Re: [PATCH v2] nbd/server: Suppress Broken pipe errors on abrupt disconnection

["Richard W.M. Jones" <rjones@redhat.com>]

On Wed, Sep 15, 2021 at 10:15:20AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 14.09.2021 19:32, Richard W.M. Jones wrote:
> >On Tue, Sep 14, 2021 at 06:21:58PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> >>14.09.2021 17:52, Richard W.M. Jones wrote:
> >>>On the
> >>>server side when the server receives NBD_CMD_DISC it must complete any
> >>>in-flight requests, but there's no requirement for the server to
> >>>commit anything to disk.  IOW you can still lose data even though you
> >>>took the time to disconnect.
> >>>
> >>>So I don't think there's any reason for libnbd to always gracefully
> >>
> >>Hmm. Me go to NBD spec :)
> >>
> >>I think, there is a reason:
> >>
> >>"The client and the server MUST NOT initiate any form of disconnect other than in one of the above circumstances."
> >>
> >>And the only possibility for client to initiate a hard disconnect listed above is "if it detects a violation by the other party of a mandatory condition within this document".
> >>
> >>So at least, nbdsh violates NBD protocol. May be spec should be updated to satisfy your needs.
> >
> >I would say the spec is at best contradictory, but if you read other
> >parts of the spec, then it's pretty clear that we're allowed to drop
> >the connection whenever we like.  This section says as much:
> >
> >https://github.com/NetworkBlockDevice/nbd/blob/5805b25ad3da96e7c0b3160cda51ea19eb518d5b/doc/proto.md#terminating-the-transmission-phase
> 
> Hmm, that was exactly the section I read and quoted :)
> 
> >
> >   There are two methods of terminating the transmission phase:
> >   ...
> >   "The client or the server drops the TCP session (in which case it
> >   SHOULD shut down the TLS session first). This is referred to as
> >   'initiating a hard disconnect'."
> 
> Right. Here the method is defined, no word that client can do it at any time.

I don't read this as a definition.

But we should probably clarify the spec to note that dropping the
connection is fine, because it is - no one has made any argument so
far that there's an actual reason to waste time on NBD_CMD_DISC.

Rich.

> Next, in same section:
> 
>    Either side MAY initiate a hard disconnect if it detects a violation by the other party of a mandatory condition within this document.
> 
> Next
>    The client MAY issue a soft disconnect at any time
> 
> And finally:
> 
>    The client and the server MUST NOT initiate any form of disconnect other than in one of the above circumstances.
> 
> 
> Or do you mean some other spec section I missed?
> 
> >
> >Anyway we're dropping the TCP connection because sometimes we are just
> >interrogating an NBD server eg to find out what it supports, and doing
> >a graceful shutdown is a waste of time and internet.
> >
> >>>shut down (especially in this case where there are no in-flight
> >>>requests), and anyway it would break ABI to make that change and slow
> >>>down the client in cases when there's nothing to clean up.
> >>
> >>Which ABI will it break?
> >
> >Our contract with callers using nbd_close(3), if nbd_shutdown(3) is
> >not called beforehand.
> >https://libguestfs.org/nbd_shutdown.3.html
> >https://libguestfs.org/nbd_create.3.html (really nbd_close ...)
> >
> >Rich.
> >
> 
> 
> -- 
> Best regards,
> Vladimir

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/