[wasm-web-api-1] Don't use fetch's "extract a MIME type"

[andreubotella]

As I was looking into refactoring some code in Deno's implementation of the wasm web APIs, I noticed that the spec calls into fetch's "extract a MIME type" algorithm, and treats its return value as if it were a string, when it is in fact a MIME type record. The WHATWG MIME sniffing spec defines an algorithm to serialize a MIME type record, but that doesn't work in this case since the parsing and serialization roundtrip normalizes application/wasm; to application/wasm (see also #1140).

Looking further, it seems like when this reference to "extract a MIME type" was introduced, that algorithm did return a string. However, we still run into a type error because it treats the return value of "extract header list values" as a string, even though it is an array.

So instead I looked at the code to see what browsers are actually doing. Chromium, WebKit and Deno all replace steps 2.2 and 2.3 of "compile a potential WebAssembly response" with:

2. Let mimeType be the result of getting `Content-Type` from response's header list.
3. If mimeType is null, or if it is not a byte-case-insensitive match for `application/wasm`, reject returnValue with a TypeError and abort these substeps.

I'm not sure whether Firefox does this or not, because the handling of responses for the wasm streaming APIs happens inside network code, and I couldn't easily follow what it does.

[Ms2ger]

cc @annevk

Gecko code seems to be here: https://searchfox.org/mozilla-central/rev/f8db81665dc2833fff09dc7eef200539ac1fd351/dom/fetch/Fetch.cpp#1344 https://searchfox.org/mozilla-central/rev/f8db81665dc2833fff09dc7eef200539ac1fd351/dom/fetch/FetchUtil.cpp#674

We'd definitely need some tests for this

[annevk]

In https://github.com/WebAssembly/design/pull/991 there was a decision that application/wasm; has to reject. There are tests for this in Gecko, but I'm not sure if there are web-platform-tests as well.

Fetch changed in https://github.com/whatwg/fetch/pull/831.

The simplest change here would be to do as @andreubotella suggests. It might be worth considering if we should align application/wasm with how we handle other MIME types, but that should be a separate decision.

[andreubotella]

There's https://github.com/web-platform-tests/wpt/blob/master/wasm/webapi/contenttype.any.js, but it doesn't test zero or multiple Content-Type headers.

It might be worth considering if we should align application/wasm with how we handle other MIME types, but that should be a separate decision.

I think #1140 would be the appropriate issue to discuss that.

[annevk]

Let's dupe this into #1138 (which was fixed).