Make C stubs more resistant to header structure

[emillon]

The existing stubs cause a warning on OCaml 5.1 because the declaration of caml_convert_signal_number can not be found.

The reason for that is that <caml/signals.h> is already included by some headers at the beginning of the file, so CAML_SIGNALS_H is already set by the time <caml/signals.h> is included manually and the extra internal definition are not included.

This used to work in previous versions of OCaml because presumably the header structure was different and <caml/signal.h> was not included transitively.

This fix just sets CAML_INTERNALS at the file level; it is not really possible to be granular in setting it.

[emillon]

cc @rgrinberg ; this replaces #48 and #53.

[rgrinberg]

Isn't this worse? Previously, we'd only open the internals of caml/signals.h, but now we need the internals of all headers.

Would it be possible to just import caml/signals.h first with CAML_INTERNALS set?

[emillon]

I don't think that the headers support this kind of modularity: if we do that, signals.h is going to include other headers (today, misc.h, mlvalues.h) with CAML_INTERNALS set, but this might change again later.

I agree with you that it's not completely satisfactory, but:

• this is scoped to just this file
• we're not duplicating the declaration (this kind of thing has changed in the past; I remember some JS stubs duplicating the definition of channels and causing an error that was pretty difficult to debug)
• this is a piece of low level code that needs to know about details of the runtime, so I'm not too shocked it needs CAML_INTERNALS

So I believe it's fine to set it.

[dra27]

It's not "worse" - you're just getting all the internals, which is the unstable part of the OCaml API. Manually declaring the declaration is worse (they do subtly change from time-to-time - linking macros, etc., and even renaming, which you then wouldn't pick up)

[rgrinberg]

Enabling CAML_INTERNALAS per header doesn't work, but it's still possible to mitigate the damage. I've moved <caml/signals.h> to the top and undef'd CAML_INTERNALS after its included. At least the headers that aren't pulled in by <caml/signals.h> won't have their internals open.

[emillon]

Fixed in #58.