Right now it is possible to have "false-positive" for this hardening,
because there is a tiny delay (that can be quite significant on CI, when
it is under pressure) between when the server sends EndOfStream and the
server removes an entry from system.processes.
But now system.processes has is_all_data_sent column, that means that
the EndOfStream was sent, and we can use it to avoid false positive.
Here is an example of such report [1]:
2022-04-25 03:47:18 00806_alter_update: [ FAIL ] 0.95 sec. - Queries left in background after the test finished:
2022-04-25 03:47:18 "elapsed": 0.100084746,
2022-04-25 03:47:18 "is_cancelled": 0,
2022-04-25 03:47:18 "query": "DROP TABLE alter_update_00806;",
2022-04-25 03:47:18 "thread_ids": [
2022-04-25 03:47:18 "8950"
2022-04-25 03:47:18 ],
2022.04.25 03:47:17.887095 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Debug> executeQuery: (from [::1]:52012) (comment: 00806_alter_update.sql) DROP TABLE alter_update_00806;
2022.04.25 03:47:17.887493 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Trace> ContextAccess (default): Access granted: DROP TABLE ON test_7ntsjn.alter_update_00806
2022.04.25 03:47:17.887765 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Trace> test_7ntsjn.alter_update_00806 (1bc92bca-10a7-444e-be5e-7f61f4650169): Found 2 old parts to remove.
2022.04.25 03:47:17.887947 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Debug> test_7ntsjn.alter_update_00806 (1bc92bca-10a7-444e-be5e-7f61f4650169): Removing part from filesystem 20180101_20180101_1_1_0
2022.04.25 03:47:17.888960 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Debug> test_7ntsjn.alter_update_00806 (1bc92bca-10a7-444e-be5e-7f61f4650169): Removing part from filesystem 20180102_20180102_2_2_0
2022.04.25 03:47:17.890620 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Debug> DatabaseCatalog: Waiting for table 1bc92bca-10a7-444e-be5e-7f61f4650169 to be finally dropped
2022.04.25 03:47:17.895046 [ 8950 ] {7c062004-4c22-486c-934a-f405846e2c81} <Debug> MemoryTracker: Peak memory usage (for query): 0.00 B.
...
2022.04.25 03:47:17.938328 [ 4422 ] {aa01985a-78f5-4c0e-b646-8d04a4a1dc77} <Debug> executeQuery: (from [::1]:59416) (comment: 00806_alter_update.sql) DROP DATABASE test_7ntsjn
2022.04.25 03:47:17.938667 [ 4422 ] {aa01985a-78f5-4c0e-b646-8d04a4a1dc77} <Trace> ContextAccess (default): Access granted: DROP DATABASE ON test_7ntsjn.*
...
2022.04.25 03:47:18.154847 [ 8950 ] {} <Debug> TCPHandler: Processed in 0.269358257 sec.
2022.04.25 03:47:18.154991 [ 8950 ] {} <Debug> TCPHandler: Done processing connection.
2022.04.25 03:47:18.155181 [ 8950 ] {} <Debug> TCP-Session: e1d8176a-ee62-4e0a-9855-fe9eb52e06dc Destroying unnamed session of user 94309d50-4f52-5250-31bd-74fecac179db
[1]: https://s3.amazonaws.com/clickhouse-test-reports/36319/a646cf76b6d4699f06aea1e8d777edb1ad6fd2c5/stateless_tests__debug__actions__[1/3]/runlog.log
So as you can see here DROP TABLE was captured when elapsed was 0.1,
while TCPHandler processes it for 0.26 seconds.
Also from the same report you are seeing that DROP DATABASE was executed
before TCPHandler stopoped processing DROP TABLE.
Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>
