Lua: global table to store state between threads

[linux-china]

Description:
Lua state is per worker and it will be non-trivial/impossible to share it between threads. A global storage for Lua is to provide a Lua API that wraps a metadata/table that calls into C++ code with proper locking to set/get items in a global table. All Lua code can access global table and share state in different Envoy Lua filters.

The original request is from #4008

[JulieMei325]

Is there any update on this feature support?

[kyessenov]

Should Envoy have a generic mechanism for filters to share state between workers? This is a common concern.

[jplevyak]

We have SharedData in C++. This acts like a global KV store.

[jplevyak]

In the Proxy-Wasm C++ SDK.

[patrickcarnahan]

Any updates on this issue? Shared state in lua would negate a potential requirement to utilize c++ simply to achieve shared state.

[ggreenway]

@patrickcarnahan nobody has showed interest in working on it or made a PR. But I don't think there's a technical reason not to do it.

[dxps]

This is also available in proxy-wasm-rust SDK and others as well.

Now, before diving into understanding how this can be done, I am wondering: did anyone evaluate the response times of a Lua vs non-Lua (like Rust, which I'm using right now) implementation ❔

I'm planning to do this, except that I really wanted to use this set & get shared data feature. Without it in Lua, I think I can happily live with a Rust based implementation, but I'm still curios to see or do and have any comparison results.