[Libguestfs] Interesting libnbd/nbdkit bug

[Eric Blake]

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

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