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

From: Markus Armbruster
Subject: Re: A brief look at deprecating our JSON extensions over RFC 8259
Date: Tue, 23 Feb 2021 10:06:30 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 22/02/21 16:24, Daniel P. Berrangé wrote:
>> This problem isn't unique to QEMU. Any app using JSON from the
>> shell will have the tedium of quote escaping. JSON is incredibly
>> widespread and no other apps felt it neccessary to introduce single
>> quoting support, because the benefit doesn't outweigh the interop
>> problem it introduces.
> The quotes were introduced for C code (and especially qtest), not for
> the shell.  We have something like
>     response = qmp("{ 'execute': 'qom-get', 'arguments': { 'path': %s, "
>                    "'property': 'temperature' } }", id);
> These are sent to QEMU as double-quoted strings (the single-quoted
> JSON is parsed to get interpolation and printed back; commit
> 563890c7c7, "libqtest: escape strings in QMP commands, fix leak",
> 2014-07-01). However, doing the interpolation requires a parser that
> recognizes the single-quoted strings.

Doing interpolation requires a parser that recognizes %-sequences.
Single quote support isn't *required*, but quite desirable to let us
avoid leaning toothpick syndrome (LTS).

Example: compare the above to

      response = qmp("{ \"execute\": \"qom-get\", \"arguments\": { \"path\": 
%s, "
                     "\"property\": \"temperature\" } }", id);

We kept the interpolation extension out of the external interfaces, but
not the single quotes.

> Markus, did you rebuild the qtests after disabling single-quoted
> strings?  "make check-qtest-x86_64" would have rebuilt them, but I'm 
> confused by the results.

I ran "make check" and looked at the failures:

* check-qjson.c

  - escaped_string() covers \'.  Naturally, this fails.

  - escaped_string() and utf8_string() try every string twice, first in
    double quotes, then in single quotes.  Naturally, the latter fails.

  - string_with_quotes() tests unquoted single quote in double-quoted
    string, and unquoted double quote in single-quoted string.
    Naturally, the latter fails.

  - large_dict() and simple_whitespace() use single quotes to avoid LTS.

  This is the test my "The unit test testing the JSON parser is of
  course excused" referred to.

* test-qobject-input-visitor.c
* qtest/qmp-test.c

  More LTS avoidance.

  This is "The remaining qtest and the unit test could perhaps be
  dismissed as atypical use of QEMU from C."

* tests/qemu-iotests/

  Unlike the tests above, these use *external* interfaces.

  In shell, we need to use double quotes to get parameter expansion.  We
  then use single quotes to avoid LTS.

  The Python code has less excuse, I think.

Still confused?

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