Re: [Qemu-devel] [PATCH 4/4] qobject: Output valid JSON for non-finite numbers

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> It's better to give downstream clients a valid JSON string,
> even if they are semantically expecting a number, than it is
> to give them a bare keyword extension that can cause a
> lexical error.

Incompatible change.  If all clients are choking on non-finite numbers,
then the incompatibility is an improvement.  If a client exists that
groks non-finite numbers, ...  Absence is always hard to show.

Moreover, it turns query-qmp-schema into a liar: the schema it returns
claims a certain member of the reply has "type": "number", and then we
go on to send a string anyway.

> Of course, as long as we don't recognize (certain) strings as valid
> numbers during a conversion to QObject,

That would be even crazier!

>                                         this means our extension
> of accepting bare keywords for non-finite numbers cannot undergo
> a round trip (once converted into a string, we never get back to
> a QFloat).  However, non-finite input is rare enough that it's
> not worth bothering with at the moment.
>
> Signed-off-by: Eric Blake <eblake@redhat.com>

I'm afraid the only sane solution is to find all uses of number in QMP
output, audit the code producing them, then assert isfinite() in the
monitor.  For commands without a side effect, we could fail the command
instead of tripping an assertion.  We'd have to declare such commands.

Let's examine the occurences of "number" in output of query-qmp-schema,
or actually in the qmp-introspect.c that gets generated with -u:

* Object q_obj_migrate_set_downtime-arg member value: input

* Builtin number: d'uh!

* Object MigrationStats member mbps: in output of query-migrate

* Object XBZRLECacheStats member overflow: likewise

* Object KeyValue case number: not a type.

* Object BlockDeviceTimedStats members avg_rd_queue_depth,
  avg_wr_queue_depth: in output of query-blockstats

* Enum CommandLineParameterType member: not a type

* Enum JSONType member: not a type

* Enum KeyValueKind: not a type

* Object PciBusInfo member: not a type

So it's just query-migrate and query-blockstats.