wslua: include Lua stack trace in startup error messages

Error messages without a stack trace are rather hard to debug for more
complex Lua dissectors. Be sure to append one, it will look like this:

    tshark: Lua: Error during loading:
     /tmp/kdnet/kdnet.lua:13: bad argument #3 to 'proto_field_constructor' (Display must be either base.NONE, base.DOT, base.DASH, base.COLON or base.SPACE)
    stack traceback:
            [C]: in function 'proto_field_constructor'
            /tmp/kdnet/kdnet.lua:13: in function 'add_field'
            /tmp/kdnet/kdnet.lua:35: in function 'add_fields'
            /tmp/kdnet/kdnet.lua:242: in main chunk

It would be nice to reuse the error handler for dissector calls as well,
but I am not sure whether this works with absolute indices which are
used almost everywhere in wslua.

Change-Id: I89b2dcd360fce3865e1bf052b9fe03e888aae167
Reviewed-on: https://code.wireshark.org/review/31763
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

epan/wslua/init_wslua.c

@@ -402,10 +402,16 @@ static const char *getF(lua_State *LS _U_, void *ud, size_t *size)
     return (*size>0) ? buff : NULL;
 }
 
-static int lua_main_error_handler(lua_State* LS) {
-    const gchar* error =  lua_tostring(LS,1);
-    report_failure("Lua: Error during loading:\n %s",error);
-    return 0;
+static int error_handler_with_callback(lua_State *LS) {
+#if LUA_VERSION_NUM >= 502
+    const char *msg = lua_tostring(LS, 1);
+    luaL_traceback(LS, LS, msg, 1);     /* push message with traceback.  */
+    lua_remove(LS, -2);                 /* remove original msg */
+#else
+    /* Return error message, unmodified */
+    (void)LS;
+#endif
+    return 1;
 }
 
 static void wslua_add_plugin(const gchar *name, const gchar *version, const gchar *filename)
@@ -523,7 +529,7 @@ static gboolean lua_load_script(const gchar* filename, const gchar* dirname, con
 
     lua_settop(L,0);
 
-    lua_pushcfunction(L,lua_main_error_handler);
+    lua_pushcfunction(L, error_handler_with_callback);
     /* The source argument should start with with '@' to indicate a file. */
     lua_pushfstring(L, "@%s", filename);
 
@@ -541,7 +547,23 @@ static gboolean lua_load_script(const gchar* filename, const gchar* dirname, con
             if (file_count > 0) {
                 numargs = lua_script_push_args(file_count);
             }
-            error = lua_pcall(L,numargs,0,1);
+            error = lua_pcall(L, numargs, 0, 1);
+            if (error != LUA_OK) {
+                switch (error) {
+                    case LUA_ERRRUN:
+                        report_failure("Lua: Error during loading:\n%s", lua_tostring(L, -1));
+                        break;
+                    case LUA_ERRMEM:
+                        report_failure("Lua: Error during loading: out of memory");
+                        break;
+                    case LUA_ERRERR:
+                        report_failure("Lua: Error during loading: error while retrieving error message");
+                        break;
+                    default:
+                        report_failure("Lua: Error during loading: unknown error %d", error);
+                        break;
+                }
+            }
             break;
 
         case LUA_ERRSYNTAX: