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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> 
> > * Markus Armbruster (armbru@redhat.com) wrote:
> >> "Daniel P. Berrange" <berrange@redhat.com> 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.
> 
> You argue syntax, I'm arguing semantics.
> 
> The command means "set parameter P to value V".  *Except* when V is "",
> it means something else, namely "reset parameter P to its default,
> whatever that may be".
> 
> This is (a) not general, because it won't do for cases where "" may
> occur as value, and (b) ugly.
> 
> Ugliness is the eye of the beholder.  Lack of generality isn't.

No, I'm questioning why it's defined as 'reset parameter P to its default';
why do we need a way to do that?

Dave

> >> 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 / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK