How to debug METAPOST inlined in ConTeXt? (Or "message" primitive and "loggingall;" in inlined METAPOST)

Question

I have a METAPOST program that works fine with mpost, but fails when I try to run it inline in ConTeXt. In order to debug it, I would like to see the usual METAPOST logging and/or to use the METAPOST "message" primitive.

However, whenever I add "loggingall;" or a message statement to my ConTeXt file, I encounter errors like this

metapost        > initializing instance 'metafun' using format 'metafun'
metapost        > loading 'metafun': D:/texlive/2012/texmf-dist/metapost/context/base/metafun.mpiv
! terminal:

What is the appropriate way to debug problems with inline METAPOST code in ConTeXt?

My example file is:

\loggingall

\starttext
\framed{Hello world.}
This is a simple_test.

\startMPpage
loggingall;
fill fullcircle scaled 1in withcolor blue;
label.top(btex This is a test. etex, origin);
\stopMPpage

\stoptext

I am using TeXLive 2012 on Windows.

I apologize for any vagueness in this question. I’m not really sure how the various pieces fit together.

Answer 1

By default MetaPost does not display the messages injected with message. However, the messages can be enabled using \enabletrackers.

To enable verbose debug output use tracingall, which prints plenty of debugging output to the console and the log file. Here's a complete example:

\enabletrackers
  [metapost.showlog]

\starttext
  \startMPpage
    tracingall;
    message "This is a debug message.";
    fill fullcircle scaled 1in withcolor blue;
    label.top(btex This is a test. etex, origin);
  \stopMPpage
\stoptext