Added `wasm32-wasi` support

[martindevans]

It looks like mlua already supports wasm through wasm32-unknown-emscripten, would it be possible to add wasm32-wasi support as well?

[khvzak]

I would love to support wasi, but currently the blocker is exception handling
See notable limitations on the wasi-sdk page
c++ exceptions or setjmp/longjmp is required for Lua VM

[martindevans]

Thanks for the response.

How does wasm32-unknown-emscripten work around the lack of exceptions or setjmp/longjmp?

[khvzak]

How does wasm32-unknown-emscripten work around the lack of exceptions or setjmp/longjmp?

Seems it support native wasm exceptions if enabled, or javascript exceptions otherwise.
See setjmp-longjmp and C++ exceptions support pages

[Bobbyjoness]

I would love to support wasi, but currently the blocker is exception handling See notable limitations on the wasi-sdk page c++ exceptions or setjmp/longjmp is required for Lua VM

That page links to this. Idk if that is a recent change. Either way this should make it possible to target wasi? I tried modifying lua-src myself but honestly I couldn't figure it out.

[bgmerrell]

I've added PoC wasi support for Lua 5.1 in my repos here:

https://github.com/bgmerrell/mlua
https://github.com/bgmerrell/lua-src-rs

Support for wasm32-wasi currently requires the main branch of wasmtime (or v37+ when released).

My mlua fork has instructions on how to build.

I'd be happy to work to get these changes upstreamed if that's desired, but I don't know how close my approach is to something that would be accepted. My use case is only for Lua 5.1 (and maybe Luau), and AI helped with a lot of the build code and commands (so there's probably some cleanup to do).

[alexcrichton]

I was curious to kick the tires on WASI support for Lua today, and similar to @bgmerrell I managed to get everything working (much to my own surprise!). I wanted to follow-up here and ask some quetions and/or provide some information about this. The high-level question I'd like to ask is if it would be possible, and how best, to support WASI. Answering such a question though requires going through a few details first, so here's what I've learned today:

• Lua fundamentally requires setjmp and longjmp which isn't supported in WebAssembly (off-the-web) until the exception-handling proposal. As @bgmerrell notes this was recently added to Wasmtime (hence my personal interest in this at this juncture), and LLVM has supported setjmp and longjmp for some time. LLVM's support requires a matrix of flags and understanding, however. As otherwise noted in this thread the main WASI C toolchain, wasi-sdk, does indeed support setjmp/longjmp as LLVM expects. I've attempted to refine the documentation here but it suffices to say that "with some extra C flags that can be specified in lua-src-rs/src/lib.rs both setjmp and longjmp now work.
• Lua requires functionality that wasi-libc does not provide. For example Lua supports the system function as well as popen, neither which are supported in wasi-libc (which is the underlying implementation detail of wasi-libc). The Lua source is easy enough to edit and say "return an error on WASI targets" and this brings me to my first question: would lua-src-the-crate support patches to the lua source code? If not this'll need to be patches to the upstream lua source itself, if so, however, the patch size isn't too large.
• Some refactoring will be necessary for various metadata/etc printed by lua-src-the-crate because WASI support will require a few extra native libraries (for example -lsetjmp needs to get passed somewhere). I don't expect this to be too contentious, just wanted to write this down.
• As-is mlua cannot be compiled with -Ctarget-feature=+exception-handling. While that's not the end of the world because setjmp/longjmp require exceptions one way or another, personally I'm interested in having this "work by default" with the exception-handling instructions and not requiring much ceremony. To that end the major blocker for mlua is a codegen error in rustc where the C-unwind ABI in Rust currently requires +exception-handling in WebAssembly to work correctly. I've sent Skip cleanups on unsupported targets rust-lang/rust#146457 to resolve this on Rust's side which means that mlua builds by default. The mlua crate will have more instrumentation if +exception-handling is passed but it won't be required to get the crate working (basically IMO it should be an orthogonal decision to compile rust code with that feature, and this helps disentangle "does rust emit standard exception instructions, or legacy ones?")

All-in-all I have a lightly edited examples/repl.rs working locally in Wasmtime (replacing rustyline-the-crate with a simple std::io implementation). The patches to lua-src are the largest ones (but still somewhat modest). Thus the main question I'd have for this repository is: how acceptable is it to patch the lua source code? I'm happy to help send PRs, work with CI, add tests, work through questions, etc, as well.

[khvzak]

Thanks for the information and poc how to add wasm32-wasi support to mlua! I have not looked to this problem yet (will do soon) but I'm looking forward to adding the WASI support.

The Lua source is easy enough to edit and say "return an error on WASI targets" and this brings me to my first question: would lua-src-the-crate support patches to the lua source code? If not this'll need to be patches to the upstream lua source itself, if so, however, the patch size isn't too large.

I'm supporting the patching lua-src code and seems it's much easier option than upstreaming them. Especially given that Lua < 5.4 unlikely will have changes backported. My only ask is to keep the Lua 5.4 diff for reference, to apply in case the source code will be updated to a next version (5.4.9+)

I'm happy to accept relevant PRs.

[alexcrichton]

As a follow-up with the lua-src changes I was able to use mlua to make Lua-based components. Just a demo, but works well! Thanks @khvzak!

[bgmerrell]

Very cool! Super happy to see this merged. Thanks @alexcrichton and @khvzak !!