fix: close dropped subscriber sockets and ping idle ones

stream_labels only noticed a disconnect on its next send, so with few labels
closed subscribers piled up in CLOSE-WAIT until the server ran out of file
descriptors. It now watches the socket and pings every 30s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/server.rs

@@ -1,4 +1,5 @@
 use std::sync::Arc;
+use std::time::Duration;
 
 use anyhow::Result;
 use axum::extract::ws::{Message, WebSocket, WebSocketUpgrade};
@@ -17,6 +18,7 @@ use crate::db::{LabelQuery, SeqLabel};
 use crate::labeler::Labeler;
 
 const REPLAY_BATCH: i64 = 500;
+const PING_INTERVAL: Duration = Duration::from_secs(30);
 
 fn bad_request(message: &str) -> GenericXrpcErrorResponse {
     GenericXrpcErrorResponse::new(StatusCode::BAD_REQUEST, "InvalidRequest", Some(message))
@@ -137,16 +139,27 @@ async fn stream_labels(
         replay(socket, labeler, &mut last).await?;
     }
 
+    // Labels are rare, so also watch the socket: otherwise a subscriber that
+    // hangs up is only noticed on the next send, and its connection leaks.
+    let mut ping = tokio::time::interval(PING_INTERVAL);
     loop {
-        match live.recv().await {
-            Ok(row) if row.seq > last => {
-                last = row.seq;
-                socket.send(labels_frame(row)?).await?;
-            }
-            Ok(_) => {}
-            // fell behind the channel; catch up from the database
-            Err(RecvError::Lagged(_)) => replay(socket, labeler, &mut last).await?,
-            Err(RecvError::Closed) => return Ok(()),
+        tokio::select! {
+            incoming = socket.recv() => match incoming {
+                None | Some(Err(_)) | Some(Ok(Message::Close(_))) => return Ok(()),
+                Some(Ok(_)) => {}
+            },
+            // keeps idle connections from being dropped by peers and proxies
+            _ = ping.tick() => socket.send(Message::Ping(Default::default())).await?,
+            published = live.recv() => match published {
+                Ok(row) if row.seq > last => {
+                    last = row.seq;
+                    socket.send(labels_frame(row)?).await?;
+                }
+                Ok(_) => {}
+                // fell behind the channel; catch up from the database
+                Err(RecvError::Lagged(_)) => replay(socket, labeler, &mut last).await?,
+                Err(RecvError::Closed) => return Ok(()),
+            },
         }
     }
 }