[BUG] Error thrown on empty body responses (e.g. 304)

[asos-craigmorten]

Issue

Setup:

• Deno Version: 1.3.1
• v8 Version: 8.6.334
• Typescript Version: 3.9.7
• SuperDeno Version: 2.2.0

Seeing 304 Not Modified responses with empty (no) body result in an error thrown by SuperDeno.

error: Uncaught Error: Request has been terminated
Possible causes: the network is offline, Origin is not allowed by Access-Control-Allow-Origin, the page is being unloaded, etc.
    at Test.Request.crossDomainError (https://dev.jspm.io/npm:superagent@5.3.1/lib/client.dew.js:680:15)
    at XMLHttpRequestSham.xhr.onreadystatechange (https://dev.jspm.io/npm:superagent@5.3.1/lib/client.dew.js:783:21)
    at XMLHttpRequestSham.xhrReceive (https://deno.land/x/superdeno@2.2.0/src/xhrSham.js:105:29)
    at XMLHttpRequestSham.send (https://deno.land/x/superdeno@2.2.0/src/xhrSham.js:64:12)

Details

Adding some logs in the xhrSham we can see that (xhrSham.js:293:34):

...

window._xhrSham.promises[self.id] = fetch(options.url, {
  method: options.method,
  headers: options.requestHeaders,
  body,
  signal: this.controller.signal,
  mode: "cors",
});

// Wait on the response, and then read the buffer.
response = await window._xhrSham.promises[self.id];

...

const buf = await response.arrayBuffer();
parsedResponse = decoder.decode(buf); // BOOM!

...

Is throwing:

err: TypeError: Cannot use 'in' operator to search for 'buffer' in null
    at TextDecoder.decode (/Users/runner/work/deno/deno/op_crates/web/08_text_encoding.js:445:18)
    at XMLHttpRequestSham.xhrSend (https://deno.land/x/superdeno@2.2.0/src/xhrSham.js:293:34)
    at async XMLHttpRequestSham.send (https://deno.land/x/superdeno@2.2.0/src/xhrSham.js:51:7)

This doesn't occur in Deno 1.3.0, so assume introduced by one of the changes to Deno core in 1.3.1, see the release notes: https://github.com/denoland/deno/releases/tag/v1.3.1

08_text_encoding line ref: https://github.com/denoland/deno/blob/master/op_crates/web/08_text_encoding.js#L445

Reproducing we can see we get the error with:

const decoder = new TextDecoder("utf-8");
const buf = null;
const decoded = decoder.decode(buf as any);

It would seem that response.arrayBuffer() in Deno 1.3.0 returned "" for a null body whereas no it returns null which causes the error in the decoder as it has no null protection.

We can protect against this in SuperDeno by adding our own null check around the decoder.