Auto merge of #19274 - Manishearth:xhr-cancel, r=jdm

Fetch cancellation

This PR implements cancellation for fetch, and uses it for XHR. This means that fetch clients can now send a message to the fetch task asking for the network request to be aborted.

Previously, clients like XHR had abort functionality but would implement it by simply ignoring future messages from the network task; and would not actually cancel the network fetch.

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components/net/http_loader.rs

@@ -1103,6 +1103,10 @@ fn http_network_fetch(request: &Request,
     let devtools_sender = context.devtools_chan.clone();
     let meta_status = meta.status.clone();
     let meta_headers = meta.headers.clone();
+    let cancellation_listener = context.cancellation_listener.clone();
+    if cancellation_listener.lock().unwrap().cancelled() {
+        return Response::network_error(NetworkError::Internal("Fetch aborted".into()))
+    }
     thread::Builder::new().name(format!("fetch worker thread")).spawn(move || {
         match StreamedResponse::from_http_response(res) {
             Ok(mut res) => {
@@ -1125,6 +1129,11 @@ fn http_network_fetch(request: &Request,
                 }
 
                 loop {
+                    if cancellation_listener.lock().unwrap().cancelled() {
+                        *res_body.lock().unwrap() = ResponseBody::Done(vec![]);
+                        let _ = done_sender.send(Data::Cancelled);
+                        return;
+                    }
                     match read_block(&mut res) {
                         Ok(Data::Payload(chunk)) => {
                             if let ResponseBody::Receiving(ref mut body) = *res_body.lock().unwrap() {
@@ -1144,6 +1153,7 @@ fn http_network_fetch(request: &Request,
                             let _ = done_sender.send(Data::Done);
                             break;
                         }
+                        Ok(Data::Cancelled) => unreachable!() // read_block doesn't return Data::Cancelled
                     }
                 }
             }