As discussed on the XCB mailing list around December 2005, we need to provide a thread-safe API for retrieving X protocol errors from XCB and correlating them with particular requests. See the discussions on the list for how we decided to do this.
XCBSendRequest now provides the extension interface needed for this; but protocol-layer API is needed to expose it to applications. Somebody needs to hack the XSLT up to do this.
Plan 7 was fully implemented as of http://gitweb.freedesktop.org/?p=xcb.git;a=commit;h=091b4c805aa454277aa6b98bee8e8ec0646a9604 .
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