dev: fix udp ring-buffer not looping

My previous version would be limited to a single thread, as `push_overwrite` would keep on returning the last element when the ring-buffer was full.

Now the ring-buffer is pre-filled and is looped over with a mutating iterator.

New handles are progressively swapped-in when the old entries are finished.

Note: I think that this implementation can be replaced with a standard vector with the same effect.

src/servers/udp/server.rs

@@ -24,7 +24,7 @@ use std::sync::Arc;
 use aquatic_udp_protocol::Response;
 use derive_more::Constructor;
 use log::{debug, error, info, trace};
-use ringbuf::traits::{Consumer, Observer, RingBuffer};
+use ringbuf::traits::{Consumer, Observer, Producer};
 use ringbuf::StaticRb;
 use tokio::net::UdpSocket;
 use tokio::sync::oneshot;
@@ -202,11 +202,21 @@ impl Launcher {
     }
 }
 
-#[derive(Default)]
 struct ActiveRequests {
     rb: StaticRb<AbortHandle, 50>, // the number of requests we handle at the same time.
 }
 
+impl ActiveRequests {
+    /// Creates a new [`ActiveRequests`] filled with finished tasks.
+    fn new() -> Self {
+        let mut rb = StaticRb::default();
+
+        let () = while rb.try_push(tokio::task::spawn_blocking(|| ()).abort_handle()).is_ok() {};
+
+        Self { rb }
+    }
+}
+
 impl std::fmt::Debug for ActiveRequests {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let (left, right) = &self.rb.as_slices();
@@ -280,15 +290,21 @@ impl Udp {
         let tracker = tracker.clone();
         let socket = socket.clone();
 
-        let reqs = &mut ActiveRequests::default();
+        let reqs = &mut ActiveRequests::new();
 
-        // Main Waiting Loop, awaits on async [`receive_request`].
         loop {
-            if let Some(h) = reqs.rb.push_overwrite(
-                Self::spawn_request_processor(Self::receive_request(socket.clone()).await, tracker.clone(), socket.clone())
-                    .abort_handle(),
-            ) {
-                if !h.is_finished() {
+            for h in reqs.rb.iter_mut() {
+                if h.is_finished() {
+                    std::mem::swap(
+                        h,
+                        &mut Self::spawn_request_processor(
+                            Self::receive_request(socket.clone()).await,
+                            tracker.clone(),
+                            socket.clone(),
+                        )
+                        .abort_handle(),
+                    );
+                } else {
                     // the task is still running, lets yield and give it a chance to flush.
                     tokio::task::yield_now().await;
 
@@ -299,6 +315,9 @@ impl Udp {
                     tracing::span!(
                         target: "UDP TRACKER",
                         tracing::Level::WARN, "request-aborted", server_socket_addr = %server_socket_addr);
+
+                    // force-break a single thread, then loop again.
+                    break;
                 }
             }
         }
@@ -396,13 +415,45 @@ mod tests {
     use std::sync::Arc;
     use std::time::Duration;
 
+    use ringbuf::traits::{Consumer, Observer, RingBuffer};
     use tokio::time::sleep;
     use torrust_tracker_test_helpers::configuration::ephemeral_mode_public;
 
+    use super::ActiveRequests;
     use crate::bootstrap::app::initialize_with_configuration;
     use crate::servers::registar::Registar;
     use crate::servers::udp::server::{Launcher, UdpServer};
 
+    #[tokio::test]
+    async fn it_should_return_to_the_start_of_the_ring_buffer() {
+        let mut a_req = ActiveRequests::new();
+
+        let mut count: usize = 0;
+        let cap: usize = a_req.rb.capacity().into();
+
+        // Add a single pending task to check that the ring-buffer is looping correctly.
+        a_req
+            .rb
+            .push_overwrite(tokio::task::spawn(std::future::pending::<()>()).abort_handle());
+
+        count += 1;
+
+        for _ in 0..2 {
+            for h in a_req.rb.iter() {
+                let first = count % cap;
+                println!("{count},{first},{}", h.is_finished());
+
+                if first == 0 {
+                    assert!(!h.is_finished());
+                } else {
+                    assert!(h.is_finished());
+                }
+
+                count += 1;
+            }
+        }
+    }
+
     #[tokio::test]
     async fn it_should_be_able_to_start_and_stop() {
         let cfg = Arc::new(ephemeral_mode_public());