Introduce an atfork hook in the runtime

This commit introduce an atfork hook in the runtime, as a follow up to the issue encountered in MPR ocaml#455 and MPR ocaml#471.

As a result, the usage of pthreads_atfork is also removed, opting to rely in both case on manually calling the runtime hook when required.  This way, fork() calls from C code no longer reset the OCaml thread machinery.  This should help with issues such as ocaml#12636.

(cherry picked from commit c6d00ee)
(and adapted to 4.14)

Co-authored-by: xavier.leroy@college-de-france.fr

otherlibs/systhreads/st_posix.h

@@ -27,6 +27,7 @@
 #ifdef __linux__
 #include <unistd.h>
 #endif
+#include <caml/domain.h>
 
 typedef int st_retcode;
 
@@ -408,7 +409,8 @@ int pthread_atfork(void (*prepare)(void), void (*parent)(void),
 
 static int st_atfork(void (*fn)(void))
 {
-  return pthread_atfork(NULL, NULL, fn);
+  caml_atfork_hook = fn;
+  return 0;
 }
 
 /* Signal handling */

otherlibs/unix/fork.c

@@ -15,6 +15,7 @@
 
 #define CAML_INTERNALS
 
+#include <caml/domain.h>
 #include <caml/mlvalues.h>
 #include <caml/debugger.h>
 #include <caml/eventlog.h>
@@ -27,6 +28,7 @@ CAMLprim value unix_fork(value unit)
   CAML_EV_FLUSH();
 
   ret = fork();
+  if (ret == 0 && caml_atfork_hook != NULL) caml_atfork_hook();
   if (ret == -1) uerror("fork", Nothing);
 
   CAML_EVENTLOG_DO({

runtime/caml/domain.h

@@ -27,6 +27,8 @@ extern "C" {
 
 void caml_init_domain(void);
 
+CAMLextern void (*caml_atfork_hook)(void);
+
 #endif /* CAML_INTERNALS */
 
 #ifdef __cplusplus

runtime/domain.c

@@ -92,3 +92,5 @@ void caml_init_domain (void)
   Caml_state->checking_pointer_pc = NULL;
   #endif
 }
+
+CAMLexport void (*caml_atfork_hook)(void) = NULL;