hive-agent: cap concurrent connections on the in-agent todo socket

hive-agent/src/todo_server.rs

@@ -47,6 +47,12 @@ const COMPACT_MIN_USAGE_FRACTION: f64 = 0.66;
 /// does not turn the accept loop into a busy spin.
 const ACCEPT_RETRY_DELAY: std::time::Duration = std::time::Duration::from_secs(1);
 
+/// Most connections served at once. The socket is mode 0666, so any process
+/// in the container can dial it; past this the accept loop stops accepting
+/// (the kernel queues the rest) instead of spawning a task and holding an fd
+/// per stalled client.
+const MAX_CONNECTIONS: usize = 64;
+
 /// Resolve the in-agent socket path from `HIVE_AGENT_SOCKET`. `None` when
 /// unset/empty (e.g. a standalone dev run) — the server then stays off.
 fn socket_path() -> Option<PathBuf> {
@@ -96,19 +102,39 @@ pub async fn run(
     };
     let listener = bind(&path)?;
     tracing::info!(socket = %path.display(), "in-agent todo socket listening");
+    accept_loop(listener, MAX_CONNECTIONS, move |stream| {
+        let (store, wake, reminders, bus) =
+            (store.clone(), wake.clone(), reminders.clone(), bus.clone());
+        async move {
+            if let Err(e) = handle_conn(stream, &store, &wake, reminders.as_deref(), &bus).await {
+                tracing::warn!(error = ?e, "in-agent todo connection failed");
+            }
+        }
+    })
+    .await
+}
+
+/// Accept connections for ever, serving at most `max` at a time with
+/// `handle`. A permit is taken before `accept`, so at the limit the loop
+/// waits there and the kernel's listen queue holds the rest.
+async fn accept_loop<F, Fut>(listener: UnixListener, max: usize, handle: F) -> Result<()>
+where
+    F: Fn(UnixStream) -> Fut,
+    Fut: std::future::Future<Output = ()> + Send + 'static,
+{
+    let permits = Arc::new(tokio::sync::Semaphore::new(max));
     loop {
+        let permit = permits
+            .clone()
+            .acquire_owned()
+            .await
+            .context("connection limit closed")?;
         match listener.accept().await {
             Ok((stream, _)) => {
-                let store = store.clone();
-                let wake = wake.clone();
-                let reminders = reminders.clone();
-                let bus = bus.clone();
+                let served = handle(stream);
                 tokio::spawn(async move {
-                    if let Err(e) =
-                        handle_conn(stream, &store, &wake, reminders.as_deref(), &bus).await
-                    {
-                        tracing::warn!(error = ?e, "in-agent todo connection failed");
-                    }
+                    served.await;
+                    drop(permit);
                 });
             }
             Err(e) => {
@@ -502,6 +528,37 @@ mod tests {
     use hive_sock_client::REQUEST_LINE_MAX_BYTES;
     use tokio::io::AsyncBufReadExt;
 
+    /// At the limit the loop stops accepting; a freed slot lets the next
+    /// connection in.
+    #[tokio::test]
+    async fn the_accept_loop_serves_no_more_than_its_limit_at_once() {
+        use std::sync::atomic::{AtomicUsize, Ordering};
+
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join("s.sock");
+        let listener = UnixListener::bind(&path).expect("bind");
+        let started = Arc::new(AtomicUsize::new(0));
+        let release = Arc::new(Notify::new());
+        let (s, r) = (started.clone(), release.clone());
+        let server = tokio::spawn(accept_loop(listener, 1, move |_stream| {
+            let (s, r) = (s.clone(), r.clone());
+            async move {
+                s.fetch_add(1, Ordering::SeqCst);
+                r.notified().await;
+            }
+        }));
+
+        let _a = UnixStream::connect(&path).await.expect("first");
+        let _b = UnixStream::connect(&path).await.expect("second");
+        tokio::time::sleep(std::time::Duration::from_millis(100)).await;
+        assert_eq!(started.load(Ordering::SeqCst), 1, "second waited its turn");
+
+        release.notify_one();
+        tokio::time::sleep(std::time::Duration::from_millis(100)).await;
+        assert_eq!(started.load(Ordering::SeqCst), 2, "freed slot admitted it");
+        server.abort();
+    }
+
     /// `saturating_age`'s own arms are exact (`hive-sh4re::inbox`); what
     /// is only checkable here is *which* stored timestamp each projection
     /// feeds it, and both read the clock internally — so these assert a