libX11: Changes to 'handoff'

[Jamey Sharp]

New branch 'handoff' available with the following commits:
commit d4e77ee5023cda7cb04afdce9be81eaf72785c2a
Author: Jamey Sharp <jamey at minilop.net>
Date:   Sun Mar 23 16:33:50 2008 -0700

    Fix XAllocID race: hold the user display lock until we have a new XID.
    
    Xlib built --without-xcb is also vulnerable to this race, and a similar
    fix might work there too.
    
    Also, use an XID that's truly invalid while waiting for the next XID to be
    requested.

commit 50d5c8363da0cd10217d4f24e8561ed9a1e1881e
Author: Josh Triplett <josh at freedesktop.org>
Date:   Sat Mar 15 17:22:23 2008 -0700

    Use XCB's new socket handoff mechanism rather than the old XCB Xlib lock.
    
    Previously, Xlib/XCB used XCB's Xlib lock to prevent XCB from sending
    requests between calls to Xlib's LockDisplay and UnlockDisplay macros.
    Xlib/XCB then sent all of its requests using XCB's xcb_send_request, and
    had to flush its requests when unlocking the display.
    
    XCB 1.2 adds a new socket handoff mechanism, xcb_take_socket.  Replace
    much of the existing Xlib/XCB implementation with the use of
    xcb_take_socket to take ownership of the write side of the X connection
    socket, and a return_socket callback which writes any outstanding requests
    with xcb_writev.  This approach allows Xlib/XCB to use the same buffering
    as traditional Xlib did.  In particular, programs which use Xlib/XCB and
    never make XCB calls will never need to hand the socket back to XCB, and
    vice versa.
    
    This allows us to discard large quantities of synchronization code from
    Xlib/XCB, together with the synchronization bugs present in that code.
    Several test cases which previously failed now work perfectly, including
    multi-threaded ico.  In addition, the infamous locking correctness
    assertions, triggered when double-locking or when unlocking without a
    previous lock, no longer exist, because Xlib/XCB no longer has any reason
    to care more about application locking than traditional Xlib does.
    
    Furthermore, the handoff approach provides great improvements to
    performance.  Results from x11perf's XNoOp test, which represented the
    worst case for the lock-based Xlib/XCB:
    
    Traditional Xlib:       average 19100000/sec
    Lock-based Xlib/XCB:    average  3350000/sec
    Handoff-based Xlib/XCB: average 17400000/sec
    
    Thus, for no-ops, the handoff mechanism provides more than a 4x speedup to
    Xlib/XCB, bringing Xlib/XCB within 9% of traditional Xlib no-op
    performance.  Of course, real-world workloads do not use no-op, so your
    mileage may vary.  In particular, since no-ops represent the worst case,
    we expect real workloads to more closely match the performance of
    traditional Xlib.
    
    While removing synchronization code, we changed _XReply to not drop
    any locks when calling xcb_wait_for_reply; previously, we had to carefully
    avoid a deadlock between the Display lock and the XCB Xlib lock. Holding
    the locks reduces implementation complexity and should not impact applications.
    
    Commit by Jamey Sharp and Josh Triplett.
    XCB's handoff mechanism inspired by Keith Packard.