RE: Multiple related lua states

Thanks Andrew,

I’m quite new to lua (just investigating) so excuse me if my questions are a bit tame.

Does using lua threads actually imply that I must use a thread? In the model I am suggesting, only one application is ‘live’ at a time – you can imagine a main menu with apps hanging off it. You go into each app one at a time. No application concurrency.

Can application access the threads global table (and by implication) access another application’s data? Or can this be prottected in the manner you have described with the global table?

Many thanks,

Greg.

PS. I have cc’d this back to the mainling list.

From: Andrew Teirney [mailto:andrew@teirney.net]
Sent: Wednesday, 24 May 2006 4:08 PM
To: Greg McCreath
Subject: Re: Multiple related lua states

Hi Greg,

    I myself have been aspiring to do something like this for stuff i work on in the embedded hardware/pos sector. I think there are several approaches to doing this, ultimately i think the implemenation comes down to what the embedded environment supports and what level of performance you are wanting.

    The approach i would probably take is using system threads and implement the lua_lock/lua_unlock macros to protect the atomic regions of the lua core. For each logical application i would create a new lua state off the master state, this being done with lua_newthread. At this point here you have a new thread of lua execution that has nothing within its global state. from there i would use metamethods to enable access to the "system" global table when the "logical application" global table doesn't have the required entry. Depending on your requirements you may/or may not need to provide means via metamethods to mutate the behaviour of the "local application" table to prevent overriding the global table entries, also i would have to suggest that the "system" global table and its children are protected from accidental overwritting/addition using metamethods and perhaps a proxy table.

    Naturally since i have not implemented this, just have done the research in the past it may or may not fit the requirements you have.

    As for when the application shuts down, getting access to the applications data/state that would be simply stored in the "threads" global table. So you can interogate that prior to the thread being destroyed, ultimately the threads global table would be garbage collected, so unless persisted you'd need to maintain a reference to it should you want to be able to reuse it.

    Alternatively if you don't support a preemptive multi threaded system and are happy with cooperative multi threading you could use coroutines and a simple scheduler.

    Hopefully this has provided some for of help to get to where you want to.

--
Andrew
Auckland, New Zealand

Greg McCreath wrote:

Hi All,

I am investigating using lua in an embedded environment where application separation and security is very important. The environment runs multiple logical applications within a single proprietary VM.

Each logical application has a virtual file system and cache etc etc. Applications will be able to create and run script. Lua seems very good for providing scripting in this environment. Except …

I want a lot of common scripted functionality to be available to all applications. But I do not want to have multiple lua states each with a lot of code loaded.

I want a single lua state with all common functions and data loaded in a central location and then each application to be able to operate on its **own** ‘virtual’ lua state without knowledge of other applications. This really does have to be secure. I do not want applications altering the global in any way unless it is using a function that exists in the global state.

As the application shuts down I also want to be able to get of an applications data and application state.

Can I achieve this?

Greg McCreath
Chief Technical Officer
TAFMO Limited
ABN: 94 109 766 592

Level 8, 342 Flinders Street
Melbourne
Victoria, 3000
Australia

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