Re: [Qemu-devel] QMP; unsigned 64-bit ints; JSON standards compliance

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, May 13, 2019 at 03:53:19PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > On Wed, May 08, 2019 at 02:44:07PM +0200, Markus Armbruster wrote:
> [...]
> >> Double-checking: do you propose to encode *all* numbers as strings, or
> >> just certain "problematic" numbers?
> >> 
> >> If the latter, I guess your idea of "problematic" is "not representable
> >> exactly as double precision floating-point".
> >
> > We have a few options
> >
> >  1. Use string format for values > 2^53-1, int format below that
> >  2. Use string format for all fields which are 64-bit ints whether
> >     signed or unsigned
> >  3. Use string format for all fields which are integers, even 32-bit
> >     ones
> >
> > I would probably suggest option 2. It would make the QEMU impl quite
> > easy IIUC, we we'd just change the QAPI visitor's impl for the int64
> > and uint64 fields to use string format (when the right capability is
> > negotiated by QMP).
> >
> > I include 3 only for completeness - I don't think there's a hugely
> > compelling reason to mess with 32-bit ints.
> 
> Agree.
> 
> > Option 1 is the bare minimum needed to ensure precision, but to me
> > it feels a bit dirty to say a given field will have different encoding
> > depending on the value. If apps need to deal with string encoding, they
> > might as well just use it for all values in a given field.
> 
> I guess that depends on what this interoperability capability does for
> QMP *input*.
> 
> For *output*, QEMU has to encode a number either as JSON number or as
> JSON string
> 
> For *input*, QEMU could accept either.  Or it could accept only the
> encoding it produces on output.
> 
> Got a preference?

IMHO if a mgmt app enables the (hypothetically named) "int64-as-string"
capability, then we should be strict and require string format on both
input & output. If QEMU accepted either, it would silently hide bugs
where the app has mistakenly not used string formatting.


Regards,
Daniel
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