[Libguestfs] [PATCH] lib: Autodetect backing format and specify it explicitly.

[Richard W.M. Jones]

On Mon, Mar 09, 2020 at 11:07:20AM +0100, Pino Toscano wrote:
> On Thursday, 6 February 2020 18:20:19 CET Richard W.M. Jones wrote:
> > In the guestfs_disk_create API we have traditionally allowed you to
> > set backingfile without setting backingformat.  The meaning of this is
> > to let qemu autodetect the backing format when opening the overlay
> > disk.
> > 
> > However libvirt >= 6.0 refuses to even pass such disks to qemu (see
> > https://bugzilla.redhat.com/show_bug.cgi?id=1798148).
> > 
> > For this reason, move the autodetection earlier and make it explicit.
> > We now autodetect the format of the backing disk at the time of
> > creation of the overlay, and set that as the backing format in the
> > overlay disk itself, allowing libvirt to open the disk later.
> > ---
> >  lib/create.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/lib/create.c b/lib/create.c
> > index e2a59b88d..e30286c39 100644
> > --- a/lib/create.c
> > +++ b/lib/create.c
> > @@ -255,6 +255,7 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
> >                     const struct guestfs_disk_create_argv *optargs)
> >  {
> >    const char *backingformat = NULL;
> > +  CLEANUP_FREE char *backingformat_free = NULL;
> >    const char *preallocation = NULL;
> >    const char *compat = NULL;
> >    int clustersize = -1;
> > @@ -302,6 +303,18 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
> >      }
> >    }
> >  
> > +  /* With libvirt >= 6.0 the backing format must be specified. */
> > +  if (backingfile != NULL && backingformat == NULL) {
> > +    backingformat = backingformat_free = guestfs_disk_format (g, backingfile);
> > +    if (!backingformat)
> > +      return -1;
> > +    if (STREQ (backingformat, "unknown")) {
> > +      error (g, _("could not auto-detect the format of the backing file %s"),
> > +             backingfile);
> > +      return -1;
> > +    }
> > +  }
> 
> I see this patch was pushed, even if it was not reviewed, and in
> general the problem was more (not totally) on libvirt side.
> 
> Was it intentional?

Yes, I pushed it intentionally.  Peter has a partial fix on the
libvirt side, but it seems to me they are heading in the direction of
requiring that the format of backing files in the chain being
specified.  Since our API specifies that a NULL backing format means
the format will be autodetected (but not by what), I moved the
detection to here.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org