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Re: [Qemu-devel] [PATCH] migration: allow clearing migration string para
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters |
Date: |
Fri, 3 Mar 2017 17:08:54 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Fri, Mar 03, 2017 at 06:05:15PM +0100, Markus Armbruster wrote:
> "Daniel P. Berrange" <address@hidden> writes:
>
> > On Fri, Mar 03, 2017 at 03:44:12PM +0100, Markus Armbruster 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".
> >>
> >> 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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- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, (continued)
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, no-reply, 2017/03/01
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, no-reply, 2017/03/01
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Eric Blake, 2017/03/01
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Dr. David Alan Gilbert, 2017/03/14
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Markus Armbruster, 2017/03/15
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Dr. David Alan Gilbert, 2017/03/15
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Markus Armbruster, 2017/03/15
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Dr. David Alan Gilbert, 2017/03/15
- Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Daniel P. Berrange, 2017/03/15