fix tenant filtering in union subqueries

Author: func25

lib/logstorage/storage_search.go

@@ -239,15 +239,15 @@ func (s *Storage) runQuery(qctx *QueryContext, writeBlock writeBlockResultFunc)
 	// A non-empty TenantIDs means the caller already resolved the exact tenants to search
 	// (e.g. the stream_context pipe scopes its subquery to a single tenant derived from the streamID).
 	// Only resolve tenants from the query filter when the caller left TenantIDs unresolved.
+	tenantIDs := qctx.TenantIDs
 	if qctx.IsMultiTenant && len(qctx.TenantIDs) == 0 {
-		tenantIDs, err := s.getTenantIDsForQuery(qctx.Context, q)
+		tenantIDs, err = s.getTenantIDsForQuery(qctx.Context, q)
 		if err != nil {
 			return fmt.Errorf("cannot get tenant IDs: %w", err)
 		}
-		qctx.TenantIDs = tenantIDs
 	}
 
-	sso := s.getSearchOptions(qctx.TenantIDs, qctx.IsMultiTenant, q, qctx.HiddenFieldsFilters)
+	sso := s.getSearchOptions(tenantIDs, qctx.IsMultiTenant, q, qctx.HiddenFieldsFilters)
 	search := func(stopCh <-chan struct{}, writeBlockToPipes writeBlockResultFunc) error {
 		workersCount := q.GetParallelReaders(s.defaultParallelReaders)
 		s.searchParallel(workersCount, sso, qctx.QueryStats, stopCh, writeBlockToPipes)