From 7447d95f1bd309a1a25cee01d527f2f3a92766ba Mon Sep 17 00:00:00 2001
From: Mathieu Duponchelle <mathieu@centricular.com>
Date: Thu, 8 Jun 2023 01:51:45 +0200
Subject: [PATCH] webrtc/signalling: fix race condition in message ordering

Spawning one task per message to send out instead of sending them out
sequentially from the one task used to poll the handler sometimes
resulted in peers receiving ICE candidates before SDP offers, triggering
hard to understand errors in the browser.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1236>
---
 net/webrtc/signalling/src/server/mod.rs | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/net/webrtc/signalling/src/server/mod.rs b/net/webrtc/signalling/src/server/mod.rs
index e6ff1162..0510437c 100644
--- a/net/webrtc/signalling/src/server/mod.rs
+++ b/net/webrtc/signalling/src/server/mod.rs
@@ -38,7 +38,7 @@ impl Server {
     #[instrument(level = "debug", skip(factory))]
     pub fn spawn<
         I: for<'a> Deserialize<'a>,
-        O: Serialize + std::fmt::Debug,
+        O: Serialize + std::fmt::Debug + Send + Sync,
         Factory: FnOnce(Pin<Box<dyn Stream<Item = (String, Option<I>)> + Send>>) -> St,
         St: Stream<Item = (String, O)>,
     >(
@@ -72,12 +72,19 @@ impl Server {
         task::spawn(async move {
             while let Some((peer_id, msg)) = handler.next().await {
                 match serde_json::to_string(&msg) {
-                    Ok(msg) => {
-                        if let Some(peer) = state_clone.lock().unwrap().peers.get_mut(&peer_id) {
-                            let mut sender = peer.sender.clone();
-                            task::spawn(async move {
-                                let _ = sender.send(msg).await;
-                            });
+                    Ok(msg_str) => {
+                        let sender = {
+                            let mut state = state_clone.lock().unwrap();
+                            if let Some(peer) = state.peers.get_mut(&peer_id) {
+                                Some(peer.sender.clone())
+                            } else {
+                                None
+                            }
+                        };
+
+                        if let Some(mut sender) = sender {
+                            trace!("Sending {}", msg_str);
+                            let _ = sender.send(msg_str).await;
                         }
                     }
                     Err(err) => {