[NEW] Add scripting languages (for EVAL, etc.) using module API

[zuiderkwast]

The problem/use-case that the feature addresses

1. Allow adding more scripting languages
2. Allow replacing the vendored Lua implementation with another one (e.g. LuaJIT or Lua 5.4)

Description of the feature

Looking at function.c, there is work started that should allows different "engines" for functions (FUNCTION CREATE, FCALL, etc.). For example, there is a function to register an engine. Currenly, Lua is the only engine, implemented in functions_lua.c. There is some separation here.

Extend the existing modularity to the module API: ValkeyModule_RegisterScriptingEngine or similar.

The module registers a callback that is invoked for executing code in commands like the EVAL, EVALSHA and FCALL.

In the beginning of an EVAL script, users can add a shebang, a line like #!lua and some optional flags or parameters, to select the scripting engine. This mechanism already exists, but currently, only "lua" exists. A module should be able to provide their own languages.

To add Lua engine in parallel to the built-in Lua implementation, the module can register with a different name like "lua5.4", "luajit". For a module to be able to replace the default "lua" engine, the built-in Lua support needs to be disabled. For that, see #1204.

Alternatives you've considered

...

Additional information

Related discussions:

• [NEW] Disable LUA integration during build #1204
• [NEW] Allow building with LuaJIT #1229
• [NEW] Support for wasm besides LUA as a scripting engine #387

[hpatro]

This will be a good addition to Valkey to provide the underlying abstraction to support new engines easily.

Few questions which comes to my mind and some discussed in the weekly meeting

1. Do we support multiple engines at a given point in time?
2. Do we plan to host other first party engines in the near future like V8 engine? KeyDB in the past created this: https://github.com/Snapchat/ModJS
3. If yes, do we route them based on the shebang (do we fail the script without shebang ?)
4. or introduce new command/sub-command for each engine ?
5. We would need certain API to describe the supported engine and it's version.

[neomantra]

Allow replacing the vendored Lua implementation with another one (e.g. LuaJIT or Lua 5.4)

One cannot easily create parallel Lua engines. This is due to symbol collisions with the statically linked Lua, discussed here and here. Disabling the Lua support makes this straight-forward at the expense of losing the built-in Lua. Otherwise, module implementors must carefully move their Lua symbols to not collide (I never tried this). But then they can't use system-installed Lua libraries in their modules (maybe that's fine).

At least once it is figured out for one Lua module, it will be figured out for all. Then it's a documentation issue =). Valkey could rename its Lua symbols since it is building and statically linking Lua from source. I'm not sure what that might break elsewhere.

I hadn't tried this since 2016, but since I just upgraded the valkey-mod_luajit, I just loaded it into a Lua-enabled Valkey and it certainly segfaults by using the wrong symbols.

[zuiderkwast]

@hpatro

1. Do we support multiple engines at a given point in time?

Yes.

2. Do we plan to host other first party engines in the near future like V8 engine? KeyDB in the past created this: https://github.com/Snapchat/ModJS

What do you mean by host? Officially support or vendor? It's not impossible. I have no answer.

3. If yes, do we route them based on the shebang (do we fail the script without shebang ?)

ModJS adds a new command EVALJS. Modules can always add their own commands, but the idea here is to provide an API for modules to hook in to EVAL and FUNCTIONs. We can extend this to triggers or events of some sort.

Scripts without a shebang are Lua scripts for backward compatibility, at least by default. We could add a config to change the default engine though, but I imagine that all other languages will use a shebang.

4. or introduce new command/sub-command for each engine ?

They should be able to take advantage of the framework provided by EVAL + EVALSHA + SCRIPT LOAD, FUNCTION CREATE + FCALL, etc.

There's a difference between scripts and functions. Scripts are part of an application and are written by the application developers while functions are assumed to be installed by a database admin. The caller of the function doesn't need to know which language the function was written in.

5. We would need certain API to describe the supported engine and it's version.

Yes. Do you have a suggestion? INFO? A new subcommand of FUNCTION or SCRIPT?

[zuiderkwast]

One cannot easily create parallel Lua engines. This is due to symbol collisions with the statically linked Lua, discussed here and here.

@neomantra We discussed this, but we were not sure why. Dynamically linked symbols don't collide with statically linked symbols, do they? Does Lua itself use dynamic linking for its modules?

Worst case, we can only have one Lua at a time. 😢

[hpatro]

2. Do we plan to host other first party engines in the near future like V8 engine? KeyDB in the past created this: https://github.com/Snapchat/ModJS

What do you mean by host? Officially support or vendor? It's not impossible. I have no answer.

Yeah, I was trying to see what's Valkey's stance on supporting other scripting engines.

[neomantra]

Dynamically linked symbols don't collide with statically linked symbols, do they? Does Lua itself use dynamic linking for its modules?

Yes, since they use the same exact symbol names and the linker can't disambiguate. LuaJIT is intended to be a drop-in replacement for Lua 5.1. A Lua 5.4 load would by be similar. There are common names like lua_State and all the API functions.

I had a bit of a conversation with Claude (for this chat, "better" IMO than ChatGPT) on how to get around it and it is tricky and platform-specific -- don't have a sharing account but wasn't something I could do in 2016. Not sure if it is gauche to share prompts:

• can I use statically linked PUC Lua 5.1 and dynamically linked LuaJIT in the same process?
• can you tell me more about the symbol space collision?
• what about a dynamically linked PUC Lua with a dynamically linked LuaJIT?
• can I hide luajit inside the shared library?
• but would my shared library know to use the luajit symbols instead of the static symbols?

I did test this on ARM/OSX versus x64/Linux and got segfaults on both.

I realized I quoted the wrong section earlier and my reply was meant to suggest you shouldn't make this a goal:

To add Lua engine in parallel to the built-in Lua implementation, the module can register with a different name like "lua5.4", "luajit".

[rjd15372]

@zuiderkwast I would like to implement the WASM engine using this approach. I can include the module API changes as part of the work I'm doing with WASM.

[zuiderkwast]

@rjd15372 sounds great, but I'd prefer a separate PR for only the module API and a dummy engine module for testing that just returns the script code back or something.

[rjd15372]

@zuiderkwast sure, I wasn't implying that all work would be in a single PR. I was thinking in the same lines as you.

[rjd15372]

@zuiderkwast @madolson I opened a PR #1277 with the changes to the module API.