Re: A brief look at deprecating our JSON extensions over RFC 8259

[Peter Krempa]

On Mon, Feb 22, 2021 at 15:57:22 +0100, Markus Armbruster wrote:
> We use JSON in several external interfaces:
> 
> * QMP
> 
> * The guest agent's QMP
> 
> * QAPIfied command line options when the option argument starts with
>   '{'
> 
> * The block layer's pseudo-protocol "json:" (which can get embedded in
>   image headers)
> 
> I *think* that's all.
> 
> The JSON parser we use for these interfaces supports extensions over RFC
> 8259.  Quoting json-lexer.c:
> 
>     - Extra escape sequence in strings:
>       0x27 (apostrophe) is recognized after escape, too
> 
>     - Single-quoted strings:
>       Like double-quoted strings, except they're delimited by %x27
>       (apostrophe) instead of %x22 (quotation mark), and can't contain
>       unescaped apostrophe, but can contain unescaped quotation mark.

[...]

> We could certainly tidy up the tests to stick to standard JSON.
> However, the prevalence of single-quoted strings in iotests makes me
> suspect that they are being used in the field as well.  Deprecating the
> extension is likely more trouble than it's worth.
> 
> Opinions?

Any user of QEMU through libvirt will not use any of the extensions even
in cases such as QMP command pasthrough (virsh qemu-monitor-command) and
the 'json:' pseudo-protocol, as libvirt parses the provided JSON to add
sequencing for QMP passthrough, and for image chain detection in case of
'json:'. Since libvirt's JSON library (yajl) doesn't support any of
those extensions users are forced to not use them.

So on behalf of libvirt, we'd be fine with deprecation/removal of those.