Fetched bodies can be on the order of gigabytes, so rather than crashing
when we hit OOM here, we can simply invoke the error callback with a DOM
exception. We use "UnknownError" here as the spec directly supports this
for OOM errors:
UnknownError: The operation failed for an unknown transient reason
(e.g. out of memory).
This is still an ad-hoc implementation. We should be using streams, and
we do have the AOs available to do so. But they need to be massaged to
be compatible with callers of Body::fully_read. And once we do use
streams, this function will become infallible - so making it infallible
here is at least a step in the right direction.
This builds on the existing ad-hoc ResourceLoader code for HTTP fetches
which works for files as well.
This also includes a test that checks that stylesheets loaded with the
"file" URL scheme actually work.
The main things missing is the CORS preflight cache and making
extract_header_list_values properly parse, validate and return split
values for the Access-Control headers.
This implements the following operations from section 4 of the Fetch
spec (https://fetch.spec.whatwg.org/#fetching):
- Fetch
- Main fetch
- Fetch response handover
- Scheme fetch
- HTTP fetch
- HTTP-redirect fetch
- HTTP-network-or-cache fetch (without caching)
It does *not* implement:
- HTTP-network fetch
- CORS-preflight fetch
Instead, we let ResourceLoader handle the actual networking for now,
which isn't ideal, but certainly enough to get enough functionality up
and running for most websites to not complain.
