Libguestfs - Aug 2018 - Re: [PATCH 4/4] mltools: JSON: unify JSON_parser type with JSON.json_t.

On Monday, 20 August 2018 18:02:06 CEST Richard W.M. Jones
wrote:
> -  } else
> -    rv = Val_none;
> +  }
> +  else {
> +    /* Previously we had a special JSON_parser_null value we could
> +     * use here, making the returned type (sort of) an option.
> +     * This is a best effort which is better than crashing /
> +     * throwing an error.
> +     */
> +    rv = caml_alloc (1, JSON_STRING_TAG);
> +    v = caml_copy_string ("");
> +    Store_field (rv, 0, v);
> +  }
NACK, this is not correct.  null is a proper type of value in JSON, and
thus JSON.json_t must represent it as well.

This is even used in other parts, for example the check of backing
files of disks (see guestfs_impl_disk_has_backing_file).

-- 
Pino Toscano

On Wed, Aug 22, 2018 at 01:20:55PM +0200, Pino Toscano
wrote:
> On Monday, 20 August 2018 18:02:06 CEST Richard W.M. Jones wrote:
> > -  } else
> > -    rv = Val_none;
> > +  }
> > +  else {
> > +    /* Previously we had a special JSON_parser_null value we could
> > +     * use here, making the returned type (sort of) an option.
> > +     * This is a best effort which is better than crashing /
> > +     * throwing an error.
> > +     */
> > +    rv = caml_alloc (1, JSON_STRING_TAG);
> > +    v = caml_copy_string ("");
> > +    Store_field (rv, 0, v);
> > +  }
> 
> NACK, this is not correct.  null is a proper type of value in JSON, and
> thus JSON.json_t must represent it as well.
> 
> This is even used in other parts, for example the check of backing
> files of disks (see guestfs_impl_disk_has_backing_file).
Can you explain more about what a "null" JSON document looks like?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
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On Wed, Aug 22, 2018 at 03:18:31PM +0100, Richard W.M. Jones
wrote:
> On Wed, Aug 22, 2018 at 01:20:55PM +0200, Pino Toscano wrote:
> > On Monday, 20 August 2018 18:02:06 CEST Richard W.M. Jones wrote:
> > > -  } else
> > > -    rv = Val_none;
> > > +  }
> > > +  else {
> > > +    /* Previously we had a special JSON_parser_null value we
could
> > > +     * use here, making the returned type (sort of) an option.
> > > +     * This is a best effort which is better than crashing /
> > > +     * throwing an error.
> > > +     */
> > > +    rv = caml_alloc (1, JSON_STRING_TAG);
> > > +    v = caml_copy_string ("");
> > > +    Store_field (rv, 0, v);
> > > +  }
> > 
> > NACK, this is not correct.  null is a proper type of value in JSON,
and
> > thus JSON.json_t must represent it as well.
> > 
> > This is even used in other parts, for example the check of backing
> > files of disks (see guestfs_impl_disk_has_backing_file).
> 
> Can you explain more about what a "null" JSON document looks
like?
Oh I see, it's actually a special value.

I think I can add this.  I've pushed patches 1-3.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

On 08/22/2018 09:18 AM, Richard W.M. Jones wrote:
>> NACK, this is not correct.  null is a proper type of value in JSON, and
>> thus JSON.json_t must represent it as well.
>>
>> This is even used in other parts, for example the check of backing
>> files of disks (see guestfs_impl_disk_has_backing_file).
> 
> Can you explain more about what a "null" JSON document looks
like?
An example: qemu will accept the following substring, as part of 
BlockdevOptionsGenricCOWFormat, when specifically asking qemu to ignore 
the backing information present in a file and instead open the file 
without any backing:

{ "backing": null }

That is, JSON has "null", "true", and "false" as
its three bare
literals, which hold the same semantic weight as literal numbers, 
strings, [] arrays, and {} objects.

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