I've rewritten my patch to lock the history with fcntl, and added
an option. It works well with my settings (INC_APPEND_HISTORY), but
I haven't tested it with other settings. I no longer get the error

zsh: failed to write history file /home/vlefevre/.zhistory: bad file descriptor

-- 
Vincent Lefèvre <vincent@vinc17.org> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)