From b4561b848d05af4908b5f493767f7cad3005610d Mon Sep 17 00:00:00 2001
From: Antonio Scandurra <me@as-cii.com>
Date: Wed, 8 Mar 2023 10:56:40 +0100
Subject: [PATCH] Limit the number of parallel messages handled for any given
 connection

---
 crates/collab/src/rpc.rs | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/crates/collab/src/rpc.rs b/crates/collab/src/rpc.rs
index d13487440f..1deaefec1a 100644
--- a/crates/collab/src/rpc.rs
+++ b/crates/collab/src/rpc.rs
@@ -53,7 +53,7 @@ use std::{
     },
     time::Duration,
 };
-use tokio::sync::watch;
+use tokio::sync::{watch, Semaphore};
 use tower::ServiceBuilder;
 use tracing::{info_span, instrument, Instrument};
 
@@ -542,8 +542,13 @@ impl Server {
             // This arrangement ensures we will attempt to process earlier messages first, but fall
             // back to processing messages arrived later in the spirit of making progress.
             let mut foreground_message_handlers = FuturesUnordered::new();
+            let concurrent_handlers = Arc::new(Semaphore::new(256));
             loop {
-                let next_message = incoming_rx.next().fuse();
+                let next_message = async {
+                    let permit = concurrent_handlers.clone().acquire_owned().await.unwrap();
+                    let message = incoming_rx.next().await;
+                    (permit, message)
+                }.fuse();
                 futures::pin_mut!(next_message);
                 futures::select_biased! {
                     _ = teardown.changed().fuse() => return Ok(()),
@@ -554,7 +559,8 @@ impl Server {
                         break;
                     }
                     _ = foreground_message_handlers.next() => {}
-                    message = next_message => {
+                    next_message = next_message => {
+                        let (permit, message) = next_message;
                         if let Some(message) = message {
                             let type_name = message.payload_type_name();
                             let span = tracing::info_span!("receive message", %user_id, %login, %connection_id, %address, type_name);
@@ -564,7 +570,10 @@ impl Server {
                                 let handle_message = (handler)(message, session.clone());
                                 drop(span_enter);
 
-                                let handle_message = handle_message.instrument(span);
+                                let handle_message = async move {
+                                    handle_message.await;
+                                    drop(permit);
+                                }.instrument(span);
                                 if is_background {
                                     executor.spawn_detached(handle_message);
                                 } else {