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Re: [Libguestfs] [PATCH 0/5] Import directly from OVA tar archive if possible

On Mon, 07 Nov 2016 15:03:36 +0100
Pino Toscano <ptoscano redhat com> wrote:

> On Friday, 4 November 2016 14:52:48 CET Tomáš Golembiovský wrote:
> > 1)  The obviously missing check whether QEMU installed in the system
> >     supports the feature we need. Do we already have such (or similar)
> >     checks in libguestfs? Should it be compile-time check or runtime
> >     check?  
> - yes, we have version checks for qemu, but only internally in the
> library (and it does not make sense to export)
> - runtime check indeed -- for example something like the following
> (taken & simplified from virt-sysprep in the rhel-6.x branches):

Thanks. I'll reuse that.

>   let qemu_img_version () =
>     let cmd = "qemu-img --version" in
>     let lines = external_command cmd in
>     match lines with
>     | [] -> error ("'qemu-img --version' returned no output")
>     | line :: _ ->
>       try
>         sscanf line "qemu-img version %d.%d" (
>           fun major minor ->
>             let minor = if minor > 9 then 9 else minor in
>             float major +. float minor /. 10.

Uhm. I know this is easy to compare with, but why not return a tuple?
QEMU 2.10 will be here sooner than we can imagine.

>         )
>       with
>         Scan_failure msg ->
>           warning (f_"warning: failed to read qemu-img version\n  line: %S\n  message: %s\n%!")
>             line msg;
>           0.9
> > 2)  Lots of 'tar' invocations that may not be easily understandable.  
> Left some comments about them.

I will try to rewrite that and do the filtering in OCaml.

> > 3)  We need a check for sparse files in tar archive. There is a GNU
> >     extension to tar format that allows space-efficient archiving of
> >     sparse files. While the OVF standard does not allow the use of GNU
> >     extensions we probably should not rely on that.  
> Is there a way to detect such kind of files in archives, falling back
> to usual "extract everything" way as done currently.

Sorry, somehow I left out this important piece of information. I didn't
find a way to get this information neither with GNU tar utility nor with
bsdtar. I didn't look at the OCaml tar module too closely yet, so I
don't know if this is possible. libarchive on the other hand is able to
tell us whether the file is sparse or not.

> > To address points 2) and 3) I see two possible options. We can either
> > use some OCaml module that understands tar. There is however a risk that
> > such module won't provide the necessary info to detect 3). The second
> > option is to use e.g. libarchive and write small set of C functions that
> > would implement the necessary magic and use those in OCaml code.
> > Richard, do you have any preferences or suggestions in this matter?  
> That's a workable way too, IMHO.
> Thanks,
> -- 
> Pino Toscano

Tomáš Golembiovský <tgolembi redhat com>

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