bug#40456: Invalid keyboard layouts pass through

["Ludovic Courtès" <ludo@gnu.org>]

Hello,

After a painful debugging session where I was blaming GDM for not using
the right keyboard layout, I found that I had specified an invalid
keyboard layout variant, as in:

  (keyboard-layout "fr" "does-not-exist")

‘ckbcomp’ would build a layout for the console just fine, only with a
warning.  Xorg would switch to the default “fr” layout (which exists)
and ignore the “does-not-exist” variant.

However, GNOME Shell would reject it in ‘keyboardManager.js’ because:

     [found, , , _layout, _variant] = this._xkbInfo.get_layout_info(id);

returns ‘found = false’ since ‘fr+does-not-exist’ is not found, and thus
goes on to use the US layout:

      if (found)
          return { layout: _layout, variant: _variant };
      else
          return { layout: DEFAULT_LAYOUT, variant: DEFAULT_VARIANT };

(This is just for GNOME Shell because again, Xorg itself installs the
“fr” layout, as can be seen if you start ratpoison or similar.)

Long story short: it would be great if invalid keyboard layouts were
caught when the system is instantiated.  It could be ‘ckbcomp’ errorring
out, or we could have additional code that browses xkeyboard-config’s
‘base.xml’ file.

Ludo’.