Daniel P. Berrangé <berrange@redhat.com> writes:
On Mon, Feb 22, 2021 at 06:47:30PM +0100, Paolo Bonzini wrote:
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.
IMHO this is the wrong solution to the problem. Consider the equivalent libvirt code that uses a standard JSON library underneath and has a high level API to serialize args into the command
qemuMonitorJSONMakeCommand("qom-get", "s:path", id, "s:property", "temperature");
Of course this example is reasonably easy since it is a flat set of arguments. Nested args get slightly more complicated, but still always preferrable to doing string interpolation IMHO.
Misunderstanding: our JSON interpolation feature is *not* string interpolation! It interpolates *objects* into the QObject built by the parser. Best explained with an example. The qmp() call above internally builds the following QObject: QDict mapping key "execute" to a QString for "qom-get" key "arguments" to a QDict mapping key "path" to a QString for @id (interpolation!) key "property" to a QString for "temperature" Unlike string interpolation, this is safe for any valid C string @id. If you think like a hacker, you might try shenanigans like "{'execute': '%s'}" You will then find that somebody has thought like a hacker before you[*]. The %-sequences are cleverly chosen to get some protection against common mistakes from the compiler. This interpolation has worked quite well for us, and I have no plans to replace it. Doesn't mean I'm against replacing it, only that I want to see benefits exceeding the costs. A bigger stumbling block for replacement is our need for a streaming interface: we feed the parser characters, and expect to be called back when an expression is complete. [*] Commit 16a4859921 "json: Improve safety of qobject_from_jsonf_nofail() & friends", fixed up in commit bbc0586ced "json: Fix % handling when not interpolating".