Allow configuration of wasmtime features

[tanishiking]

containerd-shim-spin currently constructs the underlying Wasmtime engine from spin_core::Config::default() and immediately creates the engine, and there seems no way to override the runtime configuration.

containerd-shim-spin/containerd-shim-spin/src/engine.rs

Lines 63 to 68 in de7f584

	async fn compiler() -> Option<SpinCompiler> {
	let mut config = spin_core::Config::default();
	Some(SpinCompiler(
	wasmtime::Engine::new(config.wasmtime_config()).unwrap(),
	))
	}

https://github.com/spinframework/spin/blob/a794d6e7a514af693f8779c4dfe9f59f391118d0/crates/core/src/lib.rs#L82-L199

This prevents us (scala-wasm/scala-wasm that uses GC, EH, and CM) from running our binary on spin shim.

---

I'm not very sure how should we design the configuration passing to wasmtime for spin:

There's a same issue in runwasi containerd/runwasi#1004 and I prototyped a feature containerd/runwasi#1105 that shim reads env variable in TOML format and give it to underlying wasmtime. And this way, we can configure wasmtime at the deployment site (not on cluster configuration). But not sure this applies to spin as well.

[kate-goldenring]

Thanks @tanishiking. This would be a great feature to enable. Is this a recent break for you due to these changes to pull in the defaults from Spin https://github.com/spinframework/containerd-shim-spin/pull/423/changes?

One workaround i think we could do without the runwasi changes would be to configure the environment variable via containerd environment variables:
https://github.com/spinframework/skips/tree/main/proposals/003-shim-runtime-options#spin-shim-executor-environment-variables

[tanishiking]

Thanks @kate-goldenring for reply :)

Is this a recent break for you due to these changes to pull in the defaults from Spin https://github.com/spinframework/containerd-shim-spin/pull/423/changes?

No, this is not a recent regression caused by the recent changes. IIUC, Neither runwasi nor containerd-shim-spin has ever provided a way to configure the underlying Wasmtime settings, and the wasmtime features required by the Wasm binaries generated from Scala (gc, exceptions, and function_references) have never actually been enabled so far.

One workaround i think we could do without the runwasi changes would be to configure the environment variable via containerd environment variables:

Interesting! Do I understand correctly that, environment variables configured on containerd itself would be inherited by the shim as well?.
Unfortunately, though, wasmtime does not seem to support changing those feature flags via environment variables (except things like WASMTIME_LOG, or Spin-specific configuration here: https://github.com/spinframework/spin/blob/a794d6e7a514af693f8779c4dfe9f59f391118d0/crates/core/src/lib.rs#L136-L172
), so I’m afraid setting containerd-level environment variables would not work as a workaround for our use case 😞

[kate-goldenring]

Interesting! Do I understand correctly that, environment variables configured on containerd itself would be inherited by the shim as well?.

Yes

Unfortunately, though, wasmtime does not seem to support changing those feature flags via environment variables (except things like WASMTIME_LOG, or Spin-specific configuration here

Looking at your runwasi proposed changes, i was curious if we could do a similar thing of mounting the config file into the pod as a volume and then just configuring the path to the wasmtime config as a containerd env. Therefore, it in someway becomes a "well known path" where the shim will check if the config file exists.

[tanishiking]

if we could do a similar thing of mounting the config file into the pod as a volume and then just configuring the path to the wasmtime config as a containerd env. Therefore, it in someway becomes a "well known path" where the shim will check if the config file exists.

Ah, do you mean like, for example if the "well known path" was /runtime/wasmtime/wasmtime.toml, we configure pods like this:
(and at the cluster level, we could configure the "well known path" by SPIN_WASMTIME_CONFIG_PATH=/runtime/custom/wasmtime.toml, and then the shim would try to read config from that configured path instead).

I think that makes sense 👍

kind: ConfigMap
metadata:
  name: wasmtime-config-gc
data:
  wasmtime.toml: |
    [wasm]
    gc = true
    function-references = true
---
spec:
  containers:
    - name: app
      image: test
      volumeMounts:
        - name: wasmtime-config
          mountPath: /runtime/wasmtime/wasmtime.toml
          subPath: wasmtime.toml
  volumes:
    - name: wasmtime-config
      configMap:
        name: wasmtime-config-gc

(I'm very new to k8s, let me know if the way of configuring pods looks wired 😅 )