chore: await PocketIC HTTP gateway shutdown (#2709)

This PR ensures that the PocketIC server awaits the PocketIC HTTP
gateway shutdown by shutting down the axum server handle in a drop
handler invoked in the methods `ApiState::stop_http_gateway` and
`ApiState::stop_all_http_gateways` after setting the `HttpGateway`
struct to `None`.

In particular, the function `ApiState::stop_all_http_gateways` is
awaited in the signal handler of the PocketIC server and thus all
PocketIC HTTP gateways are shut down gracefully before the PocketIC main
axum server is shut down and the main function exits (terminating the
PocketIC server process).

rs/pocket_ic_server/src/state_api/state.rs

@@ -126,6 +126,26 @@ struct Instance {
     state: InstanceState,
 }
 
+struct HttpGateway {
+    details: HttpGatewayDetails,
+    shutdown_handle: Handle,
+}
+
+impl HttpGateway {
+    fn new(details: HttpGatewayDetails, shutdown_handle: Handle) -> Self {
+        Self {
+            details,
+            shutdown_handle,
+        }
+    }
+}
+
+impl Drop for HttpGateway {
+    fn drop(&mut self) {
+        self.shutdown_handle.shutdown();
+    }
+}
+
 /// The state of the PocketIC API.
 pub struct ApiState {
     // impl note: If locks are acquired on both fields, acquire first on `instances` and then on `graph`.
@@ -136,7 +156,7 @@ pub struct ApiState {
     // PocketIC server port
     port: Option<u16>,
     // HTTP gateway infos (`None` = stopped)
-    http_gateways: Arc<RwLock<Vec<Option<HttpGatewayDetails>>>>,
+    http_gateways: Arc<RwLock<Vec<Option<HttpGateway>>>>,
 }
 
 #[derive(Default)]
@@ -1063,22 +1083,10 @@ impl ApiState {
             domains: http_gateway_config.domains.clone(),
             https_config: http_gateway_config.https_config.clone(),
         };
-        http_gateways.push(Some(http_gateway_details));
-        drop(http_gateways);
-
-        let http_gateways_for_shutdown = self.http_gateways.clone();
         let shutdown_handle = handle.clone();
-        tokio::spawn(async move {
-            loop {
-                let guard = http_gateways_for_shutdown.read().await;
-                if guard[instance_id].is_none() {
-                    shutdown_handle.shutdown();
-                    break;
-                }
-                drop(guard);
-                tokio::time::sleep(Duration::from_secs(1)).await;
-            }
-        });
+        let http_gateway = HttpGateway::new(http_gateway_details, shutdown_handle);
+        http_gateways.push(Some(http_gateway));
+        drop(http_gateways);
 
         Ok(HttpGatewayInfo {
             instance_id,
@@ -1197,9 +1205,8 @@ impl ApiState {
         self.http_gateways
             .read()
             .await
-            .clone()
-            .into_iter()
-            .flatten()
+            .iter()
+            .filter_map(|gateway| gateway.as_ref().map(|g| g.details.clone()))
             .collect()
     }