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

[Pino Toscano]

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?

-- 
Pino Toscano
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