create table t (a int primary key clustered);
insert into test.t values (1),(2),(3),(4);
set tidb_enforce_mpp=1

-- 1. With no failpoint injected, running this SQL is OK, MPPTask can be released as expected
tidb> select * from (select * from t) a join (select * from t) b;
ch>  select * from dt_tables where tidb_database='test' and tidb_table='t' \G
...
tidb_database:                            test
tidb_table:                               t
...
storage_stable_oldest_snapshot_lifetime:  0 <-- the snapshot will be released as expected

-- 2. With failpoint "exception_during_mpp_non_root_task_run" enabled
ch> DBGInvoke enable_fail_point(exception_during_mpp_non_root_task_run);
-- MPPTask can be released as expected too
tidb> select * from (select * from t) a join (select * from t) b;
ch>  select * from dt_tables where tidb_database='test' and tidb_table='t' \G
...
tidb_database:                            test
tidb_table:                               t
...
storage_stable_oldest_snapshot_lifetime:  0 <-- the snapshot will be released as expected

-- 3. With failpoint "exception_during_mpp_non_root_task_run" and "exception_during_mpp_write_err_to_tunnel" enabled
ch> DBGInvoke enable_fail_point(exception_during_mpp_non_root_task_run);
ch> DBGInvoke enable_fail_point(exception_during_mpp_write_err_to_tunnel);
-- MPPTask can not be released, it will hold the resources of IStorage, and cause other trouble, such as:
--   TiFlash stucks after applying DDL operations
--   DeltaTree can not run GC on its delta data
tidb> select * from (select * from t) a join (select * from t) b;
ch>  select * from dt_tables where tidb_database='test' and tidb_table='t' \G
...
tidb_database:                            test
tidb_table:                               t
...
storage_stable_oldest_snapshot_lifetime:  45.19153881 <-- the snapshot will not be released
...