fix: try to fix freezes 2 (#9951)

Fixes a TUI freeze caused by awaiting `mpsc::Sender::send()` that blocks
the tokio thread, stopping the consumption runtime and creating a
deadlock. This could happen if the server was producing enough chunks to
fill the `mpsc` fast enough. To solve this we try on insert using a
`try_send()` (not requiring an `await`) and delegate to a tokio task if
this does not work

This is a temporary solution as it can contain races for delta elements
and a stronger design should come here

codex-rs/tui/src/app.rs

@@ -93,6 +93,7 @@ use tokio::sync::Mutex;
 use tokio::sync::broadcast;
 use tokio::sync::mpsc;
 use tokio::sync::mpsc::error::TryRecvError;
+use tokio::sync::mpsc::error::TrySendError;
 use tokio::sync::mpsc::unbounded_channel;
 use toml::Value as TomlValue;
 
@@ -714,8 +715,23 @@ impl App {
             guard.active
         };
 
-        if should_send && let Err(err) = sender.send(event).await {
-            tracing::warn!("thread {thread_id} event channel closed: {err}");
+        if should_send {
+            // Never await a bounded channel send on the main TUI loop: if the receiver falls behind,
+            // `send().await` can block and the UI stops drawing. If the channel is full, wait in a
+            // spawned task instead.
+            match sender.try_send(event) {
+                Ok(()) => {}
+                Err(TrySendError::Full(event)) => {
+                    tokio::spawn(async move {
+                        if let Err(err) = sender.send(event).await {
+                            tracing::warn!("thread {thread_id} event channel closed: {err}");
+                        }
+                    });
+                }
+                Err(TrySendError::Closed(_)) => {
+                    tracing::warn!("thread {thread_id} event channel closed");
+                }
+            }
         }
         Ok(())
     }
@@ -2400,6 +2416,7 @@ mod tests {
     use std::sync::Arc;
     use std::sync::atomic::AtomicBool;
     use tempfile::tempdir;
+    use tokio::time;
 
     #[test]
     fn normalize_harness_overrides_resolves_relative_add_dirs() -> Result<()> {
@@ -2420,6 +2437,47 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn enqueue_thread_event_does_not_block_when_channel_full() -> Result<()> {
+        let mut app = make_test_app().await;
+        let thread_id = ThreadId::new();
+        app.thread_event_channels
+            .insert(thread_id, ThreadEventChannel::new(1));
+        app.set_thread_active(thread_id, true).await;
+
+        let event = Event {
+            id: String::new(),
+            msg: EventMsg::ShutdownComplete,
+        };
+
+        app.enqueue_thread_event(thread_id, event.clone()).await?;
+        time::timeout(
+            Duration::from_millis(50),
+            app.enqueue_thread_event(thread_id, event),
+        )
+        .await
+        .expect("enqueue_thread_event blocked on a full channel")?;
+
+        let mut rx = app
+            .thread_event_channels
+            .get_mut(&thread_id)
+            .expect("missing thread channel")
+            .receiver
+            .take()
+            .expect("missing receiver");
+
+        time::timeout(Duration::from_millis(50), rx.recv())
+            .await
+            .expect("timed out waiting for first event")
+            .expect("channel closed unexpectedly");
+        time::timeout(Duration::from_millis(50), rx.recv())
+            .await
+            .expect("timed out waiting for second event")
+            .expect("channel closed unexpectedly");
+
+        Ok(())
+    }
+
     async fn make_test_app() -> App {
         let (chat_widget, app_event_tx, _rx, _op_rx) = make_chatwidget_manual_with_sender().await;
         let config = chat_widget.config_ref().clone();