Re: [Libguestfs] Interesting libnbd/nbdkit bug
On 5/25/19 1:10 PM, Richard W.M. Jones wrote:
Try doing:
$ nbdsh
nbd> h.connect_command (["nbdkit", "-s", "null"])
At this point you may observe your laptop fan starts to spin and
nbdkit is consuming 100% of CPU. In all other respects everything
works fine, you can send commands etc.
Anyway I tracked the issue down. nbdkit sits in a loop continuously
reading stdin, each read(2) call returning EAGAIN.
The reason for that is because libnbd opens a socketpair with the
SOCK_NONBLOCK option and passes one half directly to the forked
subprocess.
It's obviously a bug, but I'm not sure if libnbd should be unsetting
the SOCK_NONBLOCK option or if nbdkit should be doing it. Maybe both!
Both wouldn't hurt for robustness, but it is more common to fix the
process sending in the non-blocking fd to an unsuspecting child than to
write child processes designed to be robust against parents that forgot.
So I see that you followed precedent and posted a patch for libnbd first :)
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
Attachments:
- signature.asc (application/pgp-signature — 488 bytes)