Re: Deriving In Lua: Weak Ref Access Crashes After Garbage Collection

Hey,

just have seen that the "[luabind] weak_ref issue" topic already
describes exactly this problem/bug, and Max McGuire already submitted
a patch here: "[luabind] weak_ref issue patch". Sorry for the double
post.

I've tested the patch and this perfectly solves the
problem/testcase/crash below. Any chance that the patch gets verified
and applied soon in the github?

Best regards,
Eduard


On Wed, Sep 29, 2010 at 15:18, eduard mueller

<[email protected]> wrote:
> Hey all,
>
> first a big thanks to Daniel Wallin & co for the great work on
> luabind! I'm a former boost::python user, so switching over to luabind
> was a breeze and am enjoying luabind a lot.
>
> Unfortunately I'm now having some troubles with the weak refs in
> luabind::wrap_base, and I'm not sure if its just me being stupid or a
> bug in either Lua or Luabind. Maybe you guys could give me a hint.
>
> What I basically do, is inheriting from a wrap_base'd class in Lua.
> Then access the lua class objects from within C++ via the wrap_base
> (which will resolve the lua object with its weak_ref). Most of the
> time this works just fine as expected and documented here:
> http://www.rasterbar.com/products/luabind/docs.html#deriving-in-lua
>
> But sometimes (really sometimes) the garbage collector seems to
> destroy the weak_refs that do point to the lua class object while the
> lua and C++ objects are still alive. I could not find out what
> destroys the weak_refs, just know that they are gone at some point. So
> accessing the Lua objects from C++ results into a crash sooner or
> later.
>
> Below is a test case for the problem. Crash was verified with luabind
> 0.9 and Lua 5.1 (Visual CPP and GCC, debug and release builds)
>
> To test, simply run the resulting binary. It should crash sooner or
> later in "call_member", because object_rep can't be resolved anymore
> (points to nil or a number - some junk?).
> "local dummy2, dummy3, dummy4" in the tests make the problem just more
> likely. Adding a "collectgarbage()" and the end of each iteration
> avoids the crash.
>
> Unfortunately its more or less impossible for me to debug this in
> luabind, because this happens more or less randomly at "some point"
> after garbagecollection. So that's all I know so far.
>
>
> Thanks for reading and any hints!
>
> Best regards,
> Eduard Müller
>
>
>
> ===<<< WeakRefTest.cpp
>
> extern "C"
> {
>  #include "lua.h"
>  #include "lualib.h"
> }
>
> #include <luabind/luabind.hpp>
>
>
> // =============================================================================
>
> class CppClass
> {
> public:
>  virtual std::string virtual_func() { return "CppClass"; }
> };
>
> class CppClassWrapper : public CppClass, public luabind::wrap_base
> {
> public:
>  virtual std::string virtual_func()
>  {
>    return call<std::string>("virtual_func");
>  }
>
>  static std::string default_virtual_func(CppClass* ptr)
>  {
>    return ptr->CppClass::virtual_func();
>  }
> };
>
> static std::string call_virtual_func(CppClass* ptr)
> {
>  return ptr->virtual_func();
> }
>
>
> // =============================================================================
>
> int main(int, char*)
> {
>  lua_State* L = lua_open();
>  luaL_openlibs(L);
>
>  luabind::open(L);
>
>  luabind::module(L)[
>    luabind::def("call_virtual_func", call_virtual_func),
>    luabind::class_<CppClass, CppClassWrapper>("CppClass")
>      .def(luabind::constructor<>())
>      .def("virtual_func", &CppClass::virtual_func,
> &CppClassWrapper::default_virtual_func)
>  ];
>
>  // segfaults sooner or later while calling "call_virtual_func":
>  luaL_dostring(L,
>    "class 'LuaClass'(CppClass)\n"
>    "function LuaClass:__init()\n"
>    "  CppClass.__init(self)"
>    "end\n"
>    "function LuaClass:virtual_func()\n"
>    "  return 'LuaClass'"
>    "end\n"
>    "\n"
>    "for i=1,10000 do\n"
>    "  local dummy = LuaClass()\n"
>    "  local dummy2 = LuaClass()\n"
>    "  local dummy3 = LuaClass()\n"
>    "  local dummy4 = LuaClass()\n"
>    "  assert(dummy:virtual_func() == 'LuaClass')\n"
>    "  assert(call_virtual_func(dummy) == 'LuaClass')\n"
>    "end\n");
>
>  lua_close(L);
>
>  return 0;
> }
>
>
> ===>>> WeakRefTest.cpp
>

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