added comment and assert about dead keys

lgc.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.c,v 2.208 2015/11/02 16:19:29 roberto Exp roberto $
+** $Id: lgc.c,v 2.209 2015/11/02 18:48:07 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -114,8 +114,13 @@ static void reallymarkobject (global_State *g, GCObject *o);
 
 
 /*
-** if key is not marked, mark its entry as dead (therefore removing it
-** from the table)
+** If key is not marked, mark its entry as dead. This allows key to be
+** collected, but keeps its entry in the table.  A dead node is needed
+** when Lua looks up for a key (it may be part of a chain) and when
+** traversing a weak table (key might be removed from the table during
+** traversal). Other places never manipulate dead keys, because its
+** associated nil value is enough to signal that the entry is logically
+** empty.
 */
 static void removeentry (Node *n) {
   lua_assert(ttisnil(gval(n)));

ltable.c

@@ -1,5 +1,5 @@
 /*
-** $Id: ltable.c,v 2.113 2015/07/04 16:32:34 roberto Exp roberto $
+** $Id: ltable.c,v 2.114 2015/11/03 15:47:30 roberto Exp roberto $
 ** Lua tables (hash)
 ** See Copyright Notice in lua.h
 */
@@ -133,6 +133,7 @@ static Node *mainposition (const Table *t, const TValue *key) {
     case LUA_TLCF:
       return hashpointer(t, fvalue(key));
     default:
+      lua_assert(!ttisdeadkey(key));
       return hashpointer(t, gcvalue(key));
   }
 }
@@ -457,7 +458,7 @@ TValue *luaH_newkey (lua_State *L, Table *t, const TValue *key) {
     Node *f = getfreepos(t);  /* get a free place */
     if (f == NULL) {  /* cannot find a free place? */
       rehash(L, t, key);  /* grow table */
-      /* whatever called 'newkey' takes care of TM cache and GC barrier */
+      /* whatever called 'newkey' takes care of TM cache */
       return luaH_set(L, t, key);  /* insert key into grown table */
     }
     lua_assert(!isdummy(f));