A brief look at deprecating our JSON extensions over RFC 8259

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. - Interpolation, if enabled: The lexer accepts %[A-Za-z0-9]*, and leaves rejecting invalid ones to the parser. Ignore interpolation; it's never enabled at external interfaces. This leaves single-quotes strings and the escape sequence to go with them. I disabled them as an experiment. Some 20 iotests, a qtest and two unit tests explode. The unit test testing the JSON parser is of course excused. The remaining qtest and the unit test could perhaps be dismissed as atypical use of QEMU from C. The iotests less so, I think. I looked at some iotest failures, and quickly found single-quoted strings used with all external interfaces except for qemu-ga's QMP. 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?