Auto merge of #22714 - aditj:patch-3, r=jdm

Added fetch_start functionality

<!-- Please describe your changes on the following line: -->
Added the fetch_start functionality in http_loader.rs (http_fetch function)

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #21258 (GitHub issue number if applicable)

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22714)
<!-- Reviewable:end -->

components/net/fetch/methods.rs

@@ -23,6 +23,7 @@ use net_traits::filemanager_thread::RelativePos;
 use net_traits::request::{CredentialsMode, Destination, Referrer, Request, RequestMode};
 use net_traits::request::{Origin, ResponseTainting, Window};
 use net_traits::response::{Response, ResponseBody, ResponseType};
+use net_traits::ResourceAttribute;
 use net_traits::{FetchTaskTarget, NetworkError, ReferrerPolicy, ResourceFetchTiming};
 use servo_url::ServoUrl;
 use std::borrow::Cow;
@@ -88,6 +89,12 @@ pub type DoneChannel = Option<(Sender<Data>, Receiver<Data>)>;
 
 /// [Fetch](https://fetch.spec.whatwg.org#concept-fetch)
 pub fn fetch(request: &mut Request, target: Target, context: &FetchContext) {
+    // Step 7 of https://w3c.github.io/resource-timing/#processing-model
+    context
+        .timing
+        .lock()
+        .unwrap()
+        .set_attribute(ResourceAttribute::FetchStart);
     fetch_with_cors_cache(request, &mut CorsCache::new(), target, context);
 }
 

components/net/http_loader.rs

@@ -525,8 +525,6 @@ pub fn http_fetch(
             request.service_workers_mode = ServiceWorkersMode::None;
         }
 
-        // Substep 3
-        // TODO(#21258) maybe set fetch_start (if this is the last resource)
         // Generally, we use a persistent connection, so we will also set other PerformanceResourceTiming
         //   attributes to this as well (domain_lookup_start, domain_lookup_end, connect_start, connect_end,
         //   secure_connection_start)
@@ -660,6 +658,13 @@ pub fn http_redirect_fetch(
         Some(Ok(url)) => url,
     };
 
+    // Step 1 of https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-fetchstart
+    context
+        .timing
+        .lock()
+        .unwrap()
+        .set_attribute(ResourceAttribute::FetchStart);
+
     // Step 5
     if request.redirect_count >= 20 {
         return Response::network_error(NetworkError::Internal("Too many redirects".into()));