Well, when I was talking about this in #lua on Freenode I imagined a scenario where someone would break out of a sandbox to get at debug.setmetatable() and then cause a segfault. I was just trying to get across the point that I'd rather have someone get at os.exit() than cause a segfault -- as you may have atexit() or on_exit() handlers or some other form of cleanup you want to happen. A segfault kind of prevents that..
Anyway -- a lot of people seem to disagree with me that a segfault should be guarded against. I've heard arguments that asserting the type with checkudata() and also the size of the userdata before accessing members would make code inefficient -- and that my worries about people causing segfaults through the debug library are unfounded as you can just make all references to the debug library disappear, etc... Sean said earlier that that there are 0 uses of rawlen() or objlen() in Lua's core used for the sort of checking I was talking about -- I'm assuming he's advocating this because upstream doesn't do it? I still disagree but all of this is kind of disconnected from what I was talking about in my original posting.
typedef union Udata {
L_Umaxalign dummy; /* ensures maximum alignment for `local' udata */
struct {
CommonHeader;
struct Table *metatable;
struct Table *env;
size_t len; /* number of bytes */
} uv;
} Udata;
Now you see, there are 2 fields/members in the userdata struct. One for the metatable and one for the environment. debug.setmetatable() sets the metatable pointer, not the env pointer. What I want is not easily possible without changing the definition of the Udata struct -- so I've given up on that since I like Roberto's idea to just contain all userdatas within one shared env table. :-) lua_get/setuservalue() set the env pointer, the Table pointer.
Does anyone happen to know where else the env pointer is used? What other parts of Lua make use of it and why? I can't easily grep for 'env' and find out. Is this something historic from previous versions of Lua? I must be missing something obvious, but what is the env table on a userdata for? As far as I see it's simply for use with get/setuservalue(), to make a GC association. That's already very useful, but does it affect the use of the userdata elsewhere?
