Contribution towards Emscripten + Wasi

[jeff-hykin]

Summary

Is it practical for a outsider/non-major-contributor to implement the emscripten + wasi environment (relevent code snippet here)?

Additional details

Current plan is to combine this wasi env with the equivlent emscripten env and just rename them to match the ___wasi_ naming

I only need it for a specific usecase, not a stable feature. Someone compiled opencv to wasm and I just want to get that working at an alpha version in Rust. I know some about implementing wasm imports in JS, which wasnt too difficult, but I don't know much about the internal structure of the wasmer crate or the wasmer runtime.

[Hywan]

Hello,

Thanks for the question.
Emscripten and WASI are really different ABI, and that's why Wasmer doesn't support both at the same time. Wasmer support Emscripten, or WASI, but separately.

Does your opencv project have been compiled to Wasm with Emscripten only?

[jeff-hykin]

Its not mine, I just pulled the wasm file out from here

It is definitely Emscripten as the main ABI, but it has a whole page of ____wasi__*function name* imports like file descriptor close, alloc, etc

[Hywan]

Well OK, I'm sorry but it's not on our agenda to support both ABI for the moment. As far as I know, it's rather unusual to get two of them at the same time (it's likely that I'm wrong here!).

Contributions are welcomed as it was your first question! I suppose it's doable, as we use the same ImportObject API. We can generate a first ImportObject with the WASI API, then generate second ImportObject with the Emscripten API, to finally merge both of them into a single ImportObject. I don't see any particular blocker, except maybe the way we manage the different environements, but I believe it's compartmentalised per imports, so it may not be an issue. @MarkMcCaskey can confirm that.

[jeff-hykin]

Okay thanks! I'll look for Mark's reply and if there's no obvious reason why it would fail I'll slowly move along on this feature.

Maybe there's a more simple way to do it like you're describing (I'm not sure how to combine import objects). I've just duplicated the Emscripten folder, and been merging the WASI folder files/code into it. It's been pretty straightforward, the only thing they have conflicting definitions of is WasmPtr, and just renaming to "WasiWasmPtr" will hopefully resolve that. There's some semi-complicated module import issues I'm working on, but that's more of a Rust issue/complication.

I agree I don't know why the API's are mixed together, but that link in the first post to the dedicated error message for this situation I guess suggests it's somewhat common.

[jcaesar]

As far as I know, it's rather unusual to get two of them at the same time

Not really: #1977 (comment) (Just linking this to organize related issues. I suspect you already know.)