Weird Lua garbage collector behavior.

testme.c

#include <stdio.h>

#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"

static int outergc(lua_State *L) {
  void * n = lua_touserdata(L, lua_upvalueindex(1));
  void * o = lua_touserdata(L, -1);
  printf("Outer gc called on %p with upvalue = %p\n", o, n);
  return 0;
}

static int innergc(lua_State *L) {
  void * n = lua_touserdata(L, -1);
  printf("Inner gc called on %p\n", n);
  return 0;
}

int main() {
  lua_State *L = luaL_newstate();
  luaL_openlibs(L);

  lua_checkstack(L, 10);
  
    lua_newtable(L);
  // 1 = {}
  lua_pushstring(L, "_proxy");
  // 1 = {}, 2 = "_proxy"
  void * o = lua_newuserdata(L, 1);
  // 1 = {}, 2 = "_proxy", 3 = proxy
  printf("Created outer object: %p\n", o);
  lua_newtable(L);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}
  lua_pushstring(L, "__gc");
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc"
  void * n = lua_newuserdata(L, 1);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = upvalue
  printf("Created inner object: %p\n", n);
  lua_newtable(L);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = upvalue, 7 = {}
  lua_pushstring(L, "__gc");
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = upvalue, 7 = {}, 8 = "__gc"
  lua_pushcfunction(L, innergc);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = upvalue, 7 = {}, 8 = "__gc", 9 = innergc
  lua_settable(L, 7);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = upvalue, 7 = {__gc = innergc}
  lua_setmetatable(L, -1);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = upvalue_with_gc
  lua_pushcclosure(L, outergc, 1);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = {}, 5 = "__gc", 6 = outergc_with_upvalue_with_gc
  lua_settable(L, 4);
  // 1 = {}, 2 = "_proxy", 3 = proxy, 4 = { __gc = outergc_with_upvalue_with_gc }
  lua_setmetatable(L, 3);
  // 1 = {}, 2 = "_proxy", 3 = proxy_with_gc
  lua_settable(L, 1);
  // 1 = { _proxy = proxy_with_gc }
  lua_pop(L, 1);
  // empty stack
  lua_gc(L, LUA_GCCOLLECT, 0);
  
  printf("Closing...\n");

  lua_close(L);
}