Hi all,Some background: Late in the cycle, some improvements to the magnifier were committed to gnome-shell; the GSoC project of Magdalen Berns (BGO#647074). Giovanni discovered a few days ago that starting gnome-control-center and switching to the display panel now resulted in the desktop freezing for up to 30 seconds. He then made a commit to gnome-shell so that it would not start the magnifier unless it was actually used. Jasper felt that the underlying bug should be fixed and tried to convince Giovanni to revert his commit, although now it would require a freeze break request to do so, and he has not made one--I don't know if he plans on doing so. The bug still affects users of the magnifier; it doesn't affect all GNOME users unless Giovanni's commit is reverted.
The problem is that gnome-control-center is making a synchronous D-Bus call to gnome-shell to get information on the monitor, and the new gnome-shell focus caret tracker responds to the focus changing by making a synchronous D-Bus call to gnome-control-center. The processes deadlock waiting on each other until the AT-SPI call times out.
I filed https://bugzilla.gnome.org/show_bug.cgi?id=708387 for this.There are a few options for handling this, and what makes the most sense might depend in part on whether the "don't start the magnifier unless it is needed" commit gets reverted.
I've made patches for at-spi2-core and at-spi2-atk (attached, and added to the bug) that fix the issue for me. They start to move in the direction that I think we should take for 3.12 (reduce the need for synchronous D-Bus calls--for instance, when sending an event, also send the information that the AT will need in order to process the event, rather than make the AT request the information). Ideally this should be configurable, but this patch sends the information that gnome-shell's state-changed:focused handler needs, avoiding the need for it to make calls over D-Bus. Having these patches in 3.10 would be nice since they eliminate the deadlock completely and don't cause the magnifier's calls to time out and fail, but they aren't trivial patches.
Alternatively, gnome-shell could lower AT-SPI's timeout values (also attached a patch to do this). This would make the AT-SPI calls time out after a relatively short amount of time, mitigating the impact of the deadlock, although it would not eliminate it (the number of users impacted on this would depend on whether or not Giovanni's commit stays), and the magnifier may behave incorrectly when switching to the display panel in gnome-control-center. It would also increase the likelihood that a call would time out if an app is busy, although I picked a timeout of 250 ms fairly arbitrarily as a timeout that would have a relatively small impact in the case of a deadlock.
Committing both sets of patches would also be an option in case there are other deadlocks that we don't know about (I haven't tried to account for the listener that is activated when the text caret moves, although I'm guessing that that is less likely to result in a D-Bus query to gnome-shell than would a focus change). We need to fix this more properly, but that will have to wait for 3.12.
Thanks, -Mike
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