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Re: [Qemu-devel] [PATCH] migration: allow clearing migration string para
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH] migration: allow clearing migration string parameters |
Date: |
Fri, 03 Mar 2017 18:05:15 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) |
"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.
- [Qemu-devel] [PATCH] migration: allow clearing migration string parameters, Daniel P. Berrange, 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, 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