bug#18986: Design bug in browse-url.el: browse-url-can-use-xdg-open's logic uses overly strict assumption

[florian@fsavigny.de (Florian v. Savigny)]

  > From: Stefan Monnier <monnier@iro.umontreal.ca>
  > Date: Sat, 08 Nov 2014 13:05:54 -0500
  > 
  > The problem was specifically that just having xdg-open installed
  > doesn't guarantee that the user has configured it.  The code
  > currently approximates "is it configured" by checking the desktop
  > environment in use.  It's clearly a gross hack (or a crude
  > approximation), so if you know a better way, we'd like to hear it.


Thank you very much for your feedback!

It would seem to me that checking if

   xdg-mime query default text/html

or, in a more general way, perhaps

   xdg-mime query default `xdg-mime query filetype URL/FILE`

returns SOMETHING should be sufficient.

I am absolutely NOT sure, I have to admit, because I have found the
documentation for the xdg system not very accessible if you are
interested in manual configuration, and lack the time to study it
hard.

I have found that what the above command returns is not necessarily
what xdg-open will actually run (i.e. it does not seem to reflect user
reconfiguration - which is probably why the query is called
"default"), but it would seem to me that if it returns SOMETHING,
there is a guarantee that there is a default, which in turn guarantees
that xdg-open will start SOMETHING.

This line of thought assumes that /usr/share/applications/*.desktop
files for any given application will ONLY be there if the application
itself is also installed, i.e. that they both come with the same
package. I think that this is the case for any distribution I have
used so far.

Even if this assumption is unsafe, I think one could check if the
return value of xdg-open is 3 or 4, which could then lead to a message
such as "Please configure your xdg system properly; see man xdg-mime
or your desktop environment's configuration tool" and/or to some
fallback action (which browse-url is already capable of). Of course,
this last measure could not be implemented within the
browse-url-can-use-xdg-open function.

An alternative approach (which would however not be mutually exclusive
with the above) might be to make the hardcoded browser list in
browse-url-default-browser customisable, which currently forces an
order of preference on the user as follows:


browse-url-gnome-moz-program
browse-url-mozilla-program
browse-url-firefox-program
browse-url-chromium-program
browse-url-galeon-program
browse-url-kde-program
browse-url-netscape-program
browse-url-mosaic-program
browse-url-xterm-program


The reason why I am somehow uncomfortable with browse-url's logic of
determining a browser to start is that I find it effectively (if most
probably not intentionally) patronising, and this is, in my view,
unusual for Emacs - out of character, as it were.

I would like to apologise if my suggestions are not sufficiently
substantial.

Best regards,

-- 

Florian von Savigny
Melanchthonstr. 41
33615 Bielefeld