[libvirt] [Qemu-devel] QMP; unsigned 64-bit ints; JSON standards compliance
Dr. David Alan Gilbert
dgilbert at redhat.com
Tue May 14 09:37:55 UTC 2019
* Daniel P. Berrangé (berrange at redhat.com) wrote:
> On Tue, May 14, 2019 at 08:02:49AM +0200, Markus Armbruster wrote:
> > Eric Blake <eblake at redhat.com> writes:
> >
> > > On 5/13/19 8:53 AM, Markus Armbruster wrote:
> > >
> > >>> 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.
> > >
> > > Other than if we ever change the type of a QMP integer. Right now, if we
> > > widen from 'int32' to 'int' (aka 'int64'), it is invisible to clients;
> > > but once we start stringizing 64-bit numbers (at client request) but NOT
> > > 32-bit numbers, then changing a type from 32 to 64 bits (or the
> > > converse) becomes an API change to clients. Introspection will at least
> > > let a client know which form to expect, but it does mean we have to be
> > > more aware of typing issues going forward.
> >
> > Thank you so much for helping my old synapses finally fire! Option 2 is
> > not what we thought it is. Let me explain.
> >
> > Introspection reports *all* QAPI integer types as "int". This is
> > deliberate.
> >
> > So, when the client that negotiated the interoperability capability sees
> > "int", it has to accept *both* integer encodings: JSON number and JSON
> > string.
> >
> > The difference between option 1 and option 2 for the client is that
> > option 2 will use only one encoding. But the client must not rely on
> > that! Another QEMU version may well use the other encoding (because we
> > narrowed or widened the QAPI integer type in the QAPI schema).
> >
> > Elsewhere in this thread, David pointed out that option 1 complicates
> > testing QEMU: full coverage requires passing both a small number (to
> > cover JSON number encoding) and a large number (to cover JSON string
> > encoding), to which I replied that there are very few places to test.
> >
> > Option 2 complicates testing clients: full coverage requires testing
> > with both a version of QEMU (or a mock-up) that uses wide integers
> > (encoded as JSON string) and narrow integers (encoded as JSON number).
> > Impractical.
> >
> > >>> 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*.
> > >
> > > "Be liberal in what you accept, strict in what you produce" - that
> > > argues we should accept both forms on input (it's easy enough to ALWAYS
> > > permit a string in place of an integer, and to take an in-range integer
> > > even when we would in turn output it as a string).
> >
> > With option 2, QEMU *has* to be liberal in what it accepts, because the
> > client cannot deduce from introspection whether the integer is wide or
> > narrow.
> >
> > [...]
> >
> > Daniel, you wrote you'd probably suggest option 2. Would you like to
> > reconsider?
>
> Based on the above, let me try & summarize what we need behaviour to be:
>
> - Integer mode (current default):
>
> - QEMU & clients MUST format integer fields as numbers
> regardless of size
>
> - QEMU & clients MUST parse number format for any integer
> fields
>
> - String mode:
>
> - QEMU & clients MUST format integer fields as strings
> if their value can not fit in a 32-bit integer.
>
> - QEMU & clients MAY format integer fields as strings
> even if their value can fit in 32-bit integer
>
> - QEMU & client MUST parse both string and number format
> for any integer fields.
>
> Unless I'm missing something, this should ensure we don't loose precision,
> can always parse large numbers, and can internally change QEMU precision
> from int8/16/32 upto full int64 without breaking clients.
But we could be stricter and simpler in string mode:
- QEMU & clients MUST format integer fields as strings, always
- QEMU & clients MUST parse only strings for integer fields.
That's (3) above, but also meets your requirements.
Dave
> Regards,
> Daniel
> --
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--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
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