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

[Markus Armbruster <armbru@redhat.com>]

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.

>>>
>>> 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.