Add component model support to the c-api

[seanisom]

Feature

Add component model support to the c-api (particularly the ability run a component from a c-api host).

Benefit

Much of the WebAssembly world is currently (re)tooling around the component model as it approaches MVP. The ability to run components will quickly become more critical with the finalization of the wasi preview 2 abi. Currently C/C++ is the largest fully-supported language with a clear path to producing component guests (through wit-bindgen) that does not have a path to run from a host in the same language.

Implementation

I can take a first pass at this modeled roughly around the Rust crate with the current c-api idioms (for example wasmtime_config_wasm_component_model(conf, true);, a component aware linker (perhaps wasmtime_linker_define_component_model(linker);), etc. I haven't looked deep enough into the implementation yet to see if there is significant core functionality to enable this past the standard glue code and lifecycle management for c objects.

Alternatives

Eventually this will become necessary - it's a matter of timing. I suspect that this has just not yet been implemented due to the instability of wasmtime::component.

CC: Discussed with @sunfishcode at the BA hackathon

[pchickey]

The Nginx Unit team is also interested in this

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[alexcrichton]

Thanks for opening an issue for this! You're right in that this hasn't quite bubbled to the top of any priority list yet which is why it isn't done, but it's something that definitely should get done!

My prediction for the main difficulty of this feature will be how to transfer values back and forth. The type grammar of core wasm is quite simple which made this a non-issue for core wasm's C API support, but for components it's significantly different. The easiest route is probably going to be to mirror Val into C more-or-less. This will not be really all that efficient in terms of calls because datatypes like lists will be copied in/out and everything will be dynamically type-checked. For example I don't know how wasmtime_func_call_unchecked could get a C/C++ equivalent that skips all the type-checks. That's not a showstopper for any sort of initial support, however. Still though there's tricky questions around how exactly memory management will work with values. I'm not sure the best answer here.

More minor it might be worth bikeshedding a bit to avoid a wasmtime_component_* prefix on everything which is quite long. Not that I have much of an idea about a better name...

[rajsite]

related #8036