[Xcb] Calling XAllocID() twice causes assertion `ret != inval_id' failed

[Matthieu Herrb]

Hi,

I'm trying to help someone understanding why an application using
the old XView (from OpenWindows) toolkit is crashing with the infamous)

xcb_io.c:378: _XAllocID: Assertion `ret != inval_id' failed.

He figured out that this happens because xview sometimes calls XAllocID() 
twice in a row.

Indeed the program below reproduces the problem. Looking at libX11
source code, it's obvious why this happens. Since XView is inherently
not thread-safe, there is no multi-threading involved and
XInitThreads() is never called, so dpy->lock is NULL and so _XIDHandler() 
is never called by XLockDisplay(), thus the xcb->next_xid field 
stays 'invalid_id'. 

I think this is a bug, and could be also one cause of 
https://bugs.freedesktop.org/show_bug.cgi?id=27552

Thoughts?

#include <stdio.h>
#include <X11/Xlib.h>

int main(int argc, char** args) {

	Display * dpy;

	XInitThreads();
	dpy = XOpenDisplay(NULL);
	printf("Display = %08x\n", dpy);
	int i;
	for (i = 0; i < 100; i++) {
		XLockDisplay(dpy);
		int xid = XAllocID(dpy);
		printf("xid = %08x\n", xid);
		XUnlockDisplay(dpy);
		XFlush(dpy);
	}
}

-- 
Matthieu Herrb