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Re: [Qemu-devel] [PATCH v3 10/18] vmstate: Use new JSON output visitor

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 10/18] vmstate: Use new JSON output visitor
Date: Tue, 3 May 2016 06:34:03 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 05/03/2016 06:26 AM, Markus Armbruster wrote:

>>> +        visit_type_int(vmdesc, "size", &size, &error_abort);
>>> +        visit_start_list(vmdesc, "fields", NULL, 0, &error_abort);
>>> +        visit_start_struct(vmdesc, NULL, NULL, 0, &error_abort);
>> Please avoid error_abort in migration code, especially on the source side.
>> You've got an apparently happily working VM, we must never kill it 
>> while attempting migration.
> These functions cannot fail, and &error_abort is a concise way to
> express that.  It's the same as
>             visit_type_int(vmdesc, "size", &size, &err);
>             assert(!err);

&error_abort is ONLY supposed to be used to flag programming errors (ie.
they should never be reachable).  I'm asserting that the errors don't
happen, and therefore this cannot make the migration fail - in other
words, this is NOT going to kill a VM that attempts migration.

> * Conditions where the JSON output visitor itself sets an error:
>   - None.

The JSON output visitor itself may be adding an error for an attempt to
output Inf or NaN for a floating point number - but since vmstate
doesn't use visit_type_number(), this is not possible.  And if we are
really worried about it, then in my next spin of the patch I may make it
user-configurable whether we stick to strict JSON or whether we relax
things and output Inf/NaN anyways.

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

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