Re: [Xen-devel] [PATCH] libvchan: create xenstore entries in one transaction

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> Sent: 31 October 2018 10:17
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: 'Marek Marczykowski-Górecki' <marmarek@xxxxxxxxxxxxxxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu <wei.liu2@xxxxxxxxxx>; Ian Jackson
> <Ian.Jackson@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] libvchan: create xenstore entries in one
> transaction
> 
> On Wed, Oct 31, 2018 at 10:08:26AM +0000, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf
> > > Of Marek Marczykowski-Górecki
> > > Sent: 30 October 2018 23:49
> > > To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>; Ian Jackson
> <Ian.Jackson@xxxxxxxxxx>;
> > > Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > Subject: [Xen-devel] [PATCH] libvchan: create xenstore entries in one
> > > transaction
> > >
> > > This will prevent race when client waits for server with xs_watch -
> all
> > > entries should appear at once.
> >
> > Watches should be put on the "state" key to avoid this kind of thing.
> That key should always be the last thing modified and hence there should
> never really be the need for xenstore transactions at all.
> 
> AIUI libxenvchan doesn't have a "state" key.

It appears you're right. OMG. How on earth is one and supposed to know the 
state of the other end? This is an awful 'protocol'.

> We can introduce one now
> but we still need to handle old code which doesn't know about the new
> key.
> 

Indeed, but this really should be replaced by a protocol which actually has 
defined state transitions.

  Paul

> I could be wrong about the protocol though.
> 
> Wei.

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