[Libguestfs] [virt-v2v PATCH 1/3] lib/nbdkit: add the "Nbdkit.with_connect_unix" helper function

[Richard W.M. Jones]

On Wed, Dec 08, 2021 at 01:20:48PM +0100, Laszlo Ersek wrote:
> Connecting to an NBD server temporarily, for a "one-shot" operation, is
> quite similar to "Std_utils.with_open_in" and "Std_utils.with_open_out",
> as there are cleanup operations regardless of whether the "one-shot"
> operation completes successfully or throws an exception.
> 
> Introduce the "Nbdkit.with_connect_unix" function, which takes a Unix
> domain socket pathname, a list of metadata contexts to request from the
> NBD server, and calls a function with the live NBD server connection.
> 
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2027598
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
> ---
>  lib/Makefile.am |  2 +-
>  lib/nbdkit.mli  | 10 ++++++++++
>  lib/nbdkit.ml   | 12 ++++++++++++
>  3 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/Makefile.am b/lib/Makefile.am
> index c274b9ecf6c7..1fab25b326f5 100644
> --- a/lib/Makefile.am
> +++ b/lib/Makefile.am
> @@ -80,7 +80,7 @@ BOBJECTS = config.cmo $(SOURCES_ML:.ml=.cmo)
>  XOBJECTS = $(BOBJECTS:.cmo=.cmx)
>  
>  OCAMLPACKAGES = \
> -	-package str,unix \
> +	-package str,unix,nbd \
>  	-I $(builddir) \
>  	-I $(top_builddir)/common/mlgettext \
>  	-I $(top_builddir)/common/mlpcre \
> diff --git a/lib/nbdkit.mli b/lib/nbdkit.mli
> index ae19295186ed..825755e61dbe 100644
> --- a/lib/nbdkit.mli
> +++ b/lib/nbdkit.mli
> @@ -107,3 +107,13 @@ val run_unix : ?socket:string -> cmd -> string * int
>      The --exit-with-parent, --foreground, --pidfile, --newstyle and
>      --unix flags are added automatically.  Other flags are set as
>      in the {!cmd} struct. *)
> +
> +val with_connect_unix : socket:string ->
> +                        meta_contexts:string list ->
> +                        f:(NBD.t -> 'a) ->
> +                        'a
> +(** [with_connect_unix socket meta_contexts f] calls function [f] with the NBD
> +    server at Unix domain socket [socket] connected, and the metadata contexts
> +    in [meta_contexts] requested (each of which is not necessarily supported by
> +    the server though). The connection is torn down either on normal return or
> +    if the function [f] throws an exception. *)

Interesting choice of module for this.  nbdkit is a server.  I would
have put it into lib/utils.mli or created a new module.

The function itself looks fine although personally I might have
abstracted the meta_context function to be a "do anything between
creation and connection" (let's call that "preparation").  What do you
think about:

val with_connect_unix : socket:string ->
                        ?prepare:(NBD.t -> unit) ->
                        f:(NBD.t -> 'a) ->
                        'a

I think you would have to replace the List.iter line:

...
> +    ~f:(fun () ->

             (match prepare with Some pf -> pf nbd | None -> ());

> +          NBD.connect_unix nbd socket;
> +          protect
> +            ~f:(fun () -> f nbd)
> +            ~finally:(fun () -> NBD.shutdown nbd)
> +       )
> +    ~finally:(fun () -> NBD.close nbd)

It would be called like this:

  let prepare nbd = NBD.add_meta_context nbd "base:allocation" in
  with_connect_unix socket ~prepare (
    fun () ->
      (* the code *)
  )

Hmm, maybe this is getting more complicated :-(

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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