Fix Windows compilation failure ppx_base because of base and ocaml_intrinsics_kernel symbol clash

[MisterDA]

Put CAMLweakdef function in its own translation unit for Windows compatibility, or it conflicts with other non-weakdef symbols with the same name from other libraries. There's a conflicting symbol in ocaml_intrinsics_kernel at: https://github.com/janestreet/ocaml_intrinsics_kernel/blob/1047d3186d0c6b498f1baa96024901cc3cbc2a19/src/conditional_stubs.c#L23-L26.

$ dune build
File "bin/dune", line 3, characters 8-12:
3 |  (names main)
            ^^^^
/usr/lib/gcc/x86_64-w64-mingw32/11/../../../../x86_64-w64-mingw32/bin/ld: C:\Users\Antonin\AppData\Local\opam\default\lib\ocaml_intrinsics_kernel\libocaml_intrinsics_kernel_stubs.a(conditional_stubs.o):/cygdrive/c/Users/Antonin/AppData/Local/opam/default/.opam-switch/build/ocaml_intrinsics_kernel.v0.17.0/_build/default/src/conditional_stubs.c:16: multiple definition of `caml_csel_value'; C:\Users\Antonin\AppData\Local\opam\default\lib\base\libbase_stubs.a(int_math_stubs.o):/cygdrive/c/Users/Antonin/AppData/Local/opam/default/.opam-switch/build/base.v0.17.0/_build/default/src/int_math_stubs.c:200: first defined here
collect2: error: ld returned 1 exit status
** Fatal error: Error during linking

Also note that the caml_ prefix for C symbols is reserved by the OCaml runtime. I suggest JS libraries pick another prefix.

[hhugo]

Why is moving the stub in another file enough to fix the issue ?

[MisterDA]

This was my initial question:

I've just noticed that the new version of JS base doesn't build anymore on Windows: it's defining a caml_csel_value function with CAMLweakdef, but their ocaml_intrinsic_kernel package also defines that same function, without CAMLweakdef. I understand there's no easy definition of weak linking on Windows?

This PR is based on @dra27's explanations:

Sadly, this is true - there's a very strange selectany, but its semantics are really odd
The best way - which is used in libcamlrun already - is to put the "weak" function in its own .o file
Because it is well defined that if you have already linked a symbol, then a .o file doesn't get pulled in when linking a .a file
(we use this with main.o in libcamlrun.a)

[MisterDA]

Hmm, it appears I had only tested ocaml_intrinsics_kernel and base, but installing ppx_base still fails (as someone reported, then removed their comment). The catch is that the same patch needs to be applied to ocaml_intrinsics_kernel.

[Chimrod]

Yes, I’ve tested the branch and got the same error. After my comment, I’ve been in doubt if I set up the branch properly when I compiled the package and I’ve removed the comment. (after checking I confirm I was right, but I didn’t had the opportunity to connect here again :)

[MisterDA]

Could you confirm that with both patches in base and ocaml_intrinsics_kernel, the compilation of ppx_base succeeds?

[Chimrod]

I just tried this morning and with the two branches I’ve been able to build the package properly. (I did not test any code using it but the compilation went fine)

[MisterDA]

I've removed the symbol from base as it is now provided by ocaml_intrinsics_kernel. This also fixes the problem.

[dkalinichenko-js]

Hi, thanks for your PR!