Where is the code responsible for running Exec parameter in *.desktop files, GNOME 2.32

[Иво Георгиев <ivo linvo org>]

I want to make a modification to the *.desktop file execution for myself. I have a custom tag in the *.desktop files (beginning with X-, as advised by the XDG specification), and I want GNOME to execute a certain command before it starts the program specified by the Exec tag. The thing specified by my custom tag in the *.desktop files will be the parameter of this command. 

After some research, I found that gnome-menus is responsible for reading *.desktop files, but I don't think that will be useful to me. I need to know where is the code for executing the program itself. I came to the conclusion that this part must be in gvfs or gnome-vfs, but I didn't find anything there, either.

Let me give an example:

foobar.desktop

Exec=foobar

X-custom-tag=barfoo

Gnome should run "command barfoo" and then "foobar". 

What's the simplest way to implement that?