rtpsend: ensure only a single rtcp pad push

Otherwise, it can occur that multiple rtcp packets may be produced out
of order.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1426>

net/rtp/src/rtpbin2/rtpsend.rs

@@ -198,29 +198,40 @@ impl SendSession {
 
     async fn rtcp_task(state: Arc<Mutex<State>>, session_id: usize) {
         let mut stream = RtcpSendStream::new(state.clone(), session_id);
+        // we use a semaphore instead of mutex for ordering
+        // i.e. we should only allow a single pad push, but still allow other rtcp tasks to
+        // continue operating
+        let sem = Arc::new(tokio::sync::Semaphore::new(1));
         while let Some(reply) = stream.next().await {
+            let send = {
                 let state = state.lock().unwrap();
                 let Some(session) = state.session_by_id(session_id) else {
                     continue;
                 };
                 match reply {
                     RtcpSendReply::Data(data) => {
-                    let Some(rtcp_srcpad) = session.rtcp_send_srcpad.clone() else {
+                        session.rtcp_send_srcpad.clone().map(|pad| (pad, data))
-                        continue;
+                    }
+                    RtcpSendReply::SsrcBye(ssrc) => {
+                        session
+                            .internal_session
+                            .config
+                            .emit_by_name::<()>("bye-ssrc", &[&ssrc]);
+                        None
+                    }
+                }
             };
-                    drop(state);
+
+            if let Some((rtcp_srcpad, data)) = send {
+                let acquired = sem.clone().acquire_owned().await;
                 RUNTIME.spawn_blocking(move || {
                     let buffer = gst::Buffer::from_mut_slice(data);
                     if let Err(e) = rtcp_srcpad.push(buffer) {
                         gst::warning!(CAT, obj: rtcp_srcpad, "Failed to send rtcp data: flow return {e:?}");
                     }
+                    drop(acquired);
                 });
             }
-                RtcpSendReply::SsrcBye(ssrc) => session
-                    .internal_session
-                    .config
-                    .emit_by_name::<()>("bye-ssrc", &[&ssrc]),
-            }
         }
     }