Understanding the differences between Spin and Wasmtime in wasip2 imports resolution

[alepvn33]

Hello everyone,

I am relatively new to Spin, so I am trying to understand how to use it. This issue is mainly a request for a technical explanation more than a problem with Spin.

I was studying the http-rust example (https://github.com/spinframework/spin/tree/main/examples/http-rust).
I noticed that while Spin can run the compiled module, Wasmtime cannot. When running the module with Wasmtime, I get:

Error: failed to run main module 'http_rust.wasm'

Caused by:
    0: failed to instantiate "http_rust.wasm"
    1: unknown import: 'wasi:io/streams@0.2.0-rc-2023-10-18::[resource-drop]output-stream' has not been defined

Since Spin is built upon Wasmtime, why is Spin able to provide this import while Wasmtime isn't? Wasmtime includes a wasi-io crate in its codebase (https://github.com/bytecodealliance/wasmtime/tree/3cc0023cb60295ee15931d40b266d723f4598eba/crates/wasi-io), so I would expect it to be able to run the example module as well.

Does anyone know why there is this difference in behavior between Spin and Wasmtime when it comes to resolving wasip2 imports?

Thanks in advance for your help :)

[lann]

The key is in the version of wasi:io/streams@0.2.0-rc-2023-10-18

Wasmtime doesn't try to maintain stable support for old RC versions while Spin does. This is just a difference in audience; Wasmtime is more of a foundational library while Spin is more of an end-developer-friendly tool.

[alepvn33]

I see. So, how can I compile to the latest versions of the proposal? Is there a way I can control which version I will end up using?

[lann]

It looks like the http-rust example is using an older version of the Rust spin-sdk crate. If you use a newer release of that crate it should import stable (non-rc) 0.2 interfaces.

Note that the patch version (0.2.0 vs 0.2.1) shouldn't matter for compatibility with wasmtime because versions are matched via (modified) semver compat rules: https://docs.rs/wasmtime/latest/wasmtime/component/struct.Linker.html#names-and-semver (but any prerelease tag like -rc* prevents this behavior)