Re: [PATCH v2] hmp: Make json format optional for qom-set

[Markus Armbruster <armbru@redhat.com>]

David Hildenbrand <david@redhat.com> writes:

> Commit 7d2ef6dcc1cf ("hmp: Simplify qom-set") switched to the json
> parser, making it possible to specify complex types. However, with this
> change it is no longer possible to specify proper sizes (e.g., 2G, 128M),
> turning the interface harder to use for properties that consume sizes.
>
> Let's switch back to the previous handling and allow to specify passing
> json via the "-j" parameter.

Two issues:

1. Makes qom-get and qom-set inconsistent

   qom-get formats as JSON, always.

   qom-set parses the string visitor's undocumented ad hoc language by
   default.  You can make it parse JSON by passing -j.

   Not a show stopper, but sure ugly.  I feel documentation should point
   it out.

2. Rearms the string visitor death trap

   If you try to qom-set a property whose ->set() uses something the
   string input visitor doesn't support, QEMU crashes.  I'm not aware of
   such a ->set(), but this is a death trap all the same.  Mind, I
   didn't actually *look* for such a ->set().  Details:

    Subject: Re: [RFC PATCH] qom: Implement qom-get HMP command
    Date: Sat, 02 May 2020 08:02:43 +0200 (6 weeks, 2 days, 4 minutes ago)
    Message-ID: <87a72q6fi4.fsf@dusky.pond.sub.org>
    https://lists.nongnu.org/archive/html/qemu-devel/2020-05/msg00178.html

   Since we've had this death trap in the code for a number of years, I
   can't call its restoration a show stopper.  It does feel like an
   unadvisable risk, though.