Test `03212_variant_dynamic_cast_or_default` is flaky

[nickitat]

Describe the bug

https://play.clickhouse.com/play?user=play#c2VsZWN0IAp0b1N0YXJ0T2ZIb3VyKGNoZWNrX3N0YXJ0X3RpbWUpIGFzIGQsCmNvdW50KCksIGdyb3VwVW5pcUFycmF5KHB1bGxfcmVxdWVzdF9udW1iZXIpLCAgYW55KHJlcG9ydF91cmwpCmZyb20gY2hlY2tzIHdoZXJlIHRvZGF5KCkgLSBJTlRFUlZBTCAnMyBtb250aHMnIDw9IGNoZWNrX3N0YXJ0X3RpbWUgYW5kIHRlc3RfbmFtZSBsaWtlICclMDMyMTJfdmFyaWFudF9keW5hbWljX2Nhc3Rfb3JfZGVmYXVsdCUnIGFuZCB0ZXN0X3N0YXR1cyBpbiAoJ0ZBSUwnLCAnRkxBS1knKSBncm91cCBieSBkIG9yZGVyIGJ5IGQgZGVzYw==

How to reproduce

No response

Error message and/or stacktrace

No response

[rschu1ze]

Example failure (from here):

2025-09-24 14:36:47 Reason: result differs with reference:  
2025-09-24 14:36:47 --- /home/ubuntu/actions-runner/_work/ClickHouse/ClickHouse/tests/queries/0_stateless/03212_variant_dynamic_cast_or_default.reference	2025-09-24 14:24:31.905870033 -0500
2025-09-24 14:36:47 +++ /home/ubuntu/actions-runner/_work/ClickHouse/ClickHouse/tests/queries/0_stateless/03212_variant_dynamic_cast_or_default.stdout	2025-09-24 14:36:47.225829234 -0500
2025-09-24 14:36:47 @@ -501,16 +501,17 @@
2025-09-24 14:36:47  1970-01-01 00:04:15
2025-09-24 14:36:47  1970-01-01 09:06:06
2025-09-24 14:36:47  1970-01-01 09:06:07
2025-09-24 14:36:47  1970-01-01 18:12:14
2025-09-24 14:36:47  1970-01-01 18:12:15
2025-09-24 14:36:47  2038-01-19 03:14:06
2025-09-24 14:36:47  2038-01-19 03:14:07
2025-09-24 14:36:47  2106-02-07 06:28:14
2025-09-24 14:36:47  2106-02-07 06:28:15
2025-09-24 14:36:47  0.0.0.0
2025-09-24 14:36:47 +114.61.8.170
2025-09-24 14:36:47  192.168.0.1
2025-09-24 14:36:47  ::
2025-09-24 14:36:47  ::1
2025-09-24 14:36:47  ::ffff:192.168.0.1
2025-09-24 14:36:47  00000000-0000-0000-0000-000000000000
2025-09-24 14:36:47  dededdb6-7835-4ce4-8d11-b5de6f2820e9
2025-09-24 14:36:47 
2025-09-24 14:36:47 
2025-09-24 14:36:47 Settings used in the test: --max_insert_threads 3 --group_by_two_level_threshold 598709 --group_by_two_level_threshold_bytes 1 --distributed_aggregation_memory_efficient 1 --fsync_metadata 1 --output_format_parallel_formatting 0 --input_format_parallel_parsing 1 --min_chunk_bytes_for_parallel_parsing 5608986 --max_read_buffer_size 575829 --prefer_localhost_replica 1 --max_block_size 21733 --max_joined_block_size_rows 25024 --max_threads 1 --optimize_append_index 0 --use_hedged_requests 0 --optimize_if_chain_to_multiif 0 --optimize_if_transform_strings_to_enum 1 --optimize_read_in_order 1 --optimize_or_like_chain 1 --optimize_substitute_columns 0 --enable_multiple_prewhere_read_steps 1 --read_in_order_two_level_merge_threshold 11 --optimize_aggregation_in_order 1 --aggregation_in_order_max_block_bytes 5527211 --use_uncompressed_cache 0 --min_bytes_to_use_direct_io 10737418240 --min_bytes_to_use_mmap_io 10737418240 --local_filesystem_read_method pread --remote_filesystem_read_method threadpool --local_filesystem_read_prefetch 1 --filesystem_cache_segments_batch_size 2 --read_from_filesystem_cache_if_exists_otherwise_bypass_cache 0 --throw_on_error_from_cache_on_write_operations 1 --remote_filesystem_read_prefetch 1 --allow_prefetched_read_pool_for_remote_filesystem 1 --filesystem_prefetch_max_memory_usage 32Mi --filesystem_prefetches_limit 10 --filesystem_prefetch_min_bytes_for_single_read_task 8Mi --filesystem_prefetch_step_marks 0 --filesystem_prefetch_step_bytes 0 --compile_expressions 0 --compile_aggregate_expressions 1 --compile_sort_description 1 --merge_tree_coarse_index_granularity 16 --optimize_distinct_in_order 0 --max_bytes_before_remerge_sort 963950786 --min_compress_block_size 1851708 --max_compress_block_size 1646123 --merge_tree_compact_parts_min_granules_to_multibuffer_read 85 --optimize_sorting_by_input_stream_properties 0 --http_response_buffer_size 5116408 --http_wait_end_of_query False --enable_memory_bound_merging_of_aggregation_results 0 --min_count_to_compile_expression 3 --min_count_to_compile_aggregate_expression 0 --min_count_to_compile_sort_description 3 --session_timezone Africa/Khartoum --use_page_cache_for_disks_without_file_cache False --page_cache_inject_eviction True --merge_tree_read_split_ranges_into_intersecting_and_non_intersecting_injection_probability 0.35 --prefer_external_sort_block_bytes 1 --cross_join_min_rows_to_compress 100000000 --cross_join_min_bytes_to_compress 1 --min_external_table_block_size_bytes 100000000 --max_parsing_threads 10 --optimize_functions_to_subcolumns 0 --parallel_replicas_local_plan 1 --query_plan_join_swap_table false --enable_vertical_final 1 --optimize_extract_common_expressions 0 --use_async_executor_for_materialized_views 0 --use_query_condition_cache 1 --secondary_indices_enable_bulk_filtering 0 --use_skip_indexes_if_final 0 --use_skip_indexes_on_data_read 1 --max_bytes_before_external_sort 10737418240 --max_bytes_before_external_group_by 10737418240 --max_bytes_ratio_before_external_sort 0 --max_bytes_ratio_before_external_group_by 0 --use_skip_indexes_if_final_exact_mode 0
2025-09-24 14:36:47 
2025-09-24 14:36:47 MergeTree settings used in test: --ratio_of_defaults_for_sparse_serialization 0.0 --prefer_fetch_merged_part_size_threshold 10737418240 --vertical_merge_algorithm_min_rows_to_activate 753752 --vertical_merge_algorithm_min_columns_to_activate 43 --allow_vertical_merges_from_compact_to_wide_parts 0 --min_merge_bytes_to_use_direct_io 1 --index_granularity_bytes 23815748 --merge_max_block_size 7648 --index_granularity 39315 --min_bytes_for_wide_part 0 --marks_compress_block_size 13355 --primary_key_compress_block_size 54409 --replace_long_file_name_to_hash 0 --max_file_name_length 81 --min_bytes_for_full_part_storage 536870912 --compact_parts_max_bytes_to_buffer 98093928 --compact_parts_max_granules_to_buffer 193 --compact_parts_merge_max_bytes_to_prefetch_part 29241633 --cache_populated_by_fetch 1 --concurrent_part_removal_threshold 100 --old_parts_lifetime 10 --prewarm_mark_cache 1 --use_const_adaptive_granularity 1 --enable_index_granularity_compression 0 --enable_block_number_column 1 --enable_block_offset_column 0 --use_primary_key_cache 1 --prewarm_primary_key_cache 0 --object_serialization_version v2 --object_shared_data_serialization_version map_with_buckets --object_shared_data_serialization_version_for_zero_level_parts advanced --object_shared_data_buckets_for_compact_part 16 --object_shared_data_buckets_for_wide_part 2 --dynamic_serialization_version v3
2025-09-24 14:36:47 
2025-09-24 14:36:47 Database: test_40z4uyjz

