[Libguestfs] [nbdkit PATCH 2/3] server: Expose final thread_model to filter's .get_ready

[Richard W.M. Jones]

On Mon, Aug 10, 2020 at 08:13:31AM -0500, Eric Blake wrote:
> On 8/10/20 8:01 AM, Richard W.M. Jones wrote:
> >On Fri, Aug 07, 2020 at 05:00:52PM -0500, Eric Blake wrote:
> >>The next patch wants to add a filter that will prevent DoS attacks
> >>from a plaintext client; to be successful, the filter must guarantee
> >>that nbdkit did not settle on SERIALIZE_CONNECTIONS.  The easiest way
> >>to solve this is to expose the final thread model to .get_ready, which
> >>is after the point where .config_complete may have altered it, and
> >>before any connections are permitted.
> >>
> >>Signed-off-by: Eric Blake <eblake at redhat.com>
> >>---
> >>  docs/nbdkit-filter.pod          | 9 ++++++++-
> >>  include/nbdkit-filter.h         | 3 ++-
> >>  server/filters.c                | 4 ++--
> >>  filters/extentlist/extentlist.c | 3 ++-
> >>  filters/log/log.c               | 2 +-
> >>  filters/rate/rate.c             | 2 +-
> >>  filters/stats/stats.c           | 2 +-
> >>  tests/test-layers-filter.c      | 2 +-
> >>  8 files changed, 18 insertions(+), 9 deletions(-)
> >
> >This is fine.  Is this something that we would also with to add to
> >plugin->get_ready in V3 API?  If so it would be a good idea to add
> >this to TODO.
> 
> We might as well, so I'll tweak TODO.  But at the moment, I'm having
> a hard time coming up with a justification for why any existing v2
> plugin would need to know this information, so without a client, I'm
> reluctant to introduce a plugin-only nbdkit_final_thread_model()
> just to be deprecated when v3 comes out.

It's certainly hard to imagine why a plugin would worry about things
being more serialized than they requested, but OTOH it does make the
filter and plugin APIs consistent (w.r.t. the informational parameters
they both receive) which is nice.

> Oh, one other oddity: I've been thinking about adding a fifth
> threading model (SERIALIZE_RETIREMENT), where commands can be
> serviced in parallel on a single connection, but replies to the
> client are reordered to match the client's submission order; for
> filters, exposing a new threading model through .get_ready is not a
> problem (all filters are in tree), but for plugins, such an action
> would mean that the plugin may get a thread model selected at
> runtime that was unknown to the plugin at compile time.  If the
> plugin is trying to make decisions on how much internal locking it
> needs to create, those decisions become more awkward when having to
> deal with an unknown thread model (then again, robust code should
> always be written to assume as much parallelism as possible, and
> only optimize for less locking on known models that are more
> serialized).

Sure; are there really clients which have this restriction?

Rich.

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