LibWeb: Support unbuffered fetch requests

Supporting unbuffered fetches is actually part of the fetch spec in its
HTTP-network-fetch algorithm. We had previously implemented this method
in a very ad-hoc manner as a simple wrapper around ResourceLoader. This
is still the case, but we now implement a good amount of these steps
according to spec, using ResourceLoader's unbuffered API. The response
data is forwarded through to the fetch response using streams.

This will eventually let us remove the use of ResourceLoader's buffered
API, as all responses should just be streamed this way. The streams spec
then supplies ways to wait for completion, thus allowing fully buffered
responses. However, we have more work to do to make the other parts of
our fetch implementation (namely, Body::fully_read) use streams before
we can do this.

Userland/Libraries/LibWeb/Fetch/Fetching/FetchedDataReceiver.cpp

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Heap/HeapFunction.h>
+#include <LibWeb/Bindings/ExceptionOrUtils.h>
+#include <LibWeb/Bindings/HostDefined.h>
+#include <LibWeb/Fetch/Fetching/FetchedDataReceiver.h>
+#include <LibWeb/Fetch/Infrastructure/FetchParams.h>
+#include <LibWeb/Fetch/Infrastructure/Task.h>
+#include <LibWeb/HTML/Scripting/ExceptionReporter.h>
+#include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
+#include <LibWeb/Streams/AbstractOperations.h>
+#include <LibWeb/WebIDL/Promise.h>
+
+namespace Web::Fetch::Fetching {
+
+JS_DEFINE_ALLOCATOR(FetchedDataReceiver);
+
+FetchedDataReceiver::FetchedDataReceiver(JS::NonnullGCPtr<Infrastructure::FetchParams const> fetch_params, JS::NonnullGCPtr<Streams::ReadableStream> stream)
+    : m_fetch_params(fetch_params)
+    , m_stream(stream)
+{
+}
+
+FetchedDataReceiver::~FetchedDataReceiver() = default;
+
+void FetchedDataReceiver::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_fetch_params);
+    visitor.visit(m_stream);
+    visitor.visit(m_pending_promise);
+}
+
+void FetchedDataReceiver::set_pending_promise(JS::NonnullGCPtr<WebIDL::Promise> promise)
+{
+    auto had_pending_promise = m_pending_promise != nullptr;
+    m_pending_promise = promise;
+
+    if (!had_pending_promise && !m_buffer.is_empty()) {
+        on_data_received(m_buffer);
+        m_buffer.clear();
+    }
+}
+
+// This implements the parallel steps of the pullAlgorithm in HTTP-network-fetch.
+// https://fetch.spec.whatwg.org/#ref-for-in-parallel④
+void FetchedDataReceiver::on_data_received(ReadonlyBytes bytes)
+{
+    // FIXME: 1. If the size of buffer is smaller than a lower limit chosen by the user agent and the ongoing fetch
+    //           is suspended, resume the fetch.
+    // FIXME: 2. Wait until buffer is not empty.
+
+    // If the remote end sends data immediately after we receive headers, we will often get that data here before the
+    // stream tasks have all been queued internally. Just hold onto that data.
+    if (!m_pending_promise) {
+        m_buffer.append(bytes);
+        return;
+    }
+
+    // 3. Queue a fetch task to run the following steps, with fetchParams’s task destination.
+    Infrastructure::queue_fetch_task(
+        m_fetch_params->controller(),
+        m_fetch_params->task_destination().get<JS::NonnullGCPtr<JS::Object>>(),
+        JS::create_heap_function(heap(), [this, bytes = MUST(ByteBuffer::copy(bytes))]() mutable {
+            HTML::TemporaryExecutionContext execution_context { Bindings::host_defined_environment_settings_object(m_stream->realm()), HTML::TemporaryExecutionContext::CallbacksEnabled::Yes };
+
+            // 1. Pull from bytes buffer into stream.
+            if (auto result = Streams::readable_stream_pull_from_bytes(m_stream, move(bytes)); result.is_error()) {
+                auto throw_completion = Bindings::dom_exception_to_throw_completion(m_stream->vm(), result.release_error());
+
+                dbgln("FetchedDataReceiver: Stream error pulling bytes");
+                HTML::report_exception(throw_completion, m_stream->realm());
+
+                return;
+            }
+
+            // 2. If stream is errored, then terminate fetchParams’s controller.
+            if (m_stream->is_errored())
+                m_fetch_params->controller()->terminate();
+
+            // 3. Resolve promise with undefined.
+            WebIDL::resolve_promise(m_stream->realm(), *m_pending_promise, JS::js_undefined());
+        }));
+}
+
+}