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, 12 insertions, 3 deletions

diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index f63addb309..0afdedf6c2 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -833,11 +833,20 @@ erts_finish_loading(Binary* magic, Process* c_p,
 
     size = stp->loaded_size;
     erts_total_code_size += size;
-    if (stp->on_load) {
-	inst_p = &mod_tab_p->old;
-    } else {
+
+    if (!stp->on_load) {
 	inst_p = &mod_tab_p->curr;
+    } else {
+	mod_tab_p->on_load =
+	    (struct erl_module_instance *)
+	    erts_alloc(ERTS_ALC_T_PREPARED_CODE,
+		       sizeof(struct erl_module_instance));
+	inst_p = mod_tab_p->on_load;
+	inst_p->nif = 0;
+        inst_p->num_breakpoints = 0;
+        inst_p->num_traced_exports = 0;
     }
+
     inst_p->code_hdr = stp->hdr;
     inst_p->code_length = size;