Libguestfs - May 2019 - Interesting libnbd/nbdkit bug

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!

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

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