Fixed race condition on queue_task_handle

dbms/src/Storages/MergeTree/BackgroundProcessingPool.cpp

@@ -60,19 +60,27 @@ BackgroundProcessingPool::BackgroundProcessingPool(int size_,
 }
 
 
-BackgroundProcessingPool::TaskHandle BackgroundProcessingPool::addTask(const Task & task)
+BackgroundProcessingPool::TaskHandle BackgroundProcessingPool::createTask(const Task & task)
 {
-    TaskHandle res = std::make_shared<TaskInfo>(*this, task);
+    return std::make_shared<TaskInfo>(*this, task);
+}
 
+void BackgroundProcessingPool::startTask(const TaskHandle & task)
+{
     Poco::Timestamp current_time;
 
     {
         std::unique_lock lock(tasks_mutex);
-        res->iterator = tasks.emplace(current_time, res);
+        task->iterator = tasks.emplace(current_time, task);
     }
 
     wake_event.notify_all();
+}
 
+BackgroundProcessingPool::TaskHandle BackgroundProcessingPool::addTask(const Task & task)
+{
+    TaskHandle res = createTask(task);
+    startTask(res);
     return res;
 }
 

dbms/src/Storages/MergeTree/BackgroundProcessingPool.h

@@ -82,9 +82,14 @@ public:
         return size;
     }
 
-    /// The task is started immediately.
+    /// Create task and start it.
     TaskHandle addTask(const Task & task);
 
+    /// Create task but not start it.
+    TaskHandle createTask(const Task & task);
+    /// Start the task that was created but not started. Precondition: task was not started.
+    void startTask(const TaskHandle & task);
+
     void removeTask(const TaskHandle & task);
 
     ~BackgroundProcessingPool();

dbms/src/Storages/StorageReplicatedMergeTree.cpp

@@ -2902,9 +2902,16 @@ void StorageReplicatedMergeTree::startup()
     /// Wait while restarting_thread initializes LeaderElection (and so on) or makes first attmept to do it
     startup_event.wait();
 
-    queue_task_handle = global_context.getBackgroundPool().addTask([this] { return queueTask(); });
+    /// If we don't separate create/start steps, race condition will happen
+    /// between the assignment of queue_task_handle and queueTask that use the queue_task_handle.
+    queue_task_handle = global_context.getBackgroundPool().createTask([this] { return queueTask(); });
+    queue_task_handle->startTask();
+
     if (areBackgroundMovesNeeded())
-        move_parts_task_handle = global_context.getBackgroundMovePool().addTask([this] { return movePartsTask(); });
+    {
+        move_parts_task_handle = global_context.getBackgroundMovePool().createTask([this] { return movePartsTask(); });
+        move_parts_task_handle->startTask();
+    }
 }