Regular files are always write-ready, so non-blocking I/O does not
give any benefits for them.
More than that - if libevent is using "epoll" to track fd events,
epoll_ctl will refuse attempt to add an fd pointing to a regular
file descriptor with EPERM.
This fix checks the object referenced by fd and avoids event_add
using event_active instead.
In the original configuration that uncovered this issue "epoll"
was used in libevent, it was triggering the following warning
message:
"[warn] Epoll ADD(1) on fd 0 failed. Old events were 0; read
change was 1 (add); write change was 0 (none): Operation not
permitted"
And the side effect was accumulation of all output in mpirun
memory and actually writing it only at mpirun exit.
Signed-off-by: Artem Polyakov <artpol84@gmail.com>
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