[tex-k] Potential bug in MF: missing output in transcript file & long diagnostics on terminal

[Dominik Leininger]

Hello everyone,

I've come across the following strange behavior of MF, which doesn't 
seem to be described in The MFbook, and therefore may not be intended 
behavior:

Running MF with `\ show tracingonline; show origin..right; end' (either 
on the command line after `mf' or immediately after `**') will give the 
following output:

     >> 0
     >> Path at line 0:
     (0,0)..controls (0.33333,0) and (0.66667,0)
     ..(1,0)

     Transcript written on mfput.log.

According to `>> 0', the path should not be displayed on the terminal, 
but it does. The second thing I noticed is that you should see the path 
in the transcript file, but mfput.log only contains the following line 
after the MF's welcoming:

     **\ show tracingonline; show origin..right; end

If you just type \ after **, hit enter and then type the show command 
after the single `*', everything works as expected.

Note that the transcript file also lacks the output of message commands 
when used immediately after `**':

terminal:

     **\ message "A";
     A
     *message "B";
     B
     *end

mfput.log:

     **\ message "A";

     *message "B";
     B
     *end

I also checked picture variables (`\ show unitpixel;') and pen variables 
(`\ show pensquare;') for tracingonline:=0 (which is default).
I also checked for tracingonline:=-1.
I tested on Windows 10 (METAFONT, Version 2.71828182 (TeX Live 2023)) 
and Ubuntu 22.04 (also METAFONT, Version 2.71828182 (TeX Live 2023)).

What am I missing here? Is this behavior intended? If so, I cannot find 
a description of it in The MFbook. The behavior feels strange, 
especially that the transcript file does not include the output of 
show/message commands when used immediately after `**'. My understanding 
is that everything after `**\' should behave the same as after `*'.

Summary of unexpected behaviors:
- output of message and show commands immediately after `**' (or after 
`mf \' in the command line) don't get logged in the transcript file.
- paths, pens and pictures are shown on the terminal even though 
tracingonline<=0 when the show command is used immediately after `**' 
(or after `mf \' in the command line)

Has anyone experienced this behavior or can anyone reproduce it?

Thanks and best regards,

Dominik