On Tue, May 07, 2019 at 10:47:06AM +0200, Markus Armbruster wrote: > > The Golang JSON parser decodes JSON numbers to float64 by default so > > will have this precision limitation too, though at least they provide > > a backdoor for custom parsing from the original serialized representation. > > > >> > QEMU, and indeed many applications, want to handle 64-bit integers. > >> > The C JSON library impls have traditionally mapped integers to the > >> > data type 'long long int' which gives a min/max of -(2^63) / 2^63-1. > >> > > >> > QEMU however /really/ needs 64-bit unsigned integers, ie a max 2^64-1. > > Correct. > > Support for integers 2^63..2^64-1 is relatively recent: commit > 2bc7cfea095 (v2.10, 2017). > > Since we really needed these, the QObject input visitor silently casts > negative integers to uint64_t. It still does for backward > compatibility. Commit 5923f85fb82 (right after 2bc7cfea095) explains > > The input visitor will cast i64 input to u64 for compatibility > reasons (existing json QMP client already use negative i64 for large > u64, and expect an implicit cast in qemu). > > Note: before the patch, uint64_t values above INT64_MAX are sent over > json QMP as negative values, e.g. UINT64_MAX is sent as -1. After the > patch, they are sent unmodified. Clearly a bug fix, but we have to > consider compatibility issues anyway. libvirt should cope fine, > because its parsing of unsigned integers accepts negative values > modulo 2^64. There's hope that other clients will, too. So QEMU reading stuff sent by libvirt in a back compatible manner is ok. The problem was specifically when a QEMU reply sent back UINT64_MAX value as a positive number. > >> > Libvirt has historically used the YAJL library which uses 'long long int' > >> > and thus can't officially go beyond 2^63-1 values. Fortunately it lets > >> > libvirt get at the raw json string, so libvirt can re-parse the value > >> > to get an 'unsigned long long'. > >> > > >> > We recently tried to switch to Jansson because YAJL has a dead upstream > >> > for many years and countless unanswered bugs & patches. Unfortunately we > >> > forgot about this need for 2^64-1 max, and Jansson also uses 'long long int' > >> > and raises a fatal parse error for unsigned 64-bit values above 2^63-1. It > >> > also provides no backdoor for libvirt todo its own integer parsing. Thus > >> > we had to abort our switch to jansson as it broke parsing QEMU's JSON: > >> > > >> > https://bugzilla.redhat.com/show_bug.cgi?id=1614569 > >> > > >> > Other JSON libraries we've investigated have similar problems. I imagine > >> > the same may well be true of non-C based JOSN impls, though I've not > >> > investigated in any detail. > >> > > >> > Essentially libvirt is stuck with either using the dead YAJL library > >> > forever, or writing its own JSON parser (most likely copying QEMU's > >> > JSON code into libvirt's git). > >> > > >> > This feels like a very unappealing situation to be in as not being > >> > able to use a JSON library of our choice is loosing one of the key > >> > benefits of using a standard data format. > >> > > >> > Thus I'd like to see a solution to this to allow QMP to be reliably > >> > consumed by any JSON library that exists. > > 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. > > >> > > >> > > >> > 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/ > 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. 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. I guess I'd have a preference for option 3 given that it has better interoperability Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
