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, 3 insertions, 0 deletions

diff --git a/erts/emulator/beam/module.c b/erts/emulator/beam/module.c
index 3eb11f1693..4f36377450 100644
--- a/erts/emulator/beam/module.c
+++ b/erts/emulator/beam/module.c
@@ -85,6 +85,7 @@ static Module* module_alloc(Module* tmpl)
     obj->old.num_breakpoints  = 0;
     obj->curr.num_traced_exports = 0;
     obj->old.num_traced_exports = 0;
+    obj->on_load = 0;
     return obj;
 }
 
@@ -201,6 +202,7 @@ void module_start_staging(void)
 
 	dst_mod->curr = src_mod->curr;
 	dst_mod->old = src_mod->old;
+	dst_mod->on_load = src_mod->on_load;
     }
 
     /*
@@ -214,6 +216,7 @@ void module_start_staging(void)
 
 	dst_mod->curr = src_mod->curr;
 	dst_mod->old = src_mod->old;
+	dst_mod->on_load = src_mod->on_load;
     }
     newsz = index_table_sz(dst);
     erts_smp_atomic_add_nob(&tot_module_bytes, (newsz - oldsz));