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

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

On Fri, Mar 03, 2017 at 06:05:15PM +0100, Markus Armbruster wrote:
> "Daniel P. Berrange" <berrange@redhat.com> writes:
> 
> > On Fri, Mar 03, 2017 at 03:44:12PM +0100, Markus Armbruster 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".
> >> 
> >> 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?
> >
> > Not really - we need this fix so libvirt can enable TLS migration,
> > in addition we'll be wanting to add it to stable branches upstream
> > and downstream.
> 
> Alternatives:
> 
> (a) We support null in 2.9, and use it to solve this problem.
> 
> (b) We use "", support null in 2.10, immediately deprecate "" in favor
>     of null.
> 
> PRO (a): Less churn, less variation, less overall work.
>
> PRO (b): Might be easier to backport to really old versions.  Soft
> freeze.
> 
> Non-alternative: use "" forever.  Sorry, I hate it.

I don't see a problem with accepting both "" & nil forever, as there's
no reason to need to distinguish them here. "" is never a valid ID for
a qobject, and "" is never a valid hostname, so treating "" as a synonym
for NULL/nil is fine.

Regards,
Daniel
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