Do thread event callback errors affect coroutine.create?

[cheesycod]

Out of curiosity, am wondering if thread event callbacks affect coroutine.create

[khvzak]

Yes

[cheesycod]

@khvzak what are the semantics when a thread event callback returns an error when a coroutine.create is called (e.g. due to a thread limit code etc)

[khvzak]

The error will be propagated to caller. Same if you ran out of memory when creating a thread.

[cheesycod]

Alright. @khvzak

Also, is it known that the runtime error contains the thread and its id and not the error itself:

MAX_THREADS=100 target/release/mlua_scheduler_cli 
Running script: ["/home/frostpaw/mlua_scheduler/examples/bench.luau"]
Batch 1 / 5
Error: runtime error: thread: 0x55ce7b1cbfd8
stack traceback:
        [C]: in function 'create'
        [string "task"]:32: in function 'spawn'
        [string "/home/frostpaw/mlua_scheduler/examples/bench.luau"]:23: in ?
Output: Ok(Some(Err(RuntimeError("thread: 0x55ce7b1cbfd8\nstack traceback:\n\t[C]: in function 'create'\n\t[string \"task\"]:32: in function 'spawn'\n\t[string \"/home/frostpaw/mlua_scheduler/examples/bench.luau\"]:23: in ?"))))
Stopping task manager

Using the following thread event callback:

        lua.set_thread_event_callback(move |_lua, value| {
            match value {
                LuaValue::Thread(_) => count_v.set(count_v.get() + 1),
                _ => count_v.set(count_v.get() - 1),
            };

            if count_v.get() > max_threads {
                // Prevent runaway threads
                Err(mlua::Error::RuntimeError(
                    "Too many threads created, possible runaway detected".to_string(),
                ))
            } else {
                Ok(())
            }
        });

This also extends to luau code itself where the error returned from pcall is the thread object with coroutine.status of dead and not the error itself

[khvzak]

Also, is it known that the runtime error contains the thread and its id and not the error itself:

This is wrong, what you got is to_string representation of Thread, that passed as an argument to RuntimeError. Do you have a minimal example to reproduce? It should not be mlua issue

[cheesycod]

I don't have a MVP (unfortunately, as my scheduler code is pretty complex), but https://github.com/Anti-Raid/mlua_scheduler/blob/master/crates/cli/src/main.rs#L143 is my thread event callback code and https://github.com/Anti-Raid/mlua_scheduler/blob/master/examples/test_thread_limit.luau is my luau code. The result from pcall is that err is a thread and not the error itself

[cheesycod]

@khvzak btw, im getting segmentation faults with the latest build of mlua:

[khvzak]

I made few changes to the thread callbacks. They are now split between "creation" and "collection" callbacks because of very different requirements. The collection callback cannot return any errors, panic or make Lua calls. The Luau GC is very fragile and does not allow interaction with the VM in GC cycle.