Duplicate MPP task

[zanmato1984]

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/tiflash_regression_test_daily/detail/tiflash_regression_test_daily/2269/pipeline

[2021-03-30T07:43:56.555Z] ERROR 1105 (HY000) at line 1: DB::Exception: The task [423910502926385155,1] has been registered

After some investigation, we found that:

• TiFlash MPP task manager maintains a map from MPP task id to a concrete MPP task;
• A MPP task is identified by its associated transaction start_ts and a intra-query integer (self-incremental within the query);
• When several queries share the same start_ts, typically in single transaction started by begin statement, MPP tasks of different queries may have the same MPP task id;
• There is a small chance that the last query finishes and the next query starts executing, but MPP task manager hasn't removed the MPP task with the same id from the previous query;
• Thus the duplicated MPP task.

[ilovesoup]

Would you please describe the issue more clear in case we want to review in the future..

[zanmato1984]

Would you please describe the issue more clear in case we want to review in the future..

Updated in issue description.