bug#34489: 25.2; pdb fails if directory contains '++'

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sun, 30 Jun 2019 18:13:19 +0200
> Cc: 34489@debbugs.gnu.org
> 
> The expected behaviour is to see TWO buffers in separate windows,
> first "*gud-pp.py" with the following content:
> 
>     Current directory is /tmp/foo++/
>     > /tmp/foo++/pp.py(1)<module>()
>     -> print(10)
>     (Pdb)
> 
> And the second a buffer visiting "pp.py".
> 
> The symptom is that you only see ONE buffer "*gud-pp.py*" with the
> following content:
> 
>     Current directory is /tmp/foo++/
> 
> The bug (half) goes away as soon as I try to debug `gud-common-init'
> using M-x edebug-defun, but only when stepping through manually using
> "n" (and not when hitting "c").  Without this instrumentation, I get
> the bug as above.  With it, I see the expected output in the
> "*gud-pp.py*" buffer (but *not* the expected "pp.py" buffer).
> 
> I tried adding strategic calls to sit-for in gud-common-init and could
> get the same result as with M-x edebug-defun.
> 
> Does anyone have any ideas what could cause this?

A stab in the dark: gud-pdb-marker-regexp?