Linking with Ssl causes segfault on multicore OCaml

[talex5]

I've been investigating why Dream segfaults when run on multicore (see aantron/dream#191).

I've managed to simplify the test-case to this (with ssl as its only dependency):

let foo = Ssl.file_descr_of_socket

let () =
  while true do
    Gc.full_major ()
  done

This segfaults immediately for me when run under ocaml-variants.4.12.0+domains.

I believe the cause is this bit of ssl.ml:

ocaml-ssl/src/ssl.ml

Lines 191 to 193 in b711cd4

	external get_client_verify_callback_ptr : unit -> verify_callback = "ocaml_ssl_get_client_verify_callback_ptr"
	let client_verify_callback = get_client_verify_callback_ptr ()

The C stub is

ocaml-ssl/src/ssl_stubs.c

Lines 562 to 565 in b711cd4

	CAMLprim value ocaml_ssl_get_client_verify_callback_ptr(value unit)
	{
	return (value)client_verify_callback;
	}

However, client_verify_callback is not an OCaml value, it's a C function. This causes the GC to crash when trying to mark it.

[patricoferris]

I just bumped into this over at https://github.com/patricoferris/dream-multicore-repro

[anmonteiro]

This should have been fixed in #83 and released in 0.5.11.