Re: [Qemu-devel] [PATCH 2/6 for-3.1] nbd: stop waiting for a NBD response with NBD_CMD_DISC

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sat, Nov 17, 2018 at 08:19:10PM -0600, Eric Blake wrote:
> On 11/16/18 9:53 AM, Daniel P. Berrangé wrote:
> > When sending a NBD_CMD_DISC message there is no reply expected,
> > however, the nbd_read_eof() coroutine is still waiting for a reply.
> > In a plain NBD connection this doesn't matter as it will just get an
> > EOF, however, on a TLS connection it will get an interrupted TLS data
> > packet. The nbd_read_eof() will then print an error message on the
> > console due to missing reply to NBD_CMD_DISC.
> > 
> > This can be seen with qemu-img
> > 
> >    $ qemu-img info \
> >        --object tls-creds-x509,dir=tlsdir,id=tls0,endpoint=client \
> >        --image-opts driver=nbd,host=127.0.0.1,port=9000,tls-creds=tls0
> >    qemu-img: Cannot read from TLS channel: Input/output error
> >    image: nbd://127.0.0.1:9000
> >    file format: nbd
> >    virtual size: 10M (10485760 bytes)
> >    disk size: unavailable
> > 
> > Simply setting the 'quit' flag after sending NBD_CMD_DISC is enough to
> > get the coroutine to stop waiting for a reply and thus supress the error
> > message.
> 
> Actually, it's not quite enough - once you actually start performing I/O,
> enough coroutines are kicked off that the error still happens:
> 
> $  qemu-io -c 'r 1m 1m' -c 'w -P 0x22 1m 1m' --image-opts \
> --object tls-creds-x509,dir=scratch/tls/client1,endpoint=client,id=tls0\
>  driver=nbd,host=localhost,port=10809,tls-creds=tls0
> read 1048576/1048576 bytes at offset 1048576
> 1 MiB, 1 ops; 0.0430 sec (23.204 MiB/sec and 23.2040 ops/sec)
> wrote 1048576/1048576 bytes at offset 1048576
> 1 MiB, 1 ops; 0.0152 sec (65.479 MiB/sec and 65.4793 ops/sec)
> Cannot read from TLS channel: Input/output error
> 
> Squashing this in on top of your patch helps, though:
> 
> diff --git i/block/nbd-client.c w/block/nbd-client.c
> index 5f63e4b8f15..e7916c78996 100644
> --- i/block/nbd-client.c
> +++ w/block/nbd-client.c
> @@ -79,7 +79,14 @@ static coroutine_fn void nbd_read_reply_entry(void
> *opaque)
>          assert(s->reply.handle == 0);
>          ret = nbd_receive_reply(s->ioc, &s->reply, &local_err);
>          if (local_err) {
> -            error_report_err(local_err);
> +            /* If we are already quitting, either another error has
> +             * already been reported, or we requested NBD_CMD_DISC and
> +             * don't need to report anything further.  */
> +            if (!s->quit) {
> +                error_report_err(local_err);
> +            } else {
> +                error_free(local_err);
> +            }
>          }
>          if (ret <= 0) {
>              break;
> 
> But I want to do more testing to make sure I'm not missing out on reporting
> an actual error if I add that.

It occurred to me that it would be nice if the TLS channel behaved the same
a normal channel when seeing EOF on the socket. Unfortunately the reason why
GNUTLS returns an error in this case does in fact make sense, so it isn't
quite as simple as just ignoring the error. Fortunately the NBD client has
decided it wants to shutdown and crucially has called qio_channel_shutdown()
at this point. Thus we can safely ignore the gnutls error code when the
channel has been shutdown for read and just return 0 for EOF as with a plain
socket.

Thus I've sent a patch which solves the problem in that way:

  https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg03616.html

With this applied, both my original patch and your extra chunk here
should be redundant.

Regards,
Daniel
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