[PATCH xf86-input-libinput] If the parent libinput_device is unavailable, create a new one

[Peter Hutterer]

On Tue, Nov 15, 2016 at 10:35:31AM +0100, Hans de Goede wrote:
> Hi,
> 
> On 15-11-16 05:37, Peter Hutterer wrote:
> > The parent device ref's the libinput device during pre_init and unref's it
> > during DEVICE_INIT, so the copy is lost. During DEVICE_ON, the libinput device
> > is re-added and ref'd, this one stays around now. But the takeaway is: unless
> > the device is enabled, no libinput device reference is available.
> > 
> > If a device is a mixed pointer + keyboard device, a subdevice is created
> > during a WorkProc. The subdevice relied on the parent's libinput_device being
> > available and didn't even check for it. This WorkProc usually runs after
> > the parent's DEVICE_ON, so in most cases all is well.
> > 
> > But when running without logind and the server is vt-switched away, the parent
> > device only runs PreInit and DEVICE_INIT but never DEVICE_ON, causing the
> > subdevice to burn, crash, and generally fail horribly when it dereferences the
> > parent's libinput device.
> > 
> > Fix this because we have global warming already and don't need to burn more
> > things and also because it's considered bad user experience to have the
> > server crash. The simple fix is to check the parent device first and if it is
> > unavailable, create a new one because it will end up disabled as well anyway,
> > so the ref goes away as well. The use-case where the parent somehow gets
> > disabled but the subdevice doesn't is a bit too niche to worry about.
> > 
> > This doesn't happen with logind because in that case we don't get a usable fd
> > while VT-switched away, so we can't even run PreInit and never get this far
> > (see the paused fd handling in the xfree86 code for that). It can be
> > reproduced by setting AutoEnableDevices off, but why would you do that,
> > seriously.
> > 
> > https://bugs.freedesktop.org/show_bug.cgi?id=97117
> > 
> > Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
> 
> Hmm, so what happens if the parent device later does get DEVICE_ON, after
> the subdevice has created its own libinputdevice ?

we don't differ between parent and subdevices during DEVICE_ON, whichever
one is the first adds the device and the rest just refcounts it. so we
should be good here.

Cheers,
   Peter