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List:       freedesktop-xorg-devel
Subject:    Re: Xwayland fatal error when Wayland output disappears
From:       Pekka Paalanen <ppaalanen () gmail ! com>
Date:       2017-10-27 8:39:28
Message-ID: 20171027113928.19ff002f () eldfell
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On Fri, 27 Oct 2017 09:54:39 +0200
Michael Thayer <michael.thayer@oracle.com> wrote:

> Hello Pekka,
> 
> On 27.10.2017 08:01, Pekka Paalanen wrote:
> > On Thu, 26 Oct 2017 15:40:13 +0200 Michael Thayer
> > <michael.thayer@oracle.com> wrote:  
> [ Discussion of a global Wayland object disappearing triggering a fatal
> error in xwl_log_handler in Xwayland.]
> > 
> > there is a known race around Wayland globals. If the Wayland server 
> > adds and removes a global in a very short time, it may succeed to 
> > remove the global (wl_output) before all clients have processed the 
> > add. If a client process an add after the server removed, you hit 
> > exactly this fatal error.
> > 
> > It's a design flaw in Wayland, gone unnoticed for years until it was 
> > too late to fix properly.
> > 
> > This issue is recorded: https://phabricator.freedesktop.org/T7722
> > 
> > There is a suggested mitigation, but I am not aware of anyone
> > working on it.  
> 
> Trigger warning: a person who does not know the code base is about to 
> make suggestions.
> 
> I am wondering why Xwayland has to call FatalError in that log handler.

Because the Wayland connection is already dead and disconnected due to
the error it is reporting.


Thanks,
pq

>  From my naive point of view, the race you mentioned is not a design 
> flaw in Wayland but part of the real world that Wayland and its clients 
> ought to deal with.  Other than that call in the log handler, I see no 
> reason why Xwayland should not just notice that the object has gone away 
> again and get on with life.
> 
> As I said, I do not know the code base well.  But for the sake of the 
> argument, and since it is not likely to make things worse, I will try 
> removing that locally and see what happens.
> 
> Regards and thanks.
> Michael


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