[Xcb] xcb_no_operation

[Yang JianJun]

I intend to get the sequence number of next request in gdk-xcb, because
gdk-x11 use the NextRequest macro. I see that the cookie returned by every
request contains the sequence number of that request. But It is impossible
to find the last request before I want the sequence number in such a
event-driven system. So I use xcb_no_operation.

Instead of NextRequest macro, I just write a XCBNextRequest macro in gdk-xcb
:
    #define XCBNextRequest(c)             ((xcb_no_operation(c)).sequence +
1)

As a result gdk-xcb send the xcb_no_operation_request frequently when window
redraw occurs. Maybe this brings down the gdk-xcb's performance? Should I
avoid using xcb_no_operation?

"This design choice causes XCBNoOperation to be something like a factor of
four times slower than XNoOperation in the current implementation. Note that
NoOperation, a completely useless request, suffers the most...", Jamey said
(How XCB's design affects its performance, solair.livejournal.com). Is it
still right now?

Thanks
Jianjun
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