Potential hang if duplicate task is registered

[windtalker]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Consider the case that task 1 is registered, and TiDB send task 1 to TiFlash again due to some issue(for example, TiDB meet rpc deadline exceeded), then the second request will fail due to "task is already registered", and go through error handling process:

tiflash/dbms/src/Flash/Mpp/MPPHandler.cpp

Lines 55 to 69 in d16a3f0

	void MPPHandler::handleError(const MPPTaskPtr & task, String error)
	{
	if (task)
	{
	try
	{
	task->handleError(error);
	}
	catch (...)
	{
	tryLogCurrentException(log, "Fail to handle error and clean task");
	}
	task->unregisterTask();
	}
	}

In L67, it will unregister the task. Note this task is not registered yet, unregister task will unregister the first "task 1" from MPPTaskManager, in this case the subsequent cancel can't not see this task, so the first "task 1" may hang forever.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)