fix download limit proxy and wait flow

seaweed-volume/src/server/handlers.rs

@@ -911,28 +911,20 @@ async fn get_or_head_handler_inner(
         return proxy_or_redirect_to_target(&state, info, vid, false).await;
     }
 
-    // Download throttling
+    // Download throttling — matches Go's checkDownloadLimit + waitForDownloadSlot
     let download_guard = if state.concurrent_download_limit > 0 {
         let timeout = state.inflight_download_data_timeout;
         let deadline = tokio::time::Instant::now() + timeout;
         let query_string = request.uri().query().unwrap_or("").to_string();
-        let mut noted_over_limit = false;
 
-        loop {
-            let current = state.inflight_download_bytes.load(Ordering::Relaxed);
-            if current <= state.concurrent_download_limit {
-                break;
-            }
-
-            if !noted_over_limit {
-                metrics::HANDLER_COUNTER
-                    .with_label_values(&[metrics::DOWNLOAD_LIMIT_COND])
-                    .inc();
-                noted_over_limit = true;
-            }
+        let current = state.inflight_download_bytes.load(Ordering::Relaxed);
+        if current > state.concurrent_download_limit {
+            metrics::HANDLER_COUNTER
+                .with_label_values(&[metrics::DOWNLOAD_LIMIT_COND])
+                .inc();
 
-            // Match Go's checkDownloadLimit: when this request has not already been
-            // proxied and the local volume has replicas, try another replica first.
+            // Go tries proxy to replica ONCE before entering the blocking wait
+            // loop (checkDownloadLimit L65). It does NOT retry on each wakeup.
             let should_try_replica =
                 !query_string.contains("proxied=true") && !state.master_url.is_empty() && {
                     let store = state.store.read().unwrap();
@@ -948,15 +940,22 @@ async fn get_or_head_handler_inner(
                 }
             }
 
-            if tokio::time::timeout_at(deadline, state.download_notify.notified())
-                .await
-                .is_err()
-            {
-                return json_error_with_query(
-                    StatusCode::TOO_MANY_REQUESTS,
-                    "download limit exceeded",
-                    raw_query.as_deref(),
-                );
+            // Blocking wait loop (Go's waitForDownloadSlot)
+            loop {
+                if tokio::time::timeout_at(deadline, state.download_notify.notified())
+                    .await
+                    .is_err()
+                {
+                    return json_error_with_query(
+                        StatusCode::TOO_MANY_REQUESTS,
+                        "download limit exceeded",
+                        raw_query.as_deref(),
+                    );
+                }
+                let current = state.inflight_download_bytes.load(Ordering::Relaxed);
+                if current <= state.concurrent_download_limit {
+                    break;
+                }
             }
         }
         // We'll set the actual bytes after reading the needle (once we know the size)