Re: Does Libvirt's json parser support single quoted string in qmp json string?

[Eric Blake <eblake@redhat.com>]

[adding Markus]

On 2/3/20 4:13 AM, Daniel P. Berrangé wrote:
> On Fri, Jan 31, 2020 at 06:44:42AM -0600, Eric Blake wrote:
>> On 1/31/20 4:38 AM, Peter Luo wrote:
>>
>>> error: internal error: cannot parse json {"execute": "block-commit", "arguments": { "device": "drive-virtio-disk2", "job-id": "job100", "base":'json:{"encrypt.key-secret":"vol-38973xjl.secret","driver":"qcow2","file":{"driver":"file","filename":"/pitrix/data/container/vol-38973xjl.img"}}', "top": "/pitrix/data/container/vol-38973xjl_ss-2tw7v0mm.img"}}: lexical error: invalid char in json text.
>>>
>>>             , "job-id": "job100", "base":'json:{"encrypt.key-secret":"vo
>>>
>>>                        (right here) ------^
>>>
>>
>> qemu's QMP language has an extension where it accepts 'string' in addition
>> to the JSON "string".  But it uses a hand-rolled parser, so it can do
>> whatever it wants.
> 
> Can we deprecate & remove this extension in QEMU ?

We could start a deprecation clock, if desired, but I don't know how 
many external users would be impacted (at least qemu's testsuite heavily 
relies on the extension of single quotes).

Are there any third-party libraries that parse JSON5? 
https://json5.org/ documents that one of the nice features of JSON5 is 
that single quotes work out of the box.  Right now, even though qemu 
does NOT parse the same thing as pure JSON (which is 
https://datatracker.ietf.org/doc/rfc8259/), it does appear to parse a 
subset of JSON5, where the additional features of JSON5 (such as 
allowing a trailing comma, escaped newlines within strings, allowing 
bareword key:"value" syntax, a larger set of accepted numeric values, 
allowing comments) don't seem that onerous.

> 
> If we're going to call QMP protocol JSON, then IMHO QEMU should follow
> the JSON spec as closely as possible, without extensions.

As it is, qemu explicitly does NOT parse all valid JSON - it rejects 
non-ASCII bytes (whether as UTF-8 or as \u escape sequences) in strings. 
  So the fact that it already extensions in some places and limitations 
in others is a burden for clients to be aware of.

> 
> On the flip side, if we're going to support extensions like single quoting,
> then we should make it clear to applications that this is not really JSON
> and that they need to provide an impl that is 100% matching QEMU's dialect.
> This effectively means they need just import a copy of QEMU's code.
> 
> Regards,
> Daniel
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org