Erlang (OTP 21+) Logger Console Output Question (Garbled Prompt)

[Lukas Larsson]

Hello!

This issue has to do with which group_leader that the process that is
writing the output has.

When doing `io:format` in a process that either is a shell (or is started
by a shell) the group_leader is set to a process that is aware of the shell
and can rewrite it when output comes its way.

However, since logger does not use the group_leader of the calling process
to print log messages, those messages get sent directly to the `user`
process which has no notion of whether a shell is actually running or not,
so it cannot rewrite the output.

It is not only logger that has this problem, if you do an `io:format` in a
process started as part of an application hierarchy you will see the same
behaviour. (or by doing `spawn(fun F() -> timer:sleep(1000),
io:format(user,"hello hello~n",[]), F() end)`)

I'm not sure how easy it is to do anything about this, as it would require
different parts of the group_leader hierarchy to interact in ways that I do
not think it does today.

A possibly short-term solution for you might be hitting "ctrl-l" (ctrl and
non-capital L) when the situation happens. That should rewrite the current
line so that you can see what you are doing.

Lukas


On Fri, May 7, 2021 at 9:21 PM Brett Hemes <brhemes@REDACTED> wrote:

> While developing I often use console prints as debugging/verification
> tools.  In the past I have used io:format for this purpose but I would like
> to use the new OTP logger to do this and then be able to filter based on
> severity and/or send them to a file instead of the console as the project
> matures.  However, I am having trouble currently preventing logger’s output
> from destroying my console interactivity.  I have logger setup to use the
> console for output but **the logging prints output AT the current cursor
> position and thus destroy any input-in-progress** making interaction during
> busy logging near impossible.  In contrast, using io:format performs as
> desired where the output happens (and scrolls) right ABOVE the cursor line
> (i.e., without destroying the prompt).  Is this known behavior and/or is
> there any way to fix it (e.g., with some sys.config settings)?
>
>
>
> For example,
>
>
>
> With io:format:
>
>     37> some_intput.
>
>     some_info_i_would_like_to_display
>
>     some_more_info
>
>     info_etc
>
>     38> my-next-input.<enter>
>
>     39> <enter>
>
>     39>
>
>
>
> With logger:
>
>     37> some_input.
>
>     38> some_info_i_would_like_to_display
>
>     my-nexsome_more_info
>
>     t-input.<enter>
>
>
>
>     39> info_etc<enter>
>
>
>
>     39>
>
>
>
>
>
>
>
> I would very much like to figure this out.
>
>
>
> Thanks,
>
> Brett
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20210510/d3b882bc/attachment.htm>