It'd be useful to enable rtldebug if you don't have it already. There'd probably also be some useful debug printout if you uncomment the #define POLL_DEBUG line in backend.cmod. Lastly, stack frames got to be missing from that backtrace, so please don't use -fomit-frame-pointer or whatever optimizations you have that cause that.
Ok, I'll do that this week.
This bug aside, the Shuffler module really ought to move away from the old compat callback interface and use fd_callback_box directly. Didn't Stephen fix that recently?
I don't know but I looked in the 7.7 code of Shuffle because the autodoc of the code was much better here so maybe one should backport the doc.
I'm also thinking, is it possible to put both the doc for stable and development branch on the website?
/ David