fix(server): keep shutdown responsive during sends

crates/irtt-server/AGENTS.md

@@ -202,15 +202,14 @@ Do not rewrite the protocol evidence to match them.
   The only interoperability constraint is negative — expiry must never be
   signalled — and silence satisfies it. A zero idle timeout expires a session at
   the next evaluation; it is not "never expire".
-- **Expiry is packet-driven until the runtime exists.** Logical expiry is
+- **Logical expiry is request-exact; physical reclamation also runs on the
-  timestamp-based, but reclamation happens in a global sweep run after
+  runtime's fixed maintenance cadence.** Every authenticated, structurally valid
-  authentication and before dispatch, so an expired session may stay resident
+  request runs the global sweep before dispatch, so a request at the deadline
-  until the next authenticated, structurally valid request. Nothing observable
+  finds the session expired and a stale session cannot deny an open. The Tokio
-  turns on it: the table is already bounded, and the sweep runs before capacity
+  runtime invokes that same core sweep once per second so dead sessions do not
-  is judged, so a stale session cannot deny an open. Because the sweep is global
+  remain resident when there is no further traffic. This cadence is internal
-  it also runs for a no-test open, where upstream was observed not to reclaim.
+  housekeeping, not protocol policy or public configuration, and the runtime
-  Do not add a public timer or tick abstraction for this; the Tokio slice may add
+  must not inspect session internals or duplicate the expiry rule.
-  scheduled sweeps.
 - **The maximum-duration deadline is `first served echo + maximum + 2 s`.** The
   origin is measured behavior: neither the open, nor a rejected first echo, nor a
   rate-limited one starts it. The two-second grace matches the measured upstream

crates/irtt-server/src/runtime.rs

@@ -62,9 +62,10 @@ impl Server {
     /// Serves datagrams until `shutdown` completes.
     ///
     /// Graceful shutdown sends no session-close packets and leaves no hidden
-    /// receive task behind. Per-packet send failures and short sends drop only
+    /// receive task behind, even while a reply send is waiting for socket
-    /// that reply; receive failures and internal core failures terminate the
+    /// writability. Per-packet send failures and short sends drop only that
-    /// loop with an error.
+    /// reply; receive failures and internal core failures terminate the loop
+    /// with an error.
     pub async fn run<F>(&mut self, shutdown: F) -> Result<(), ServerRuntimeError>
     where
         F: Future<Output = ()>,
@@ -89,7 +90,15 @@ impl Server {
                     }
 
                     if let Some(reply) = self.core.handle_datagram(peer, &self.recv_buffer[..len])? {
-                        let sent = self.socket.send_to(&reply, peer).await;
+                        let send = self.socket.send_to(&reply, peer);
+                        tokio::pin!(send);
+                        let sent = loop {
+                            tokio::select! {
+                                _ = &mut shutdown => return Ok(()),
+                                _ = maintenance.tick() => self.core.maintain(),
+                                sent = &mut send => break sent,
+                            }
+                        };
                         if !matches!(sent, Ok(len) if len == reply.len()) {
                             continue;
                         }