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

[Laszlo Ersek]

On 02/04/22 09:48, Richard W.M. Jones wrote:
> On Fri, Feb 04, 2022 at 09:41:33AM +0100, Laszlo Ersek wrote:
>> 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.
> 
> Relevant fact: libnbd OCaml wrapper turns Assert_failure in callbacks
> into abort:
> 
> https://gitlab.com/nbdkit/libnbd/-/blob/b80780e980275a879c13d27aff1449f91f883ce6/ocaml/helpers.c#L144

Haha, nice! :)
Laszlo