I found that one issue was left unaddressed:

On Thu, 20 Jul 2023 at 17:59, Sergei Golubchik <serg@mariadb.org> wrote:
> diff --git a/sql/log_event.cc b/sql/log_event.cc
> index 10180a95d99..0c4c284a40f 100644
> --- a/sql/log_event.cc
> +++ b/sql/log_event.cc
> @@ -761,16 +761,21 @@ Log_event::Log_event(const uchar *buf,

>  int Log_event::read_log_event(IO_CACHE* file, String* packet,
>                                const Format_description_log_event *fdle,
> -                              enum enum_binlog_checksum_alg checksum_alg_arg)
> +                              enum enum_binlog_checksum_alg checksum_alg_arg,
> +                              size_t max_allowed_packet)
>  {
>    ulong data_len;
>    char buf[LOG_EVENT_MINIMAL_HEADER_LEN];
>    uchar ev_offset= packet->length();
>  #if !defined(MYSQL_CLIENT)
> -  THD *thd=current_thd;
> -  ulong max_allowed_packet= thd ? thd->slave_thread ? slave_max_allowed_packet
> -                                                    : thd->variables.max_allowed_packet
> -                                : ~(uint)0;
> +  if (max_allowed_packet == 0)

may be better pass THD as an argument?
or even correct value of max_allowed_packet. As

  ulong max_allowed_packet(THD *thd)
  { return thd ? ... : ~(uint)0; }

I thought about it, but let me avoid it this time. There are ~15 calls of both read_log_event functions, and it's hard to say what's called from where.
I'd better leave this insane knot as it is now. 

Also as a bonus mysql_bin_log will not depend on an external max_allowed_packet, which is fixed as 1GiB.


on the second thought, do you need to ignore max_allowed_packet
here for online ALTER? Is there no event size limit when the
event is written into binlog?

> +  {
> +    THD *thd=current_thd;
> +    max_allowed_packet= thd ? thd->slave_thread
> +                              ? slave_max_allowed_packet
> +                              : thd->variables.max_allowed_packet
> +                            : ~(uint)0;
> +  }
>  #endif
>    DBUG_ENTER("Log_event::read_log_event(IO_CACHE*,String*...)");


Regards,
Sergei
VP of MariaDB Server Engineering
and security@mariadb.org


--
Yours truly,
Nikita Malyavin