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Re: [Qemu-devel] [PATCH v5 22/28] qapi: Whitelist commands that don't re

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v5 22/28] qapi: Whitelist commands that don't return dictionary
Date: Fri, 27 Mar 2015 14:20:26 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 03/27/2015 03:11 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
>> ...or an array of dictionaries.  Although we have to cater to
>> existing commands, returning a non-dictionary means the command
>> is not extensible (no new name/value pairs can be added if more
>> information must be returned in parallel).  By making the
>> whitelist explicit, any new command that falls foul of this
>> practice will have to be self-documenting, which will encourage
>> developers to either justify the action or rework the design to
>> use a dictionary after all.
>> Signed-off-by: Eric Blake <address@hidden>
>> ---

> Since there's just one whitelist, all schemata share it, and that means
> it's too permissive for each of them.  Sloppy, but good enough.
> If the sloppiness bothers us, here are two alternatives:

Doesn't bother me enough to use either alternative, but I _will_ update
the commit message to at least remind someone of the potential for
tightening things, should the need arise in the future.

> * Program takes the whitelist as argument, say
>     scripts/qapi-commands.py --legacy-returns qmp-legacy-returns ...

Early exit (generator fails if you don't whitelist), but noisy (have to
touch a Makefile in addition to the .json schema).

> * Leave enforcing to C
>   If a command 'frobnicate' returns a non-dictionary, generate something
>   like
>       #error Command 'frobnicate' should return a dictionary
>       #endif
>   Then manually define the macros necessary to keep the current use
>   working in a suitable header.

Late exit (failure does not occur at python generation time, but only at
C compilation time) - and the point of this series was to promote
several late exits into early exits.  But slightly cleaner than having
to touch Makefiles to add in the whitelist.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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