Current versions of the Linux kernel don't seem to remove the struct
epitem for a given (file,fd) combo when the fd is closed unless the
file itself is also completely closed. This means that if you do:
fd = dup(fd_orig);
add(fd);
close(fd);
dup2(fd_orig, fd);
add(fd);
you will get an EEXIST when you should have gotten a success. This
could cause warnings and dropped events when using dup and epoll.
The solution is pretty simple: when we get an EEXIST from
EPOLL_CTL_ADD, we retry with EPOLL_CTL_MOD.
Unit test included to demonstrate the bug.
Found due to the patient efforts of Gilad Benjamini; diagnosed with
help from Nicholas Marriott.
c281aba30e