feat: [torrust#508] add health check for UDP tracker

Using the `connect` UDP request. If the UDP trackers replies to a
connection request we assume is healthy.

src/servers/health_check_api/handlers.rs

@@ -1,12 +1,14 @@
 use std::net::SocketAddr;
 use std::sync::Arc;
 
+use aquatic_udp_protocol::{ConnectRequest, Response, TransactionId};
 use axum::extract::State;
 use axum::Json;
 use torrust_tracker_configuration::Configuration;
 
 use super::resources::Report;
 use super::responses;
+use crate::shared::bit_torrent::udp::client::new_udp_tracker_client_connected;
 
 /// If port 0 is specified in the configuration the OS will automatically
 /// assign a free port. But we do now know in from the configuration.
@@ -23,6 +25,8 @@ pub(crate) async fn health_check_handler(State(config): State<Arc<Configuration>
     // running service, after starting it as we do for testing with ephemeral
     // configurations.
 
+    // Health check for API
+
     if config.http_api.enabled {
         let addr: SocketAddr = config.http_api.bind_address.parse().expect("invalid socket address for API");
 
@@ -35,6 +39,8 @@ pub(crate) async fn health_check_handler(State(config): State<Arc<Configuration>
         }
     }
 
+    // Health check for HTTP Trackers
+
     for http_tracker_config in &config.http_trackers {
         if !http_tracker_config.enabled {
             continue;
@@ -56,7 +62,22 @@ pub(crate) async fn health_check_handler(State(config): State<Arc<Configuration>
         }
     }
 
-    // todo: for all UDP  Trackers, if enabled, check if is healthy
+    // Health check for UDP Trackers
+
+    for udp_tracker_config in &config.udp_trackers {
+        if !udp_tracker_config.enabled {
+            continue;
+        }
+
+        let addr: SocketAddr = udp_tracker_config
+            .bind_address
+            .parse()
+            .expect("invalid socket address for UDP tracker");
+
+        if addr.port() != UNKNOWN_PORT && !can_connect_to_udp_tracker(&addr.to_string()).await {
+            return responses::error(format!("UDP Tracker is not healthy. Can't connect to: {addr}"));
+        }
+    }
 
     responses::ok()
 }
@@ -67,3 +88,18 @@ async fn get_req_is_ok(url: &str) -> bool {
         Err(_err) => false,
     }
 }
+
+/// Tries to connect to an UDP tracker. It returns true if it succeeded.
+async fn can_connect_to_udp_tracker(url: &str) -> bool {
+    let client = new_udp_tracker_client_connected(url).await;
+
+    let connect_request = ConnectRequest {
+        transaction_id: TransactionId(123),
+    };
+
+    client.send(connect_request.into()).await;
+
+    let response = client.receive().await;
+
+    matches!(response, Response::Connect(_connect_response))
+}