[incfly]

Example

2025-09-26 22:39:37 Reason: result differs with reference:  
2025-09-26 22:39:37 --- /home/ubuntu/actions-runner/_work/ClickHouse/ClickHouse/tests/queries/0_stateless/03212_variant_dynamic_cast_or_default.reference	2025-09-26 22:30:32.916257771 -0400
2025-09-26 22:39:37 +++ /home/ubuntu/actions-runner/_work/ClickHouse/ClickHouse/tests/queries/0_stateless/03212_variant_dynamic_cast_or_default.stdout	2025-09-26 22:39:37.205485899 -0400
2025-09-26 22:39:37 @@ -504,13 +504,14 @@
2025-09-26 22:39:37  1970-01-01 18:12:14
2025-09-26 22:39:37  1970-01-01 18:12:15
2025-09-26 22:39:37  2038-01-19 03:14:06
2025-09-26 22:39:37  2038-01-19 03:14:07
2025-09-26 22:39:37  2106-02-07 06:28:14
2025-09-26 22:39:37  2106-02-07 06:28:15
2025-09-26 22:39:37  0.0.0.0
2025-09-26 22:39:37  192.168.0.1
2025-09-26 22:39:37  ::
2025-09-26 22:39:37  ::1
2025-09-26 22:39:37 +::ffff:111.22.2.233
2025-09-26 22:39:37  ::ffff:192.168.0.1
2025-09-26 22:39:37  00000000-0000-0000-0000-000000000000
2025-09-26 22:39:37  dededdb6-7835-4ce4-8d11-b5de6f2820e9

[PedroTadim]

Another one: https://s3.amazonaws.com/clickhouse-test-reports/json.html?PR=88034&sha=eafefa22e3f55fc313a6026a59f1ba87ff4d13da&name_0=PR

[zlareb1]

Potential change to fix this test failure - #88269

My current hypothesis is that this behavior might be due to toIPv4OrDefault() / toIPv6OrDefault() reading from non–null-terminated buffers when operating on Dynamic values backed by FixedString.
Depending on memory layout, compiler, or platform, the parser could interpret adjacent bytes as valid IP addresses — leading to nondeterministic results in CI.