An attempt at fixing a bug involving DGA and mouse buttons

Steven Elliott selliott4 at
Sat Aug 18 18:46:01 PDT 2012

I'd like to describe a bug I've encountered involving mouse events and
DGA as well as my attempt at fixing it.  Hopefully either my patch is
acceptable, or people will have ideas about how it can be improved.
I'll mention some specifics about my setup at the end.

Recently I debugged a problem where an application I used seemed to
break the X Server so that it was no longer possible to cause windows to
gain focus by clicking on them.  The application is MCEdit.  It's a
pygame based python program that edits Minecraft maps.

In MCEdit clicking the right mouse button engages a freelook mode so
that moving the mouse changes the user's perspective instead of moving
the mouse pointer.  Engaging freelook mode in MCEdit causes the problem
which persists after MCEdit has been shutdown.

Debugging has revealed that the reason I'm no longer able cause windows
to gain focus by clicking on them once freelook mode has been used in
MCEdit is that device->button->buttonsDown ("buttonsDown" only occurs in
one structure) is not decremented when the mouse button is released
which causes CheckDeviceGrabs() to return early instead of sending the
mouse button event to the window being clicked on.

Further debugging revealed that the reason buttonsDown is not
decremented when the mouse is released has to do with the fact that DGA
mode is active during freelook mode.  When DGA mode is active  (when it
has registered a hook that is called for events so that DGAHandleEvent()
is called for keyboard and mouse events instead of
ProcessKeyboardEvent() and ProcessPointerEvent()) the following seems to
be true:
  1) DGAProcessPointerEvent() calls UpdateDeviceState() with mouse
button events where ev.detail.key is not set.
  2) DGAHandleEvent() refuses to process events that are not master
        if (!IsMaster(device))
     which causes UpdateDeviceState() to ignore the master event as it
thinks the child event has not been processed.

My tentative fix can be seen in the attached patch.  Even though my
patch fixes the problem for me without any apparent side effects I
suspect that additional research is needed.  Although the part of my
patch that addresses 1) above (the part in xf86DGA.c) is straightforward
the part of my patch that addresses 2) above (the part in exevents.c)
involves deleting code that was deliberately added.  Unless the point of
the code I removed was only perceived efficiency (avoiding processing
the mouse release event because the developer thought it would have done
nothing) then my patch is breaking something that I have not noticed

I have not yet been successful in writing a standalone test program to
recreate the problem (to recreate it without MCEdit), but I can try
things and report back.

I've reproduced the problem with Fedora 16 64 bit and Fedora 17 64 bit.
In both cases (same hardware) I was only able to reproduce the problem
with the proprietary driver for my Nvidia GeForce GTX 560 Ti, but that
may just that it's an issue of timing rather than a Nvidia specific

The RPMs and their versions where I've been able to reproduce the
  OS         RPM                   Version
  --         ---                   -------
  Fedora 16  xorg-x11-server-Xorg  1.11.4-3
  Fedora 17  xorg-x11-server-Xorg  1.12.2-4

Unfortunately it's not obvious to me how the RPM versions relate to
X.Org versions even after reading the change log for the RPM, but Fedora
17 is fairly new.

|  Steven Elliott  |  |  selliott4 at  |
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