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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> Eric Blake <eblake@redhat.com> writes:
> 
> > On 5/7/19 4:39 AM, Daniel P. Berrangé wrote:
> >
> >>> JSON is terrible at interoperability, so good luck with that.
> >>>
> >>> If you reduce your order to "the commonly used JSON libraries we know",
> >>> we can talk.
> >> 
> >> I don't particularly want us to rely on semantics of small known set
> >> of JSON libs. I really do want us to do something that is capable of
> >> working with any JSON impl that exists in any programming language.
> >> 
> >> My suggested option 2 & 3 at least would manage that I believe, as
> >> any credible JSON impl will be able to represent 32-bit integers
> >> or strings without loosing data.
> >> 
> >> Option 1 would not cope as some impls can't even cope with
> >> signed 64-bit ints.
> >> 
> >>>>>> 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.
> >
> > Having qemu accept signed ints in place of large unsigned values is easy
> > enough. But you are right that it loses precision when doubles are
> > involved on the receiving end, and we cross the 2^53 barrier.
> >
> >>>
> >>>>>>
> >>>>>>
> >>>>>>   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.
> >
> > Particularly ugly as we turn 1<<55 from:
> >
> > "value":36028797018963968
> >
> > into
> >
> > "value":[8388608,0]
> >
> > and now both qemu and the client end have to agree that an array of two
> > integers is a valid replacement for any larger 64-bit quantity
> > (presumably, we'd always accept the array form even for small integer
> > values, but only produce the array form for large values).  And while it
> > manages just fine for uint64_t values, what rules would you place on
> > int64_t values? That the resulting 2-integer array is combined with the
> > first number as a 2's-complement signed value, and the second being a
> > 32-bit unsigned value?
> 
> There's more than one way to encode integers as a list of 53 bit signed
> integers.  Any of them will do, we just have to specify one.
> 
> >>>>>>
> >>>>>>
> >>>>>>   3. Encode all 64-bit integers as strings
> >>>>>>
> >>>>>>      The application has todo all parsing/formatting client
> >>>>>>      side.
> >>>
> >>> Yet another ugly.
> >
> > But less so than option 2.
> >
> > "value":36028797018963968
> >
> > vs.
> >
> > "value":"36028797018963968"
> >
> > is at least tolerable.
> 
> Yes.
> 
> >>>>>> 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.
> >
> > If we're going to negotiate something, I'd lean towards option 3
> > (anywhere the introspection states that we accept 'int64' or similar, it
> > is also appropriate to send a string value in its place). We'd also have
> > to decide if we want to allow "0xabcd", or strictly insist on 43981,
> > when stringizing an integer.  And while qemu should accept a string or a
> > number on input, we'd still have to decide/document whether it's
> > response to the client capability negotiation is to output a string
> > always, or only for values larger than the 2^53 threshold.
> 
> Picking option 3 is no excuse for complicating matters further.  QMP is
> primarily for machines.  So my first choice would be to keep everything
> decimal.  I could be persuaded to have QEMU parse integers from strings
> with base 0, i.e. leading 0x gets you hex, leading 0 gets you octal.

as I said in an earlier reply, my preference would also be to keep it
very strict and simple; although my preference is to stick to hex.
My suggestion was 0x%x format, always with the leading 0x, always lower
case, allowing but not requiring 0 padding, and never more than 18
characters (including the 0x). Always using the string format.

Dave

> >>>
> >>> There's no such thing.  You mean "we maximize interoperability with
> >>> common implementations of JSON".
> >> 
> >> s/common/any/
> >> 
> >>> 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.
> >
> > JSON requires decimal-only, but I'm okay if we state that when
> > negotiating the alternative representation, that we output hex-only.
> > (JSON5 adds hex support among other things, but it is not an RFC
> > standard, and even fewer libraries exist that parse JSON5 in addition to
> > straight JSON).
> >
> >>>
> >>>
> >>> 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.
> >> 
> >> This is close to the approach libvirt takes with YAJL parser today. YAJL
> >> parses as a int64 and we then ignore its result, and re-parse the string
> >> again in libvirt as uint64. When generating json we format as uint64
> >> in libvirt and ignore YAJLs formatting for int64.
> >> 
> >>> Can we agree to eliminate option 2 from the race?
> >> 
> >> I'm fine with eliminating option 2.
> >
> > Same here.
> 
> Noted.
> 
> >> I guess I'd have a preference for option 3 given that it has better
> >> interoperability
> >
> > Likewise - if we're going to bother with a capability that changes
> > output and allows the input validators to accept more forms, I'd prefer
> > a string form with correct sign over a negative integer that depends on
> > 64-bit 2's-complement arithmetic to intepret correctly.
> 
> Noted.
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK