Re: Fix for timestamp lag issue from emit_log_hook when GUC log_line_prefix has '%m'

On Sat, Mar 09, 2024 at 09:09:39PM +0530, Kambam Vinay wrote:
> We observed a slight lag in timestamp for a few logs from the emit_log_hook
> hook implementation when the log_line_prefix GUC has '%m'.
>
> Upon debugging, we found that the saved_timeval_set variable is set to
> 'true' in get_formatted_log_time() but is not reset to 'false' until the
> next call to send_message_to_server_log(). Due to this, saved_timeval_set
> will be true during the execution of hook emit_log_hook() which prefixes
> the saved timestamp 'saved_timeval' from the previous log line (our hook
> implementation calls log_line_prefix()).
>
> Attached patch sets the saved_timeval_set to false before executing the
> emit_log_hook()

Indeed.  If you rely on log_line_prefix() in your hook before the
server side elog, the saved timestamp would not match with what could
be computed in the follow-up send_message_to_server_log or
send_message_to_frontend.

Hmm.  Shouldn't we remove the forced reset of formatted_log_time for
the 'm' case in log_status_format() and remove the reset done at the
beginning of send_message_to_server_log()?  One problem with your
patch is that we would still finish with a different saved_timeval in
the hook and in send_message_to_server_log(), but we should do things
so as the timestamps are the same for the whole duration of
EmitErrorReport(), no?  It seems to me that a more correct solution
would be to reset saved_timeval_set and formatted_log_time[0] once
before the hook, at the beginning of EmitErrorReport().
--
Michael