Re: [Qemu-devel] [PATCH v2 2/6] qapi: support nested structs in OptsVisitor

[Gerd Hoffmann]

On Mi, 2015-06-17 at 09:50 +0200, Markus Armbruster wrote:
> Copying László because his fingerprints are on OptsVisitor.
> 
> "Kővágó, Zoltán" <address@hidden> writes:
> 
> > The current OptsVisitor flattens the whole structure, if there are same 
> > named
> > fields under different paths (like `in' and `out' in `Audiodev'), the 
> > current
> > visitor can't cope with them (for example setting `frequency=44100' will 
> > set the
> > in's frequency to 44100 and leave out's frequency unspecified).
> >
> > This patch fixes it, by the following changes:
> > 1) Specifying just the field name will apply to all fields that has the
> >    specified name (this means it would set both in's and out's frequency to
> >    44100 in the above example).
> > 2) Optionally user can specify the path in the hierarchy. Names are 
> > separated by
> >    a dot (e.g. `in.frequency', `foo.bar.something', etc). The user need not
> >    specify the whole path, only the last few components (i.e. 
> > `bar.something' is
> >    equivalent to `foo.bar.something' if only `foo' has a `bar' field). This 
> > way
> >    1) is just a special case of this when only the last component is 
> > specified.
> > 3) In case of an ambiguity (e.g `frequency=44100,in.frequency=8000') the 
> > longest
> >    matching (the most specific) path wins (so in this example, in's 
> > frequency
> >    would become 8000, because `in.frequency' is more specific that 
> > `frequency',
> >    and out's frequency would become 44100, because only `frequency' matches 
> > it).
> 
> Can you explain why the complexity is needed, i.e. why we can't just
> require full paths always?

Keeping the short names is required for -netdev backward compatibility.

Restricting to short or full (i.e. something= or foo.bar.something=, but
disallow bar.something=) should not be a problem.  I'm not sure this
simplifies things much though.  We have to build the full path anyway,
and I think bar.something= is just a convenient thing we get almost for
free ...

cheers,
  Gerd