app-server: Add back pressure and batching to command/exec (#15547)

* Add
`OutgoingMessageSender::send_server_notification_to_connection_and_wait`
which returns only once message is written to websocket (or failed to do
so)
* Use this mechanism to apply back pressure to stdout/stderr streams of
processes spawned by `command/exec`, to limit them to at most one
message in-memory at a time
* Use back pressure signal to also batch smaller chunks into ≈64KiB ones

This should make commands execution more robust over
high-latency/low-throughput networks

codex-rs/app-server/src/command_exec.rs

@@ -42,6 +42,7 @@ use crate::outgoing_message::ConnectionRequestId;
 use crate::outgoing_message::OutgoingMessageSender;
 
 const EXEC_TIMEOUT_EXIT_CODE: i32 = 124;
+const OUTPUT_CHUNK_SIZE_HINT: usize = 64 * 1024;
 
 #[derive(Clone)]
 pub(crate) struct CommandExecManager {
@@ -577,13 +578,19 @@ fn spawn_process_output(params: SpawnProcessOutputParams) -> tokio::task::JoinHa
         let mut buffer: Vec<u8> = Vec::new();
         let mut observed_num_bytes = 0usize;
         loop {
-            let chunk = tokio::select! {
+            let mut chunk = tokio::select! {
                 chunk = output_rx.recv() => match chunk {
                     Some(chunk) => chunk,
                     None => break,
                 },
                 _ = stdio_timeout_rx.wait_for(|&v| v) => break,
             };
+            // Individual chunks are at most 8KiB, so overshooting a bit is acceptable.
+            while chunk.len() < OUTPUT_CHUNK_SIZE_HINT
+                && let Ok(next_chunk) = output_rx.try_recv()
+            {
+                chunk.extend_from_slice(&next_chunk);
+            }
             let capped_chunk = match output_bytes_cap {
                 Some(output_bytes_cap) => {
                     let capped_chunk_len = output_bytes_cap
@@ -597,8 +604,8 @@ fn spawn_process_output(params: SpawnProcessOutputParams) -> tokio::task::JoinHa
             let cap_reached = Some(observed_num_bytes) == output_bytes_cap;
             if let (true, Some(process_id)) = (stream_output, process_id.as_ref()) {
                 outgoing
-                    .send_server_notification_to_connections(
-                        &[connection_id],
+                    .send_server_notification_to_connection_and_wait(
+                        connection_id,
                         ServerNotification::CommandExecOutputDelta(
                             CommandExecOutputDeltaNotification {
                                 process_id: process_id.clone(),
@@ -809,6 +816,7 @@ mod tests {
         let OutgoingEnvelope::ToConnection {
             connection_id,
             message,
+            ..
         } = envelope
         else {
             panic!("expected connection-scoped outgoing message");
@@ -891,6 +899,7 @@ mod tests {
         let OutgoingEnvelope::ToConnection {
             connection_id,
             message,
+            ..
         } = envelope
         else {
             panic!("expected connection-scoped outgoing message");