node: Add a max capacity to fetch queues

radicle-node/src/service.rs

@@ -1017,12 +1017,9 @@ where
                         timeout,
                         channel,
                     };
+                    debug!(target: "service", "Queueing fetch for {rid} with {from} (already fetching)..");
 
-                    if self.queue_fetch(fetch) {
-                        debug!(target: "service", "Queueing fetch for {rid} with {from} (already fetching)..");
-                    } else {
-                        debug!(target: "service", "Fetch for {rid} with {from} is already queued..");
-                    }
+                    self.queue_fetch(fetch);
                 }
             }
             Err(TryFetchError::SessionCapacityReached) => {
@@ -1047,12 +1044,15 @@ where
         false
     }
 
-    fn queue_fetch(&mut self, fetch: QueuedFetch) -> bool {
+    fn queue_fetch(&mut self, fetch: QueuedFetch) {
         let Some(s) = self.sessions.get_mut(&fetch.from) else {
             log::error!(target: "service", "Cannot queue fetch for unknown session {}", fetch.from);
-            return false;
+            return;
         };
-        s.queue_fetch(fetch)
+        if let Err(e) = s.queue_fetch(fetch) {
+            let fetch = e.inner();
+            log::debug!(target: "service", "Unable to queue fetch for {} with {}: {e}", &fetch.rid, &fetch.from);
+        }
     }
 
     // TODO: Buffer/throttle fetches.

radicle-node/src/service/session.rs

@@ -15,6 +15,8 @@ pub use crate::node::{PingState, State};
 
 /// Time after which a connection is considered stable.
 pub const CONNECTION_STABLE_THRESHOLD: LocalDuration = LocalDuration::from_mins(1);
+/// Maximum items in the fetch queue.
+pub const MAX_FETCH_QUEUE_SIZE: usize = 128;
 
 #[derive(thiserror::Error, Debug, Clone, Copy)]
 pub enum Error {
@@ -46,6 +48,27 @@ impl Error {
     }
 }
 
+/// Error when trying to queue a fetch.
+#[derive(thiserror::Error, Debug, Clone)]
+pub enum QueueError {
+    /// The item already exists in the queue.
+    #[error("item is already queued")]
+    Duplicate(QueuedFetch),
+    /// The queue is at capacity.
+    #[error("queue capacity reached")]
+    CapacityReached(QueuedFetch),
+}
+
+impl QueueError {
+    /// Get the inner [`QueuedFetch`].
+    pub fn inner(&self) -> &QueuedFetch {
+        match self {
+            Self::Duplicate(f) => f,
+            Self::CapacityReached(f) => f,
+        }
+    }
+}
+
 /// Fetch waiting to be processed, in the fetch queue.
 #[derive(Debug, Clone)]
 pub struct QueuedFetch {
@@ -131,7 +154,7 @@ impl Session {
             subscribe: None,
             persistent,
             last_active: LocalTime::default(),
-            queue: VecDeque::new(),
+            queue: VecDeque::with_capacity(MAX_FETCH_QUEUE_SIZE),
             attempts: 1,
             rng,
             limits,
@@ -205,15 +228,17 @@ impl Session {
 
     /// Queue a fetch. Returns `true` if it was added to the queue, and `false` if
     /// it already was present in the queue.
-    pub fn queue_fetch(&mut self, fetch: QueuedFetch) -> bool {
+    pub fn queue_fetch(&mut self, fetch: QueuedFetch) -> Result<(), QueueError> {
         assert_eq!(fetch.from, self.id);
 
-        if self.queue.contains(&fetch) {
-            false
-        } else {
-            self.queue.push_back(fetch);
-            true
+        if self.queue.len() >= MAX_FETCH_QUEUE_SIZE {
+            return Err(QueueError::CapacityReached(fetch));
+        } else if self.queue.contains(&fetch) {
+            return Err(QueueError::Duplicate(fetch));
         }
+        self.queue.push_back(fetch);
+
+        Ok(())
     }
 
     pub fn dequeue_fetch(&mut self) -> Option<QueuedFetch> {