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Date: Thu, 15 May 2014 09:10:16 -0400
From: Jeff Cody <jcody@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 5/5] block: extend block-commit to accept a
 string for the backing file
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To: Eric Blake <eblake@redhat.com>
Cc: =?iso-8859-1?Q?Beno=EEt?= Canet <benoit.canet@irqsave.net>, kwolf@redhat.com, pkrempa@redhat.com, famz@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com

On Thu, May 15, 2014 at 06:57:11AM -0600, Eric Blake wrote:
> On 05/15/2014 06:26 AM, Beno=EEt Canet wrote:
> > The Wednesday 14 May 2014 =E0 23:20:19 (-0400), Jeff Cody wrote :
> >> On some image chains, QEMU may not always be able to resolve the
> >> filenames properly, when updating the backing file of an image
> >> after a block commit.
> >>
>=20
> >>      } else {
> >>          commit_start(bs, base_bs, top_bs, speed, on_error, block_jo=
b_cb, bs,
> >> -                    &local_err);
> >> +                     backing_file, &local_err);
> >=20
> > I don't know QAPI well enough to be sure but are we certain that if=20
> > has_backing_file =3D=3D false then backing_file =3D=3D NULL and not s=
ome
> > random pointer ?
> >=20
> > If am thinking to add has_backing_file ? backing_file : NULL here.
>=20
> We are moving towards having qapi guarantee sane defaults for FOO when
> has_FOO is false; but aren't there yet.  You are correct that this need=
s
> to guarantee that we aren't passing random memory.
>

The QAPI code generator for the QMP input marshaller initializes all
pointers to NULL, and all bools to false.  If has_ is false, then the
associated pointer will also be NULL, so it is safe to just pass
backing_file.