Re: [PATCH 3/5] block/nbd.c: Add yank feature

[Daniel P . Berrangé]

On Fri, May 15, 2020 at 11:48:18AM +0200, Lukas Straub wrote:
> On Tue, 12 May 2020 09:54:58 +0100
> Daniel P. Berrangé <address@hidden> wrote:
> 
> > On Mon, May 11, 2020 at 07:05:24PM +0200, Lukas Straub wrote:
> > > On Mon, 11 May 2020 17:19:09 +0100
> > > "Dr. David Alan Gilbert" <address@hidden> wrote:
> > >   
> > > > * Lukas Straub (address@hidden) wrote:  
> > > > > Add yank option, pass it to the socket-channel and register a yank
> > > > > function which sets s->state = NBD_CLIENT_QUIT. This is the same
> > > > > behaviour as if an error occured.
> > > > > 
> > > > > Signed-off-by: Lukas Straub <address@hidden>    
> > > >   
> > > > > +static void nbd_yank(void *opaque)
> > > > > +{
> > > > > +    BlockDriverState *bs = opaque;
> > > > > +    BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
> > > > > +
> > > > > +    atomic_set(&s->state, NBD_CLIENT_QUIT);    
> > > > 
> > > > I think I was expecting a shutdown on the socket here - why doesn't it
> > > > have one?  
> > > 
> > > For nbd, we register two yank functions: This one and we enable
> > > the yank feature on the qio channel (see function
> > > nbd_establish_connection below).  
> > 
> > As mentioned on the earlier patch, I don't want to see any yank
> > code in the QIOChannel object directly. This nbd_yank function
> > can simply call the qio_channel_shutdown() function directly
> > and avoid need for modifying the QIOChannel object with yank
> > support.
> 
> Hi,
> Looking at it again, the problem is not with registering the yank functions, 
> but with tracking the lifetime of it. Suppose we add qio_channel_shutdown to 
> the yank_nbd function. Then we need to unregister it whenever the QIOChannel 
> object is freed.
> 
> In the code that would lead to the following constructs in a lot of places:
>      if (local_err) {
>          yank_unregister_function(s->yank_name, yank_nbd, bs);
>          object_unref(OBJECT(sioc));
>          error_propagate(errp, local_err);
>          return NULL;
>      }

The nbd patch here already has a yank_unregister_function() so I'm
not seeing anything changes in that respect. The "yank_nbd" function
should check that the I/O channel is non-NULL before calling the
qio_channel_shutdown method.

> If you don't want the code in QIOChannel I guess I can create a
> subclass (YankableChannelSocket?) of QIOChannelSocket. What do
> you think?

That's no better, and I don't see any compelling need for it as calling
yank_unregister_function is something nbd already does in its nbd_close
function. It isn't a burden for the other backends to do similarly.



Regards,
Daniel
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