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

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

On Mon, May 13, 2019 at 01:29:34PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > On Wed, May 08, 2019 at 02:44:07PM +0200, Markus Armbruster wrote:
> > > Daniel P. Berrangé <berrange@redhat.com> writes:
> > > 
> > > > On Tue, May 07, 2019 at 10:47:06AM +0200, Markus Armbruster wrote:
> > > >
> > > >> >> > I can think of some options:
> > > >> >> > 
> > > >> >> >   1. Encode unsigned 64-bit integers as signed 64-bit integers.
> > > >> >> > 
> > > >> >> >      This follows the example that most C libraries map JSON ints
> > > >> >> >      to 'long long int'. This is still relying on undefined
> > > >> >> >      behaviour as apps don't need to support > 2^53-1.
> > > >> >> > 
> > > >> >> >      Apps would need to cast back to 'unsigned long long' for
> > > >> >> >      those QMP fields they know are supposed to be unsigned.
> > > >> 
> > > >> Ugly.  It's also what we did until v2.10, August 2017.  QMP's input
> > > >> direction still does it, for backward compatibility.
> > > >> 
> > > >> >> > 
> > > >> >> > 
> > > >> >> >   2. Encode all 64-bit integers as a pair of 32-bit integers.
> > > >> >> >     
> > > >> >> >      This is fully compliant with the JSON spec as each half
> > > >> >> >      is fully within the declared limits. App has to split or
> > > >> >> >      assemble the 2 pieces from/to a signed/unsigned 64-bit
> > > >> >> >      int as needed.
> > > >> 
> > > >> Differently ugly.
> > > >> 
> > > >> >> > 
> > > >> >> > 
> > > >> >> >   3. Encode all 64-bit integers as strings
> > > >> >> > 
> > > >> >> >      The application has todo all parsing/formatting client
> > > >> >> >      side.
> > > >> 
> > > >> Yet another ugly.
> > > >> 
> > > >> >> > 
> > > >> >> > 
> > > >> >> > None of these changes are backwards compatible, so I doubt we could make
> > > >> >> > the change transparently in QMP.  Instead we would have to have a
> > > >> >> > QMP greeting message capability where the client can request enablement
> > > >> >> > of the enhanced integer handling.
> > > >> 
> > > >> We might be able to do option 1 without capability negotiation.  v2.10's
> > > >> change from option 1 to what we have now produced zero complaints.
> > > >> 
> > > >> On the other hand, we made that change for a reason, so we may want a
> > > >> "send large integers as negative integers" capability regardless.
> > > >> 
> > > >> >> > 
> > > >> >> > Any of the three options above would likely work for libvirt, but I
> > > >> >> > would have a slight preference for either 2 or 3, so that we become
> > > >> >> > 100% standards compliant.
> > > >> 
> > > >> There's no such thing.  You mean "we maximize interoperability with
> > > >> common implementations of JSON".
> > > >
> > > > s/common/any/
> > > 
> > > info: error correction applied, future applications will be silent ;-P
> > > 
> > > >> Let's talk implementation for a bit.
> > > >> 
> > > >> Encoding and decoding integers in funny ways should be fairly easy in
> > > >> the QObject visitors.  The generated QMP marshallers all use them.
> > > >> Trouble is a few commands still bypass the generated marshallers, and
> > > >> mess with the QObject themselves:
> > > >> 
> > > >> * query-qmp-schema: minor hack explained in qmp_query_qmp_schema()'s
> > > >>   comment.  Should be harmless.
> > > >> 
> > > >> * netdev_add: not QAPIfied.  Eric's patches to QAPIfy it got stuck
> > > >>   because they reject some abuses like passing numbers and bools as
> > > >>   strings.
> > > >> 
> > > >> * device_add: not QAPIfied.  We're not sure QAPIfication is feasible.
> > > >> 
> > > >> netdev_add and device_add both use qemu_opts_from_qdict().  Perhaps we
> > > >> could hack that to mirror what the QObject visitor do.
> > > >> 
> > > >> Else, we might have to do it in the JSON parser.  Should be possible,
> > > >> but I'd rather not.
> > > >> 
> > > >> >> My preference would be 3 with the strings defined as being
> > > >> >> %x lower case hex formated with a 0x prefix and no longer than 18 characters
> > > >> >> ("0x" + 16 nybbles). Zero padding allowed but not required.
> > > >> >> It's readable and unambiguous when dealing with addresses; I don't want
> > > >> >> to have to start decoding (2) by hand when debugging.
> > > >> >
> > > >> > Yep, that's a good point about readability.
> > > >> 
> > > >> QMP sending all integers in decimal is inconvenient for some values,
> > > >> such as addresses.  QMP sending all (large) integers in hexadecimal
> > > >> would be inconvenient for other values.
> > > >> 
> > > >> Let's keep it simple & stupid.  If you want sophistication, JSON is the
> > > >> wrong choice.
> > > >> 
> > > >> 
> > > >> Option 1 feels simplest.
> > > >
> > > > But will still fail with any JSON impl that uses double precision floating
> > > > point for integers as it will loose precision.
> > > >
> > > >> Option 2 feels ugliest.  Less simple, more interoperable than option 1.
> > > >
> > > > If we assume any JSON impl can do 32-bit integers without loss of
> > > > precision, then I think we can say it is guaranteed portable, but
> > > > it is certainly horrible / ugly.
> > > >
> > > >> Option 3 is like option 2, just not quite as ugly.
> > > >
> > > > I think option 3 can be guaranteed to be loss-less with /any/ JSON impl
> > > > that exists, since you're delegating all string -> int conversion to
> > > > the application code taking the JSON parser/formatter out of the equation.
> > > 
> > > 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.
> 
> What about when the size is architecture dependent?

The QAPI visitor for 'int' uses an 'int64_t' parameters, so I think
that will want to be string encoded, as if it was a 64-bit int, even
if built on a 32-bit platform.

> 
> > 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.
> 
> Yeh, 1 is horrid; it's too easy to miss a case which forgot to handle
>  the 2^53-1 because we hadn't forced a large value down that check.

Regards,
Daniel
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