[Libguestfs] [libnbd PATCH v2 2/5] ocaml: tests: Fix error handling

[Laszlo Ersek]

On 02/03/22 21:25, Eric Blake wrote:
> Like a lot of the C examples, the aio copy test ignores read and write
> errors in the completion callback, which can cause silent data
> corruption. The failure in the test is not critical, but this is a bad
> example that may be copied by developers to a real application.
> 
> The test dies with an assertion failure now if completion callback
> fails.  Tested with the temporary patch of:
> 
> | diff --git i/ocaml/tests/test_590_aio_copy.ml w/ocaml/tests/test_590_aio_copy.ml
> | index a0339c8b..332bf31b 100644
> | --- i/ocaml/tests/test_590_aio_copy.ml
> | +++ w/ocaml/tests/test_590_aio_copy.ml
> | @@ -120,7 +120,8 @@ let
> |    let dst = NBD.create () in
> |    NBD.set_handle_name dst "dst";
> |    NBD.connect_command src ["nbdkit"; "-s"; "--exit-with-parent"; "-r";
> | -                           "pattern"; sprintf "size=%d" disk_size];
> | +                           "--filter=error"; "pattern"; "error-pread-rate=1";
> | +                           sprintf "size=%d" disk_size];
> |    NBD.connect_command dst ["nbdkit"; "-s"; "--exit-with-parent";
> |                             "memory"; sprintf "size=%d" disk_size];
> |    asynch_copy src dst
> ---
>  ocaml/tests/test_590_aio_copy.ml | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/ocaml/tests/test_590_aio_copy.ml b/ocaml/tests/test_590_aio_copy.ml
> index 11d89256..a0339c8b 100644
> --- a/ocaml/tests/test_590_aio_copy.ml
> +++ b/ocaml/tests/test_590_aio_copy.ml
> @@ -1,6 +1,6 @@
>  (* hey emacs, this is OCaml code: -*- tuareg -*- *)
>  (* libnbd OCaml test case
> - * Copyright (C) 2013-2019 Red Hat Inc.
> + * Copyright (C) 2013-2022 Red Hat Inc.
>   *
>   * This library is free software; you can redistribute it and/or
>   * modify it under the terms of the GNU Lesser General Public
> @@ -45,7 +45,8 @@ let
>     * next iteration of the loop.
>     *)
>    let writes = ref [] in
> -  let read_completed buf offset _ =
> +  let read_completed buf offset err =
> +    assert (!err = 0);
>      bytes_read := !bytes_read + NBD.Buffer.size buf;
>      (* Get ready to issue a write command. *)
>      writes := (buf, offset) :: !writes;
> @@ -56,7 +57,8 @@ let
>    (* This callback is called when any pwrite to the destination
>     * has completed.
>     *)
> -  let write_completed buf _ =
> +  let write_completed buf err =
> +    assert (!err = 0);
>      bytes_written := !bytes_written + NBD.Buffer.size buf;
>      (* By returning 1 here we auto-retire the pwrite command. *)
>      1
> 

My (older) OCaml book calls "assert" an "operator"; whereas Real World
OCaml calls "assert" a "directive":
<https://dev.realworldocaml.org/error-handling.html#exceptions>. Either
way, AIUI it cannot be compiled out, and if the assertin fails,
Assert_failure is raised, which (= throwing an exception) is the proper
way for an OCaml-language completion callback to report an error, IIUC
Rich's explanation.

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