Re: [Qemu-devel] [PATCH 05/21] qmp: Clean up how we enforce capability negotiation

[Eric Blake]

On 02/23/2017 03:45 PM, Markus Armbruster wrote:
> To enforce capability negotiation before normal operation,
> handle_qmp_command() inspects every command before it's handed off to
> qmp_dispatch().  This is a bit of a layering violation, and results in
> duplicated code.
> 
> Before capability negotiation (!cur_mon->in_command_mode), we fail
> commands other than "qmp_capabilities".  This is what enforces
> capability negotiation.
> 
> Afterwards, we fail command "qmp_capabilities".
> 
> Clean this up as follows.
> 
> The obvious place to fail a command is the command itself, so move the
> "afterwards" check to qmp_qmp_capabilities().
> 
> We do the "before" check in every other command, but that would be
> bothersome.  Instead, start without all the other commands, by
> registering only qmp_qmp_capabilities().  Register the others in
> qmp_qmp_capabilities().
> 
> Additionally, replace the generic human-readable error message for
> CommandNotFound by one that reminds the user to run qmp_capabilities.
> Without that, we'd regress commit 2d5a834.
> 
> Signed-off-by: Markus Armbruster <address@hidden>
> ---
>  monitor.c | 70 
> ++++++++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 40 insertions(+), 30 deletions(-)
> 

> @@ -3781,12 +3782,21 @@ static void handle_qmp_command(JSONMessageParser 
> *parser, GQueue *tokens)
>      cmd_name = qdict_get_str(qdict, "execute");
>      trace_handle_qmp_command(mon, cmd_name);
>  
> -    if (invalid_qmp_mode(mon, cmd_name, &err)) {
> -        goto err_out;
> -    }
> -
>      rsp = qmp_dispatch(req);
>  
> +    qdict = qdict_get_qdict(qobject_to_qdict(rsp), "error");
> +    if (qdict) {
> +        if (!mon->qmp.in_command_mode
> +            && !strcmp(qdict_get_try_str(qdict, "class"),

Ouch - this can call strcmp(NULL, ...) if the error is ill-formed.  Is
that a problem?  Can we assert that "class" will always exist as a string?

> +                       
> QapiErrorClass_lookup[ERROR_CLASS_COMMAND_NOT_FOUND])) {
> +            /* Provide a more useful error message */
> +            qdict_del(qdict, "desc");
> +            qdict_put(qdict, "desc",
> +                      qstring_from_str("Expecting capabilities negotiation"
> +                                       " with 'qmp_capabilities'"));
> +        }
> +    }
> +

Otherwise, I like it.

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

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