fix(admin): keep data usage endpoint available (#5494)

rustfs/src/app/admin_usecase.rs

@@ -276,10 +276,8 @@ impl DefaultAdminUsecase {
             .await
             .map_err(|_| Self::app_error(S3ErrorCode::InternalError, "list_bucket failed"))?;
         if !Self::data_usage_snapshot_covers_namespace(&info, buckets.into_iter().map(|bucket| bucket.name)) {
-            return Err(Self::app_error(
-                S3ErrorCode::ServiceUnavailable,
-                "authoritative data usage snapshot is not available yet",
-            ));
+            // The default wire shape distinguishes unknown usage from a confirmed zero snapshot.
+            info = DataUsageInfo::default();
         }
 
         let storage_info = StorageAdminApi::storage_info(store.as_ref()).await;

rustfs/src/app/data_usage_snapshot_gating_test.rs

@@ -274,9 +274,20 @@ async fn data_usage_endpoint_treats_legacy_partial_stats_as_unknown_until_covere
     let before_endpoint = live_bucket_usage_computations();
     let unknown = DefaultAdminUsecase::query_data_usage_info_with_store(ecstore.clone())
         .await
-        .expect_err("an uncovered snapshot must be reported as temporarily unavailable");
+        .expect("an uncovered snapshot must be reported as unknown without failing the endpoint");
 
-    assert_eq!(unknown.code, s3s::S3ErrorCode::ServiceUnavailable);
+    assert!(unknown.last_update.is_none());
+    assert!(!unknown.usage_snapshot_complete);
+    assert_eq!(unknown.buckets_count, 0);
+    assert_eq!(unknown.objects_total_count, 0);
+    assert_eq!(unknown.objects_total_size, 0);
+    assert!(unknown.buckets_usage.is_empty());
+    assert!(unknown.bucket_sizes.is_empty());
+    assert!(unknown.total_capacity > 0);
+    assert_eq!(
+        unknown.total_used_capacity,
+        unknown.total_capacity.saturating_sub(unknown.total_free_capacity)
+    );
     assert_eq!(
         live_bucket_usage_computations(),
         before_endpoint,