Re: [gtk-list] Re: GraphicsExpose (was Re: About the About Box)

[Raph Levien <raph acm org>]

On Mon, 15 Dec 1997, Owen Taylor wrote:

> 
> Raph Levien <raph@acm.org> writes:
>
> > I had another thought on this and wanted to throw it out there. Since 
> > this is at heart a race condition caused by asynchronous GraphicsExpose 
> > events following XCopyArea, wouldn't the problem be fixed by doing an 
> > XSync right before the scroll? I can't imagine the performance 
> > degradation would be that serious, and it sounds a whole lot simpler 
> > than some of the other proposals.
> 
> I don't think this will help. XSync (or gdk_flush()) only affect
> the output queue. Since the problem is GraphicsExpose events sitting
> in the input queue, the only way to fix it is to process those events
> before doing the XCopyArea.

Yeah, I tried the hack (it's only one line) and found it didn't help. 
It's clear to me now why - even though the XSync causes the app to have 
access to all pending GraphicsExpose events, it has to actively look 
ahead in the queue to process them, something that Gtk is not doing.

It still seems to me that the XSync is required to avoid the race 
condition, though. The manpage for XSync says that it both flushes the 
output queue and waits until all requests have been received and 
processed by the X server. Actually, that still sounds a little vague to 
me - is the generation of the GraphicsExpose event part of "processing" 
the XCopyArea request, or may it happen later?

When I'm thinking about the input queue, I find it helpful to think of it 
as having two parts. The first part is what the X server has written down 
the socket but is not available to the app. The second part is what is 
available to the app on a read call. Logically, the input queue is the 
concatenation of the two. It seems to me very plausible that 
GraphicsExpose events may be present in the first of these sections, in 
which case the XSync would seem to be necessary.

If I'm right about this, then it seems that there's just no way to scroll 
without at least one roundtrip to the server :(

Raph