Found in [1]:
2021.03.12 23:32:58.897190 [ 436606 ] {97a08a8f-4630-44f8-ba18-a94c6fabbded} <Information> test_13.lc_empty_part_bug: Waiting mutation: mutation_3.txt
...
2021.03.12 23:32:58.900444 [ 374 ] {} <Debug> test_13.lc_empty_part_bug: Current max source part size for mutation is 0 but part size 1. Will not mutate part all_1_1_0_2. Max size depends not only on available space, but also on settings 'number_of_free_entries_in_pool_to_execute_mutation' and 'background_pool_size'
...
Hang check:
query: alter table lc_empty_part_bug modify column s LowCardinality(String);
elapsed: 2156.396613506
[1]: https://clickhouse-test-reports.s3.yandex.net/21511/d7d9638ceb6bd702f34a88ee54f8f83197e90af5/stress_test_(address).html#fail1
