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

[Richard W.M. Jones]

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

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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