Curious : Thread safety?

[shohnwal]

I'm working with the Unreal 4 engine, which uses seperate threads in C++ for gameplay tasks and AI-stuff. My sol2-lua state is in the gameplay-thread.

I read on stackoverlow that normally accessing the lua state is not thread safe and can even mess up the lua stack.

I'm wondering : Is sol2s implementation of sol::thread and sol::coroutine providing thread-safety the the lua state?
If not, could you provide any advice how accessing a sol2 lua state from several threads could be done (mutexes in c++?)?

Right now i'm calling the lua state from the ai thread via

sol::thread thread = sol::thread::create(*this->GameModeLua);
sol::state_view view = thread.state();
sol::coroutine function = view["decide_command"];
sol::protected_function_result r = function(uniqueID, characterID, sol::as_table(this->combatevents));

but i'm unsure whether this actually provides any thread-safety.
The documentation about coroutines and threads
http://sol2.readthedocs.io/en/latest/api/thread.html
http://sol2.readthedocs.io/en/latest/api/coroutine.html
have sentences like "see thread for an example on how to get a coroutine that runs on a thread separate from your usual “main” lua_State." and "sol::thread is a separate runnable part of the Lua VM that can be used to execute work separately from the main thread," but it's not very clear if that means that these are c++ thread-safe.

Or should i use sol::main_thread( ... ) for a multithreaded c++ program?
If so, could you give an example how to use it? The documentation doesn't have any examples for this one.

If it's important, i'm using LuaJit.

[ThePhD]

Lua has no thread safety. sol2 does not force thread safety bottlenecks anywhere.

Assume any access or any call on Lua affects the whole global state (because it does, in a fair bit of cases). Therefore, every call to a state should be blocked off in C++ with some kind of access control, when you start hitting the same state from multiple threads. Individual Lua coroutines might be able to run on separate C++-created threads without tanking the state utterly, since each Lua coroutine runs on a thread, and that thread has its own stack and some other associated bits and pieces that won't quite interfere with the global state.

The sol::thread documentation needs a real overhaul, and I do need a multithreading section in the top level. I haven't gotten around to it because, to be quite frank, I have no idea what the best practices are.

I'm going to leave this issue open for other people to put their ideas here, as well as mark that I need to update the docs with some more concrete examples about multithreading (another person ask this question not more than 2 days ago, too).

[ThePhD]

Threading tips: http://sol2.readthedocs.io/en/latest/threading.html

This is about the best I can do. Maybe somebody else has some tips and tricks...? But until then, closing things out...