[Xcb] [RFC PATCHv2] New XCB socket handoff mechanism for Xlib/XCB and other libraries

Colin Guthrie gmane at colin.guthr.ie
Wed Apr 30 16:41:17 PDT 2008

Jamey Sharp wrote:
> On Sat, Mar 15, 2008 at 09:36:37PM -0700, we wrote:
>> Libraries like Xlib/XCB, some XCB language bindings, and potentially
>> others have a common problem: they want to share the X connection with
>> XCB. This requires coordination of request sequence numbers. XCB
>> currently has an Xlib-specific lock, and allows Xlib to block XCB from
>> making requests. The attached patches to XCB replace that lock with a
>> handoff mechanism, xcb_take_socket, allowing external code to ask XCB
>> for permission to take over the write side of the socket and send raw
>> data with xcb_writev. The caller of xcb_take_socket must supply a
>> callback which XCB can call when it wants the write side of the socket
>> back to make a request. This callback synchronizes with the external
>> socket owner, flushes any output queues if appropriate, and then returns
>> the sequence number of the last request sent over the socket.
> After reviewing and testing these patches for the last week, we shook
> out a few small bugs.  The attached patches represent version 2 of the
> handoff implementation, which fixes most of the bugs we're aware of.  As
> before, when applying the Xlib/XCB patches, please make sure you use
> current Xlib from Git or have the patch from Git commit
> 64325f38bab082a8e0e9ce779a8e582de5c8588e applied.
> Changes to the XCB side since version 1 of the handoff implementation:
> * Fix bad invariant asserts in _xcb_in_replies_done: external owners
>   will not have a pending_reply unless they set default flags.
> * Fix missed wakeup bug in get_socket_back by using
>   pthread_cond_broadcast rather than pthread_cond_signal.
> * Use _xcb_in_expect_reply rather than adding a new function
>   _xcb_in_expect_replies that does almost the same thing.
> On the Xlib/XCB side:
> * Fix a race condition in the way XAllocID allocates XIDs.  Previously,
>   if a thread allocated an XID with XAllocID, another thread could run
>   between the subsequent UnlockDisplay and SyncHandle, before the
>   _XIDHandler obtained another XID.  Prevent this by holding the user
>   display lock between XAllocID and the subsequent SyncHandle.
>   This race condition also exists in traditional Xlib, and a similar fix
>   should work there too.
> * Use an XID that's truly invalid as a placeholder before requesting a
>   new XID.  Previously, XID allocation used 0 as a placeholder, but that
>   represents a potentially valid XID.
> - Josh Triplett and Jamey Sharp

Was this ever officially accepted/committed?

I see it on:

but is this planned to be merged to master on the official repos?

I want to update the Mandriva cooker package but as a lot of other
packages link against libxcb-xlib and will require rebuilt, I don't want
to push it out only to find that this wont actually happen and I'll have
to rebuild everything again :s


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