[Libguestfs] nbdkit worker thread shutdown

[Daniel P. Berrangé]

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.

Perhaps try running the program under GDB and capture the output
from 'thread apply all backtrace' when the assert fires. It should
give some insight into what threads are doing

With regards,
Daniel
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