[Xcb] How to Debug a missing xcb reply message?

[Bertrand Wind]

We have a UI application hanging on an xcb_wait_for_reply message with a
backtrace:

the backtrace starts like this:
/home/views/qt/lib/liblfpCore.so.1(_Z13dumpBacktraceP10sigcontexti+0x30)[0x7faec6c0da10]
[0x623c00]
/lib64/libpthread.so.0(pthread_cond_wait+0xbd)[0x31e200c02d]
/usr/lib64/libxcb.so.1[0x363ba0a749]
/usr/lib64/libxcb.so.1[0x363ba0b9ff]
/usr/lib64/libxcb.so.1(xcb_wait_for_reply+0x62)[0x363ba0bb02]
/usr/lib64/libX11.so.6(_XReply+0x107)[0x31e1040c57]
/usr/lib64/libGL.so.1(+0x44f53)[0x7faec30d5f53]
/usr/lib64/libGL.so.1(+0x4270f)[0x7faec30d370f]
/usr/lib64/dri/i965_dri.so(+0x324510)[0x7faeaf293510]
/usr/lib64/dri/i965_dri.so(+0x3248cd)[0x7faeaf2938cd]
/usr/lib64/dri/i965_dri.so(+0x318ff7)[0x7faeaf287ff7

What's the best way to debug xcb transactions? How to find from which
thread the reply should be issued? Any debug flag to activate to track
these transactions?

Thanks
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