Libguestfs - Mar 2020 - nbdkit Assertion `h->can_flush == 1' failed on Arch

https://aur.archlinux.org/packages/nbdkit/#comment-733981 reports that
tests-parallel-nbd.sh hangs.  Looking at the log file from the test
reveals:

nbdkit: backend.c:523: backend_flush: Assertion `h->can_flush == 1'
failed.

I've attached the full log.  If you want to see other logs from the
run (probably not relevant) then download
https://svenne.dk/nbdkit-20200315/nbdkit-logs-20200315.tar.gz

I believe the assertion comes from calling flush when can_flush
previously returned false, but also I don't quite understand why the
server is doing this.

However I think the wider problem is that the reporter is using the
non-libnbd "standalone" nbd plugin.  I basically never test this any
longer.  I wonder if we should think about either having a way to
routinely test it despite libnbd being installed, or simply dropping
it?

I will ask the reporter if they can try installing libnbd.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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On 3/16/20 3:39 AM, Richard W.M. Jones wrote:
> 
> https://aur.archlinux.org/packages/nbdkit/#comment-733981 reports that
> tests-parallel-nbd.sh hangs.  Looking at the log file from the test
> reveals:
> 
> nbdkit: backend.c:523: backend_flush: Assertion `h->can_flush == 1'
failed.
I can confirm the hang.
> 
> I've attached the full log.  If you want to see other logs from the
> run (probably not relevant) then download
> https://svenne.dk/nbdkit-20200315/nbdkit-logs-20200315.tar.gz
> 
> I believe the assertion comes from calling flush when can_flush
> previously returned false, but also I don't quite understand why the
> server is doing this.
> 
> However I think the wider problem is that the reporter is using the
> non-libnbd "standalone" nbd plugin.  I basically never test this
any
> longer.  I wonder if we should think about either having a way to
> routinely test it despite libnbd being installed, or simply dropping
> it?
My plan is to patch the bug, backport the patch to the stable branches, 
and then remove the file for the next major release.  libnbd is now 
stable enough that I'm comfortable enough having nbdkit 1.20 depend on 
libnbd (and only the stable releases, like 1.18.2, will still have the 
option of building a limited nbd plugin without libnbd).
> 
> I will ask the reporter if they can try installing libnbd.
> 
> Rich.
> 
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org