to get to the bottom of it.

[tschoartschi]

I tried to debug this problem a little bit because we are facing the following issue: miragejs/miragejs#339

I think our issue involves even more "problems" but for the binary data problem I found out, that when I do the following in the fetch implementation of https://github.com/pretenderjs/pretender everything works fine 🙂

    // inside fetch(input, init)
    if (xhr.url.indexOf('.wasm') !== -1) {
      xhr.responseType = 'arraybuffer';
    }

The main problem I face is, that I do not know how to set the correct responseType because in the fetch implementation I have no information about what responseType to use 🤔 If I would know where to get this information from, I probably could create a PR

[tschoartschi]

@skarger I found a workaround that works for us 🙂we use MirageJS without Ember-CLI but maybe it helps you to find a workaround for you as well. The details are here: miragejs/miragejs#339 (comment)

[nerdkid93]

I think this is related to this bug in pretender: pretenderjs/pretender#305