Re: [Caml-list] Justifying a breaking 4.03 change, strong dependency on modules with "external" declarations

[whitequark <whitequark@whitequark.org>]

On 2016-05-16 01:09, Gabriel Scherer wrote:
> As Roberto Di Cosmo points out in the thread "parmap package broken in
> opam switch 4.03.0", using an "external" declaration provided by a
> module A now counts as a dependency on A -- in particular, A has to be
> linked in the final executable.
> 
> This breaks some user code (such as Parmap, but I heard reports from
> other breakages) in the case where A was used solely to provide such
> "external" declarations, for C functions implemented in a C stub that
> was linked in the application. In that case it was neither necessary
> nor useful to link A, and projects did not do it. They need to do it
> explicitly now, and break at linking-time otherwise.
> 
> I realize that users may not understand why this change, which looks
> like a regression, happened in 4.03. It is actually a bugfix, see
> PR#4166 ( http://caml.inria.fr/mantis/view.php?id=4166 ): if A does
> not only provide those external declarations, but also initializes
> some state that the C functions rely on (such as exception
> declarations), then forgetting to link A may result in a crash at
> runtime.
> 
> In other words, the pre-4.03 situation could result in some rare cases
> of user error in a very subtle, very difficult to understand bug.

I am highly fond of this change. This was a substantial problem with
the LLVM OCaml bindings--I as well as other contributors have introduced
a bug more than once, but even beyond that, not having the `external`
declarations in the .mli unnecessarily pessimizes some hot paths.

-- 
whitequark