Re: Problems with lifecycle of states

["Peter Cawley" <lua@...>]

The comment on the second line might explain why they are not garbage collected:
lua_pushvalue(hostLuaState, -1);        // Prevent the new thread from
being garbage collected

If a lua object (in this case a thread) is in a stack, being a key in
a table or being a value in a table, then it will not be garbage
collected. If you want it garbage collected:
1) Make sure it isn't on the stack anywhere (the stack of the main
state, or the stack of any threads)
2) Make sure it isn't the key or value of a key/value pair in a table

Removing a key from a table is the same as removing a key/value pair;
"x.y = nil" removes the "y" key and the associated value from "x".

Weak tables (http://www.lua.org/manual/5.1/manual.html#2.10.2) are an
exception to this rule, in that a table can be defined so that the
keys or values in the table are not counted as references to objects.
When a key or value of a key/value pair is garbage collected, then the
entire key/value pair is removed from the table automatically.

On 30/10/2007, Tom Miles <Tom@creative-assembly.co.uk> wrote:
> Hi,
>
> Having just started using Decda to debug my lua project I've noticed
> that my threads are never getting garbage collected.  I've searched the
> archives about this, and found some vaguely relevant posts, but none
> which can address my situation, so apologies if I've missed something.
>
> The states are set up thusly:
>
> state.m_state = lua_newthread(hostLuaState);
> lua_pushvalue(hostLuaState, -1);        // Prevent the new thread from
> being garbage collected
>
> // Add to our list of registered states, keeping a reference to the
> state so we can remove it later on
> m_RegisteredStates.push_back(ThreadRef(&state, hostLuaState,
> luaL_ref(hostLuaState, LUA_REGISTRYINDEX)));
>
> // Retrieve our lookup table from the global index table
> lua_pushstring(m_MainState, "lookup");
> lua_gettable(m_MainState, LUA_GLOBALSINDEX);
>
> // Set lookup[state.m_state] = &state
> lua_pushlightuserdata(m_MainState, state.m_state);
> lua_pushlightuserdata(m_MainState, &state);
>
> lua_settable(m_MainState, -3);
> // Remove the table from the stack
> lua_pop(m_MainState, 1);
>
>
> And "freed":
>
> CA_STD::LIST<ThreadRef>::iterator iter =
> std::find_if(m_RegisteredStates.begin(), m_RegisteredStates.end(),
> Finder(&state));
> if (iter != m_RegisteredStates.end())
>         luaL_unref((*iter).host, LUA_REGISTRYINDEX, (*iter).ref);
>
> This has been working for ages, but I realised I still used the lookup
> table, so I added:
>
> // Remove the state from the lookup table by setting
> lookup[state.m_state] = nil
> lua_pushstring(m_MainState, "lookup");
> lua_gettable(m_MainState, LUA_GLOBALSINDEX);
> lua_pushlightuserdata(m_MainState, state.m_state);
> lua_pushnil(m_MainState);
> lua_settable(m_MainState, -3);
>
> Which has added lots of random lockups and crashes within lua, usually
> during garbage colled.  It would seem that if I remove the value from my
> lookup table then states are being pulled from under my feet.  Or the
> lookup table removal code is corrupting the state somehow.
>
> So, a couple of questions:
> 1.  What am I doing wrong?
> 2.  If I use the state as a key in a table, does that count as a
> reference to the object as far as the GC is concerned?  If so, how do I
> remove a key from the table?
>
> Any help would be very greatfully recieved.
>
> Tom
>
>
>
> Tom Miles
>  creative assembly ----------------------------------------------------------------------------
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