Previously, when loading a document, we would try to sniff the MIME
type by reading from the response body's source. However, for streaming
HTTP responses, the body source is Empty (the data comes through the
stream instead), so we had no bytes to sniff.
This caused pages like hypr.land (which sends no Content-Type header)
to be misidentified as plain text instead of HTML, since the MIME
sniffing algorithm would receive zero bytes and fall back to the
default type.
The fix captures the first bytes of the response body during fetch,
storing them on the Body object. These bytes are the "resource header"
defined by the MIME Sniffing spec - up to 1445 bytes, which is enough
to identify any MIME type the spec can detect.
Since bytes may arrive asynchronously during streaming, we use a
callback mechanism: if bytes aren't ready yet when load_document()
needs them, it registers a callback that fires once enough bytes have
been captured (or the stream ends).
The flow is:
1. FetchedDataReceiver receives network bytes, buffers them
2. When Body is created, buffered bytes are flushed to Body's sniff
buffer, and subsequent bytes are appended as they arrive
3. Before calling load_document(), Navigable waits for sniff bytes
4. load_document() passes the bytes to MimeSniff::Resource::sniff()
The spec declares these as a byte sequence, which we then implemented as
a ByteBuffer. This has become pretty awkward to deal with, as evidenced
by the plethora of `MUST(ByteBuffer::copy(...))` and `.bytes()` calls
everywhere inside Fetch. We would then treat the bytes as a string
anyways by wrapping them in StringView everywhere.
We now store these as a ByteString. This is more comfortable to deal
with, and we no longer need to continually copy underlying storage (as
ByteString is ref-counted).
This work is largely preparatory for an upcoming HTTP header refactor.
Note that it's not actually executing tasks in parallel, it's still
throwing them on the HTML event loop task queue, each with its own
unique task source.
This makes our fetch implementation a lot more robust when HTTP caching
is enabled, and you can now click links on https://terminal.shop/
without hitting TODO assertions in fetch.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
