flambda-backend: Add some checks that the minor GC does not recurse (#1743)

Author: stedolan

otherlibs/systhreads/st_stubs.c

@@ -247,6 +247,8 @@ static void memprof_ctx_iter(th_ctx_action f, void* data)
 
 CAMLexport void caml_thread_save_runtime_state(void)
 {
+  if (Caml_state->in_minor_collection)
+    caml_fatal_error("Thread switch from inside minor GC");
 #ifdef NATIVE_CODE
   curr_thread->top_of_stack = Caml_state->top_of_stack;
   curr_thread->bottom_of_stack = Caml_state->bottom_of_stack;

runtime/minor_gc.c

@@ -417,6 +417,8 @@ void caml_empty_minor_heap (void)
 #endif
     if (caml_minor_gc_begin_hook != NULL) (*caml_minor_gc_begin_hook) ();
     prev_alloc_words = caml_allocated_words;
+    if (Caml_state->in_minor_collection)
+      caml_fatal_error("Minor GC triggered recursively");
     Caml_state->in_minor_collection = 1;
     caml_gc_message (0x02, "<");
     CAML_EV_BEGIN(EV_MINOR_LOCAL_ROOTS);