wslua: add support for negative indices

Internal change, this allows use of negative indices for
lua_rawgetfield/lua_rawsetfield convenience functions and is closer to
lua_getfield/lua_setfield semantics in terms of indices.

Add lua_absindex for compatibility with Lua 5.1 (do not bother exporting
it yet since it is only used in this file, also do not rename it since
it is the same functionality with no Wireshark-specific changes.)

Change-Id: I322954ef461164ec514ed9f034ae2691775e67da
Reviewed-on: https://code.wireshark.org/review/18025
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>

1 files changed, 21 insertions, 11 deletions

diff --git a/epan/wslua/wslua_internals.c b/epan/wslua/wslua_internals.c
index 782d5bae68..ff61a74dd9 100644
--- a/epan/wslua/wslua_internals.c
+++ b/epan/wslua/wslua_internals.c
@@ -30,6 +30,16 @@
 #include "wslua.h"
 #include <wsutil/ws_printf.h> /* ws_debug_printf */
 
+#if LUA_VERSION_NUM == 501
+/* Compatibility with Lua 5.1, function was added in 5.2 */
+static
+int lua_absindex(lua_State *L, int idx) {
+  return (idx > 0 || idx <= LUA_REGISTRYINDEX)
+         ? idx
+         : lua_gettop(L) + 1 + idx;
+}
+#endif
+
 WSLUA_API int wslua__concat(lua_State* L) {
     /* Concatenate two objects to a string */
     if (!luaL_callmeta(L,1,"__tostring"))
@@ -160,18 +170,22 @@ WSLUA_API void wslua_setfuncs(lua_State *L, const luaL_Reg *l, int nup) {
   lua_pop(L, nup);  /* remove upvalues */
 }
 
-/* identical to lua_getfield but without triggering metamethods
-   warning: cannot be used directly with negative index (and shouldn't be changed to)
-   decrement your negative index if you want to use this */
+/**
+ * Identical to lua_getfield, but without triggering the __newindex metamethod.
+ * The resulting value is returned on the Lua stack.
+ */
 static void lua_rawgetfield(lua_State *L, int idx, const char *k) {
+    idx = lua_absindex(L, idx);
     lua_pushstring(L, k);
     lua_rawget(L, idx);
 }
 
-/* identical to lua_setfield but without triggering metamethods
-   warning: cannot be used with negative index (and shouldn't be changed to)
-   decrement your negative index if you want to use this */
+/**
+ * Identical to lua_setfield, but without triggering the __newindex metamethod.
+ * The value to be set is taken from the Lua stack.
+ */
 static void lua_rawsetfield (lua_State *L, int idx, const char *k) {
+    idx = lua_absindex(L, idx);
     lua_pushstring(L, k);
     lua_insert(L, -2);
     lua_rawset(L, idx);
@@ -208,11 +222,9 @@ const gchar* wslua_typeof(lua_State *L, int idx) {
 /* this gets a Lua table of the given name, from the table at the given
  * location idx. If it does not get a table, it pops whatever it got
  * and returns false.
- * warning: cannot be used with pseudo-indeces like LUA_REGISTRYINDEX
  */
 gboolean wslua_get_table(lua_State *L, int idx, const gchar *name) {
     gboolean result = TRUE;
-    if (idx < 0) idx--;
     lua_rawgetfield(L, idx, name);
     if (!lua_istable(L,-1)) {
         lua_pop(L,1);
@@ -224,11 +236,9 @@ gboolean wslua_get_table(lua_State *L, int idx, const gchar *name) {
 /* this gets a table field of the given name, from the table at the given
  * location idx. If it does not get a field, it pops whatever it got
  * and returns false.
- * warning: cannot be used with pseudo-indeces like LUA_REGISTRYINDEX
  */
 gboolean wslua_get_field(lua_State *L, int idx, const gchar *name) {
     gboolean result = TRUE;
-    if (idx < 0) idx--;
     lua_rawgetfield(L, idx, name);
     if (lua_isnil(L,-1)) {
         lua_pop(L,1);
@@ -373,7 +383,7 @@ int wslua_reg_attributes(lua_State *L, const wslua_attribute_table *t, gboolean
         if (cfunc) {
             /* if there's a previous methods table, make sure this attribute name doesn't collide */
             if (nup > 1) {
-                lua_rawgetfield(L, -2, t->fieldname);
+                lua_rawgetfield(L, -1, t->fieldname);
                 if (!lua_isnil(L,-1)) {
                     fprintf(stderr, "'%s' attribute name already exists as method name for the class\n", t->fieldname);
                     exit(1);