bug#70910: xdot has stopped working after update to 1.3

[Ada Stevenson]

Hi Guix,

I was trying to use `xdot` to view to output of `guix graph` (`guix graph bash | xdot -`). However, it crashed instantly with the following stack trace:

> dot: No such file or directory
>         Traceback (most recent call last):
>           File
>         "/gnu/store/r9mrqfpci65hsypgm7r5kpzxlm6fcd1l-xdot-1.3/bin/.xdot-real",
>         line 33, in <module>
>             sys.exit(load_entry_point('xdot==1.3', 'gui_scripts',
>         'xdot')())
>           File
> "/gnu/store/r9mrqfpci65hsypgm7r5kpzxlm6fcd1l-xdot-1.3/lib/python3.10/site-packages/xdot/__main__.py",
>         line 80, in main
>             win.set_dotcode(sys.stdin.buffer.read())
>           File
> "/gnu/store/r9mrqfpci65hsypgm7r5kpzxlm6fcd1l-xdot-1.3/lib/python3.10/site-packages/xdot/ui/window.py",
>         line 712, in set_dotcode
>             if self.dotwidget.set_dotcode(dotcode, filename):
>           File
> "/gnu/store/r9mrqfpci65hsypgm7r5kpzxlm6fcd1l-xdot-1.3/lib/python3.10/site-packages/xdot/ui/window.py",
>         line 148, in set_dotcode
>             if self._set_dotcode(dotcode, filename, center=center):
>           File
> "/gnu/store/r9mrqfpci65hsypgm7r5kpzxlm6fcd1l-xdot-1.3/lib/python3.10/site-packages/xdot/ui/window.py",
>         line 134, in _set_dotcode
>             xdotcode = self.run_filter(dotcode)
>           File
> "/gnu/store/r9mrqfpci65hsypgm7r5kpzxlm6fcd1l-xdot-1.3/lib/python3.10/site-packages/xdot/ui/window.py",
>         line 129, in run_filter
>             return xdotcode
>         UnboundLocalError: local variable 'xdotcode' referenced before
>         assignment
>

Using `guix time-machine` I was able to identify the regression commit as 2aeb9faa421635c0531d96658414e5bf288a3142 (when Ludo' updated it to 1.3). Using `xdot` within the time machine inside the parent commit works.

I've cc'd Ludo' and hopefully we can fix this!

Warmly,

Ada