open upvalues are never black

upvalue的类型为

typedef struct UpVal {

  CommonHeader;
  TValue *v;  /* points to stack or to its own value */
  union {
    TValue value;  /* the value (when closed) */
    struct {  /* double linked list (when open) */
      struct UpVal *prev;
      struct UpVal *next;
    } l;
  } u;

} UpVal;

当其为open时，是对当前栈上数据的引用.

lua采用mark and sweep GC. 其中main invariant (white objects cannot point to black ones)

新创建的upvalue默认为关闭的，并加入allgc

UpVal *luaF_newupval (lua_State *L) {
  UpVal *uv = &luaC_newobj(L, LUA_TUPVAL, sizeof(UpVal), NULL, 0)->uv;
  uv->v = &uv->u.value;
  setnilvalue(uv->v);
  return uv;
}