MSAN use-of-uninitialized-value on Unknown_log_event::read_checksum_alg

To reproduce, handcraft a malformed event in a binary log and read it with mariadb-binlog --force-read, like in MDEV-39485's new test, main.binlog_invalid_row_v2_tag.

  Uninitialized value was stored to memory at

    #0 in Log_event::print_header(st_io_cache*, st_print_event_info*, bool) /path/to/src/sql/log_event_client.cc:349:7

    #1 in Unknown_log_event::print(_IO_FILE*, st_print_event_info*) /path/to/src/sql/log_event_client.cc:2611:9

[...]

  Memory was marked as uninitialized

[...]

    #2 in Log_event::operator new(unsigned long) /path/to/src/sql/log_event.h:1477:12

    #3 in Log_event::read_log_event(unsigned char const*, unsigned int, char const**, Format_description_log_event const*, char, char) /path/to/src/sql/log_event.cc

    #4 in Log_event::read_log_event(st_io_cache*, int*, Format_description_log_event const*, char, char, unsigned long) /path/to/src/sql/log_event.cc:926:13

[...]

Code Analysis

The Log_event constructor used to always initialize the checksum algorithm field to UNDEF; the actual algorithm is set later.

MDEV-31273 Commit 2 removed this constructor initialization, but missed that the creation of mariadb-binlog --force-read's Unknown_log_event substitutes for invalid events is after the recording of the actual algorithm.

Checksum failures (in mariadb-binlog --verify-binlog-checksum --force-read) also has a similarly problem:
An Unknown_log_event is immediately outputted, bypassing the post-processing that records the algorithm.
In practice, this case encounters MDEV-40544 first, but should face the same problem after MDEV-40544 is fixed.

Not-events are not affected because they fail in the outer Log_event::read_log_event(st_io_cache*, ...) and become (for whatever reason) encrypted event substitutes that do not output a checksum.

Fix Recommendations

Choose One...

• Restore pre-MDEV-31273 "behaviour" (read: implementation detail) by always setting (or initializing) Unknown_log_event::read_checksum_alg to UNDEF.
• Reörder the code to also set the actual algorithm for an Unknown_log_event since it's known at this point.
  • This improvement may be worth applying on pre-MDEV-31273 (10.11) as well.

Blocker priority:
This bug causes the aforementioned test to brick the required MSAN check.