Merge pull request #15987 from filimonov/background-pool-count_no_work_done-bug

Fix the bug when NOTHING_TO_DO events wrongly increment count_no_work_done

src/Storages/MergeTree/BackgroundProcessingPool.cpp

@@ -217,8 +217,11 @@ void BackgroundProcessingPool::workLoopFunc()
 
             if (task_result == TaskResult::SUCCESS)
                 task->count_no_work_done = 0;
-            else
+            else if (task_result == TaskResult::ERROR)
                 ++task->count_no_work_done;
+            /// NOTHING_TO_DO should not increment count_no_work_done
+            /// otherwise error after period of inactivity (lot of NOTHING_TO_DO)
+            /// leads to 5-10 min replication hang
 
             /// If task has done work, it could be executed again immediately.
             /// If not, add delay before next run.