fix(rustproxy-http): validate pooled HTTP/2 connections asynchronously before reuse and evict stale senders

rust/crates/rustproxy-http/src/proxy_service.rs

@@ -659,17 +659,40 @@ impl HttpProxyService {
                 h2: use_h2,
             };
 
-            // H2 pool checkout (H2 senders are Clone and multiplexed)
+            // H2 pool checkout with async readiness validation.
+            // checkout_h2 does synchronous is_closed()/is_ready() checks, but these
+            // reflect cached state — the H2 connection driver (a separate tokio task)
+            // may not have processed a pending GOAWAY/RST yet. The ready().await
+            // forces the runtime to yield, giving the driver a chance to detect failures.
             if use_h2 {
-                if let Some(sender) = self.connection_pool.checkout_h2(&pool_key) {
-                    self.metrics.backend_pool_hit(&upstream_key);
-                    self.metrics.set_backend_protocol(&upstream_key, "h2");
-                    let result = self.forward_h2_pooled(
-                        sender, parts, body, upstream_headers, &upstream_path,
-                        route_match.route, route_id, &ip_str, &pool_key, domain_str, &conn_activity,
-                    ).await;
-                    self.upstream_selector.connection_ended(&upstream_key);
-                    return result;
+                if let Some((mut sender, age)) = self.connection_pool.checkout_h2(&pool_key) {
+                    match tokio::time::timeout(
+                        std::time::Duration::from_millis(500),
+                        sender.ready(),
+                    ).await {
+                        Ok(Ok(())) => {
+                            self.metrics.backend_pool_hit(&upstream_key);
+                            self.metrics.set_backend_protocol(&upstream_key, "h2");
+                            let result = self.forward_h2_pooled(
+                                sender, parts, body, upstream_headers, &upstream_path,
+                                route_match.route, route_id, &ip_str, &pool_key, domain_str, &conn_activity,
+                            ).await;
+                            self.upstream_selector.connection_ended(&upstream_key);
+                            return result;
+                        }
+                        Ok(Err(e)) => {
+                            warn!(backend = %upstream_key, age_secs = age.as_secs(),
+                                "Pooled H2 sender failed ready check (GOAWAY/RST): {}, evicting", e);
+                            self.connection_pool.remove_h2(&pool_key);
+                            // Fall through to fresh connection
+                        }
+                        Err(_) => {
+                            warn!(backend = %upstream_key, age_secs = age.as_secs(),
+                                "Pooled H2 sender ready check timed out (500ms), evicting");
+                            self.connection_pool.remove_h2(&pool_key);
+                            // Fall through to fresh connection
+                        }
+                    }
                 }
             }
         }