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

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):

  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 :: _ ->
        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.
        Scan_failure msg ->
          warning (f_"warning: failed to read qemu-img version\n  line: %S\n  message: %s\n%!")
            line msg;

> 2)  Lots of 'tar' invocations that may not be easily understandable.

Left some comments about them.

> 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.

> 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.

Pino Toscano

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