Stream: wasi

Topic: Help: Wasmtime's runner for WASI-HTTP p2 guest

Chris Suszynski (Jul 01 2025 at 13:33):

Hi there. I'm trying to develop a PoC of having the Wasmtime-based runner to execute wasi-http p2 guest. For the guest, I tried doing a similar thing that Dan did here: https://github.com/sunfishcode/hello-wasi-http, but instead of generating all the bindings from WIT files, I tried using wasi 0.14 crate.

Here you could see my code:

• runner
• guest

While trying to execute the runner, I get:

Error: component imports instance `wasi:cli/environment@0.2.4`, but a matching implementation was not found in the linker

Caused by:
    0: instance export `get-environment` has the wrong type
    1: function implementation is missing

Does this could be done, in a way I'm trying? Can someone help with this?

bjorn3 (Jul 01 2025 at 13:59):

You will likely need to call wasmtime_wasi::p2::add_to_linker_async on the linker too. Your wasm component depends on the wasi-cli interfaces like all components compiled for the wasm32-wasip2 rustc target.

Lann Martin (Jul 01 2025 at 14:00):

You are linking wasmtime_wasi_http into your host but that doesn't include all of the interfaces needed by Rust wasi targets. You would either need your guest to target wasm32-unknown-unknown or ... do what bjorn3 just said :slight_smile:

Chris Suszynski (Jul 01 2025 at 15:07):

@Lann Martin I'm building the guest like this: build/tasks/build.go#L24-L25, so using the wasm32-wasip2 target. So, I thought it would have those interfaces.

Lann Martin (Jul 01 2025 at 16:13):

Yes, by targetting wasm32-wasip2 your guest is implicitly importing a bunch of wasi interfaces that aren't currently provided by your host, which currently only implements the wasi:http/proxy world by linking with wasmtime_wasi_http.

Lann Martin (Jul 01 2025 at 16:16):

I haven't worked on this for the latest versions of wasmtime but I think you would need to switch to instead use these two functions:

• https://docs.rs/wasmtime-wasi/34.0.1/wasmtime_wasi/p2/fn.add_to_linker_async.html
• https://docs.rs/wasmtime-wasi-http/34.0.1/wasmtime_wasi_http/fn.add_only_http_to_linker_async.html

Chris Suszynski (Jul 01 2025 at 17:57):

@Lann Martin Yes, it works!! Thanks a lot! You can see the change here: https://github.com/cardil/knative-serving-wasm/pull/4

Chris Suszynski (Jul 01 2025 at 18:09):

The wasmtime_wasi_http crate, shows in the usage example - https://docs.rs/wasmtime-wasi-http/34.0.1/wasmtime_wasi_http/index.html to just use:

wasmtime_wasi_http::add_to_linker_async(&mut linker)?;

But that does not work either. I wonder if the doc is wrong on this? Probably yes, as the tests are using this working tandem (I should have been looking at the tests, they never lie!):

https://github.com/bytecodealliance/wasmtime/blob/c953cc8fd16df3ff73853101402f343436ab4cef/crates/wasi-http/tests/all/p2/async_.rs#L15-L16

Chris Suszynski (Jul 01 2025 at 18:10):

Once again, thanks for your help, folks! I appreciate it very much!

Ralph (Jul 01 2025 at 18:15):

Chris Suszynski said:

The wasmtime_wasi_http crate, shows in the usage example - https://docs.rs/wasmtime-wasi-http/34.0.1/wasmtime_wasi_http/index.html to just use:

wasmtime_wasi_http::add_to_linker_async(&mut linker)?;

But that does not work either. I wonder if the doc is wrong on this? Probably yes, as the tests are using this working tandem (I should have been looking at the tests, they never lie!):

https://github.com/bytecodealliance/wasmtime/blob/c953cc8fd16df3ff73853101402f343436ab4cef/crates/wasi-http/tests/all/p2/async_.rs#L15-L16

file a repro if you have a moment against the docs! that should ALWAYS work. :-)

Pat Hickey (Jul 01 2025 at 18:24):

ill fix the docs rn

Pat Hickey (Jul 01 2025 at 18:24):

thank you for catching our error

Pat Hickey (Jul 01 2025 at 18:27):

unfortunately that doc is no_run, because it starts a server that never gets any requests and never dies, so it would just spin forever

Pat Hickey (Jul 01 2025 at 18:27):

otherwise cargo doc would have caught this bug

Pat Hickey (Jul 01 2025 at 18:27):

im not brave enough to fix the no_running-ness right now. we regret the error.

Lann Martin (Jul 01 2025 at 18:29):

Is the example actually wrong? It might be confusing that your guest can't target wasm32-wasip2 but it would work fine for a guest that just uses proxy right? Or am I missing something...?

Pat Hickey (Jul 01 2025 at 18:35):

its just missing the add_to_linker for the wasmtime-wasi parts

Pat Hickey (Jul 01 2025 at 18:35):

which are required, even in proxy, for the io package, stdio from cli, and others

Lann Martin (Jul 01 2025 at 18:37):

I'm pretty sure wasmtime_wasi_http::add_to_linker_async does add those, which is what differentiates it from add_only_http_to_linker_async

Lann Martin (Jul 01 2025 at 18:38):

Either way it seems reasonable for the example to link the rest of wasi just to avoid this kind of confusion.

Pat Hickey (Jul 01 2025 at 18:39):

oh, you know what, you're right.

Pat Hickey (Jul 01 2025 at 18:39):

but, unfortunately, its very hard to actually target the proxy world, virtually all guests end up being proxy + cli imports

Pat Hickey (Jul 01 2025 at 18:39):

the wasmtime server -Scli set

Lann Martin (Jul 01 2025 at 18:40):

Spin always links both so I'm not going to argue there. :slight_smile:

Pat Hickey (Jul 01 2025 at 18:40):

once we have 0.3 we can finally solve that problem in wasm with wasi-virt, but we gave up on it in 0.2 because too much pain for too little gain

Pat Hickey (Jul 01 2025 at 18:42):

https://github.com/bytecodealliance/wasmtime/pull/11171

Chris Suszynski (Jul 01 2025 at 18:58):

@Pat Hickey :heart:

Last updated: Dec 06 2025 at 06:05 UTC