Don't leak old code when loading a modules with an on_load function

Normally, calling code:delete/1 before re-loading the code for a
module is unnecessary but causes no problem.

But there will be be problems if the new code has an on_load function.
Code with an on_load function will always be loaded as old code
to allowed it to be easily purged if the on_load function would fail.
If the on_load function succeeds, the old and current code will be
swapped.

So in the scenario where code:delete/1 has been called explicitly,
there is old code but no current code. Loading code with an
on_load function will cause the reference to the old code to be
overwritten. That will at best cause a memory leak, and at worst
an emulator crash (especially if NIFs are involved).

To avoid that situation, we will put the code with the on_load
function in a special, third slot in Module.

ERL-240

1 files changed, 10 insertions, 8 deletions

diff --git a/erts/emulator/beam/erl_nif.c b/erts/emulator/beam/erl_nif.c
index 559b4017e7..3a547982da 100644
--- a/erts/emulator/beam/erl_nif.c
+++ b/erts/emulator/beam/erl_nif.c
@@ -3203,18 +3203,20 @@ BIF_RETTYPE load_nif_2(BIF_ALIST_2)
     if (init_func != NULL)
       handle = init_func;
 
+    this_mi = &module_p->curr;
+    prev_mi = &module_p->old;
     if (in_area(caller, module_p->old.code_hdr, module_p->old.code_length)) {
-	if (module_p->old.code_hdr->on_load_function_ptr) {
-	    this_mi = &module_p->old;
+	ret = load_nif_error(BIF_P, "old_code", "Calling load_nif from old "
+			     "module '%T' not allowed", mod_atom);
+	goto error;
+    } else if (module_p->on_load) {
+	ASSERT(module_p->on_load->code_hdr->on_load_function_ptr);
+	if (module_p->curr.code_hdr) {
 	    prev_mi = &module_p->curr;
 	} else {
-	    ret = load_nif_error(BIF_P, "old_code", "Calling load_nif from old "
-				 "module '%T' not allowed", mod_atom);
-	    goto error;
+	    prev_mi = &module_p->old;
 	}
-    } else {
-	this_mi = &module_p->curr;
-	prev_mi = &module_p->old;
+	this_mi = module_p->on_load;
     }
 
     if (init_func == NULL &&