[Libguestfs] [PATCH libnbd 1/2] ocaml/examples: Fix extents example

[Laszlo Ersek]

On 12/11/21 14:41, Richard W.M. Jones wrote:
> Laszlo Ersek noted that the original example we had did not work
> properly if the size of the disk was larger than around 2G.
> 
> The nbd_block_status API is really difficult to use correctly!  In
> particular it is not guaranteed that the server will return extents
> covering the size requested.  It's also not guaranteed that a bad
> server will return any base:allocation extents at all (although such a
> server would not be conforming - the protocol says that servers must
> always make forward progress).
> 
> This commit attempts a fix, although it is not complete especially if
> the server is badly behaved.  It also makes the output look a bit
> better by aligning the columns.  Also we use nbdkit-sparse-random-
> plugin with a larger size to test the > 2G case.
> ---
>  ocaml/examples/extents.ml | 53 ++++++++++++++++++++-------------------
>  1 file changed, 27 insertions(+), 26 deletions(-)
> 
> diff --git a/ocaml/examples/extents.ml b/ocaml/examples/extents.ml
> index 6fa70e087..e4422b270 100644
> --- a/ocaml/examples/extents.ml
> +++ b/ocaml/examples/extents.ml
> @@ -4,32 +4,33 @@ let () =
>    let nbd = NBD.create () in
>    NBD.add_meta_context nbd "base:allocation";
>    NBD.connect_command nbd
> -                      ["nbdkit"; "-s"; "--exit-with-parent"; "memory"; "size=128K"];
> -
> -  (* Write some sectors. *)
> -  let data_sector = Bytes.make 512 'a' in
> -  let zero_sector = Bytes.make 512 '\000' in
> -  NBD.pwrite nbd data_sector 0_L;
> -  NBD.pwrite nbd zero_sector 32768_L;
> -  NBD.pwrite nbd data_sector 65536_L;
> +                      ["nbdkit"; "-s"; "--exit-with-parent"; "-r";
> +                       "sparse-random"; "8G"];
>  
>    (* Read the extents and print them. *)
>    let size = NBD.get_size nbd in
> -  NBD.block_status nbd size 0_L (
> -    fun meta _ entries err ->
> -      printf "err=%d\n" !err;
> -      if meta = "base:allocation" then (
> -        printf "index\tlength\tflags\n";
> -        for i = 0 to Array.length entries / 2 - 1 do
> -          let flags =
> -            match entries.(i*2+1) with
> -            | 0_l -> "data"
> -            | 1_l -> "hole"
> -            | 2_l -> "zero"
> -            | 3_l -> "hole+zero"
> -            | i -> sprintf "unknown (%ld)" i in
> -          printf "%d:\t%ld\t%s\n" i entries.(i*2) flags
> -        done
> -      );
> -      0
> -  )
> +  let fetch_offset = ref 0_L in
> +  while !fetch_offset < size do
> +    let remaining = Int64.sub size !fetch_offset in
> +    let fetch_size = min remaining 0x8000_0000_L in
> +    NBD.block_status nbd fetch_size !fetch_offset (
> +      fun meta _ entries err ->
> +        printf "nbd_block_status callback: meta=%s err=%d\n" meta !err;
> +        if meta = "base:allocation" then (
> +          printf "index\t%-20s %-20s %s\n" "offset" "length" "flags";
> +          for i = 0 to Array.length entries / 2 - 1 do
> +            let len = Int64.of_int32 entries.(i*2)
> +            and flags =
> +              match entries.(i*2+1) with
> +              | 0_l -> "data"
> +              | 1_l -> "hole"
> +              | 2_l -> "zero"
> +              | 3_l -> "hole+zero"
> +              | i -> sprintf "unknown (%ld)" i in
> +            printf "%d:\t%-20Ld %-20Ld %s\n" i !fetch_offset len flags;

Should we use right-justification for the numbers?

Reviewed-by: Laszlo Ersek <lersek at redhat.com>

Thanks!
Laszlo

> +            fetch_offset := Int64.add !fetch_offset len
> +          done;
> +        );
> +        0
> +    ) (* NBD.block_status *)
> +  done
>