How to use async eval in tokio multi-thread model

[yongman]

I use mlua with tokio async framework, use mlua async feature.
It works when I use eval() to chunk, but when I try to use eval_async(), it tells me as follows

error: future cannot be sent between threads safely
   --> src/server.rs:274:13
    |
274 |             tokio::spawn(async move {
    |             ^^^^^^^^^^^^ future created by async block is not `Send`
    |
    = help: within `Lua`, the trait `Sync` is not implemented for `*mut lua_State`
note: future is not `Send` as this value is used across an await
   --> src/tikv/lua.rs:101:48
    |
100 |         let chunk = lua.load(script);
    |             ----- has type `Chunk<'_, '_>` which is not `Send`
101 |         let resp: LuaValue = chunk.eval_async().await?;
    |                                                ^^^^^^ await occurs here, with `chunk` maybe used later
...
107 |     }
    |     - `chunk` is later dropped here
note: required by a bound in `tokio::spawn`
   --> /home/dev/.cargo/registry/src/gitproxy.zycloud.tk-1ecc6299db9ec823/tokio-1.17.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ^^^^ required by this bound in `tokio::spawn`

error: future cannot be sent between threads safely
   --> src/server.rs:274:13
    |
274 |             tokio::spawn(async move {
    |             ^^^^^^^^^^^^ future created by async block is not `Send`
    |
    = help: the trait `std::marker::Send` is not implemented for `dyn futures::Future<Output = std::result::Result<LuaValue<'_>, LuaError>>`
note: future is not `Send` as it awaits another future which is not `Send`
   --> src/tikv/lua.rs:101:30
    |
101 |         let resp: LuaValue = chunk.eval_async().await?;
    |                              ^^^^^^^^^^^^^^^^^^ await occurs here on type `Pin<Box<dyn futures::Future<Output = std::result::Result<LuaValue<'_>, LuaError>>>>`, which is not `Send`
note: required by a bound in `tokio::spawn`
   --> /home/dev/.cargo/registry/src/gitproxy.zycloud.tk-1ecc6299db9ec823/tokio-1.17.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ^^^^ required by this bound in `tokio::spawn`

How can I use mlua async feature with tokio multi-thread ?

Thanks.

[yongman]

And if I can use eval to execute async_functions？
It just return runtime error when I do call async_functions created by create_async_funtion in lua script.

(error) runtime error: attempt to yield across metamethod/C-call boundary
stack traceback:
	[C]: in ?
	[C]: in function 'yield'
	[string "_mlua_async_poll"]:9: in main chunk
	(tail call): in function <(tail call):-1>

[khvzak]

You need to use LocalSet to execute non-Send futures.

And it's not possible to use eval instead of eval_async to execute async functions because only eval_async returns Future.

[yongman]

Thanks for your reply.
It works after I use tokio LocalSet.

[yongman]

@khvzak Is it possible to use feature send in mlua to make the Chunk Sendable in order to use tokio spawn than spawn_local, because the performance is falling down after using LocalSet , it just consuming one cpu core.