Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters

[Dr. David Alan Gilbert]

* Markus Armbruster (address@hidden) wrote:
> "Daniel P. Berrange" <address@hidden> writes:
> 
> > On Wed, Mar 01, 2017 at 08:36:03AM -0600, Eric Blake wrote:
> >> On 03/01/2017 06:32 AM, Daniel P. Berrange wrote:
> >> >      }
> >> >      if (params->has_tls_creds) {
> >> >          g_free(s->parameters.tls_creds);
> >> > -        s->parameters.tls_creds = g_strdup(params->tls_creds);
> >> > +        if (*params->tls_creds == '\0') {
> >> > +            s->parameters.tls_creds = NULL;
> >> 
> >> I'm wondering if you should also do s->parameters.has_tls_creds = false
> >> at this point?  The visitors expect that if has_tls_creds is true, then
> >> the string is non-NULL.
> >
> > The fact that s->parameters contains has_* fields is completely ignored
> > by the migration code afaict. IOW the code behaves as if all the has_*
> > fields are hardwired to true in s->parameters, even though that is not
> > the case :-) The has_* fields are only used when the various migration
> > QMP methods are executed, and those all use a separate MigrationParameters
> > struct instance.
> 
> Not keeping the has_ members up-to-date is harmless as long as you don't
> pass the thing to visitors, including the one hiding in qapi_free_FOO().
> That one ignores scalars, though.
> 
> 
> From a more abstract point of view, we have two related data types: one
> for the state, and one for state changes requests.
> 
> In state, members are always present.
> 
> A state change request is a bag of state member change requests, and
> each request can either specify the new value or ask for a reset to
> default.  Absent member means no change.
> 
> We press the same QAPI type into service for both by making all members
> optional.
> 
> For the state case, we hardwire the has_ to true.  Or even ignore them
> completely.
> 
> For the state change request, we use has_ = false for "no change", has_
> = true with a special value for "reset to default" (new in this patch)
> and has_ = true with a non-special value for "set to this value".

I'm confused why we need a 'reset to default' - all we need is the ability
to change each parameter, and for the new value of that parameter
to be an empty string.

> Requires a special value outside the set of non-special values.  The
> obvious one is JSON null, but the QAPI generator doesn't quite support
> that, yet.  "" works here, but is not general.
> 
> I think I can get you null support in 2.10.  Would that work for you?

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK