Re: [Qemu-devel] [PATCH 1/2] commit: Add top-node/base-node options

[Eric Blake]

On 08/13/2018 04:35 AM, Markus Armbruster wrote:

> > > Or, here's an idea:
> > >
> > > Keep the name @base and @top, but add a new '*by-node':'bool' parameter,
> > > defaulting to false for now, but perhaps with a deprecation warning that
> > > we'll switch the default to true in one release and delete the parameter
> > > altogether in an even later release. When false, @base and @top are
> > > filenames, as before; when true, @base and @top are node names instead.
> > > Introspectible, nicer names in the long run, and avoids having to detect a
> > > user providing two mutually-exclusive options at once.
> >
> > I don't like options that completely change the semantics of other
> > options, but maybe that's just personal preference.
>
> I happen to share it.

Okay, we'll ditch that idea as a non-starter.

> > Anyway, thinking about the long term for block-commit is useless, the
> > command is just broken (for example, the @device option doesn't make any
> > sense) and will have to be replaced. But libvirt needs something _now_
> > for the -blockdev support, so I decided to add this as a quick hack
> > before we get the proper replacement.
> >
> > I think it makes more sense to create a new blockdev-commit (which
> > would be a name more in line with the other block job commands) and
> > deprecate the old block-commit command as a whole.

Okay, looks like a good plan for the long term, and thus a good 
rationale for the short-term choices. The commit message could call that 
out.


> > Has any progress been made on the plan to support defaults in QAPI, so
> > that we could get rid of the has_* parameters?
>
> No.  It's one of those things that keep getting pushed out by more
> important or urgent stuff.
>
> I expect it to be straightforward, if tedious.

In part, Marc-Andre's work to get conditional compilation in has gotten 
us closer, in that we can have 'name':{'type':'foo','if':'...'} instead 
of 'name':'type', since that dict for conditional compilation is also 
where we would stick in default values.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org