add another debug and overflow comment

proxy/src/batch.rs

@@ -52,7 +52,10 @@ impl<P: QueueProcessing> BatchQueue<P> {
     pub async fn call(&self, req: P::Req) -> P::Res {
         let (id, mut rx) = self.inner.lock_propagate_poison().register_job(req);
         let guard = scopeguard::guard(id, move |id| {
-            self.inner.lock_propagate_poison().queue.remove(&id);
+            let mut inner = self.inner.lock_propagate_poison();
+            if inner.queue.remove(&id).is_some() {
+                tracing::debug!("batched task cancelled before completion");
+            }
         });
 
         let resp = loop {
@@ -112,6 +115,12 @@ impl<P: QueueProcessing> BatchQueueInner<P> {
         let (tx, rx) = tokio::sync::oneshot::channel();
 
         let id = self.version;
+
+        // Overflow concern:
+        // This is a u64, and we might enqueue 2^16 tasks per second.
+        // This gives us 2^48 seconds (9 million years).
+        // Even if this does overflow, it will not break, but some
+        // jobs with the higher version might never get prioritised.
         self.version += 1;
 
         self.queue.insert(id, BatchJob { req, res: tx });