[Libguestfs] nbdkit worker thread shutdown

[Richard W.M. Jones]

On Thu, May 05, 2022 at 02:24:12PM +0100, Nikolaus Rath wrote:
> On May 05 2022, "Richard W.M. Jones" <rjones at redhat.com> wrote:
> > On Thu, May 05, 2022 at 08:59:56AM +0100, Nikolaus Rath wrote:
> >> Hello,
> >> 
> >> When nbdkit calls a plugin's unload() method, is it guaranteed that all
> >> pending requests have been handled (and all worker threads exited)?
> >> 
> >> (I would have expected this to be the case, but I'm getting errors from
> >> threads accessing data that my unload() handler frees, so I wanted to
> >> confirm my assumption).
> >
> > .unload is literally called just before dlclose (see
> > server/backend.c:backend_unload) so if a thread in nbdkit was to
> > access something in your plugin after .unload then that would be quite
> > a serious bug in nbdkit.  It would be bound to crash because the code
> > and data has been unloaded from the process.  This implies too that
> > all connections would have been closed already.
> >
> > Normally the server has no view into your plugin except through the
> > pointers in the plugin structure, and it shouldn't be using those
> > after .unload.  Can you give us some clue as to what exactly nbdkit is
> > accessing in your plugin?
> 
> It *looks like* one of the nbdkit-started worker threads is still
> executing the plugin's read() handler at the time that a different
> thread is executing the plugin's unload() handler (background:
> https://github.com/archiecobbs/s3backer/issues/180).
> 
> However, this may not actually be the case. I wanted to confirm that
> this is not expected behavior before diving into this deeply.
> 
> Now that I know that it shouldn't happen (this is what you're saying,
> right?), I'll investigate in more detail and hopefully either find a
> different explanation for the symptoms or provide a proper nbdkit bug
> report.

It shouldn't happen, but I can't discount there's some sort of bug.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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