Add OCI spec aware compiler hook (for precompile)

[tanishiking]

spinframework/containerd-shim-spin#444 adds Wasmtime TOML feature configuration. Runtime configuration can be provided through a Pod mounted /runtime/wasmtime.toml for example, but precompilation currently creates the compiler before the shim has access to the configuration.

This commits adds Shim::compiler_with_spec(&Spec). Instance creation now loads config.json before loading OCI Wasm layers and passes the spec to the compiler through that new API.
Downstream shims precompilation should be able to read the spec to configure the wasmtime engine.

Concerns, this might add a small per-container-start cost: 🤔

• we read config.json before precopmile
• construct a precompiler per container rather than once per shim process.

[jsturtevant]

If I understand correctly from spinframework/containerd-shim-spin#444 you using the spec to find the spin toml which contains the wasmtime configuration options?

I think you have that information when we call async fn compiler on the trait? Could you read that file then and hand back the properly configured compiler?

[tanishiking]

If I understand correctly from spinframework/containerd-shim-spin#444 you using the spec to find the spin toml which contains the wasmtime configuration options?

Yes, that is what I’m trying to do. I’d like to read the OCI spec to find the path of the Wasmtime configuration file on the node.
Aat precompile time, the container mounts are not available via the container-visible path (/runtime/wasmtime.toml in spinframework/containerd-shim-spin#444), so the shim needs to use the OCI spec to resolve where the configuration file is located on the node.

I think you have that information when we call async fn compiler on the trait? Could you read that file then and hand back the properly configured compiler?

Yeah... but IIUC it's not possible with the current API (?). To make it possible, I think we need a new API that passes OCI spec information to the shim.

• runwasi have that information (cfg.bundle) at the call site.
• However, the current Shim::compiler() API does not pass any spec information to the shim.
• Therefore, containerd-shim-spin cannot create a "properly configured compiler": it does not know where the Wasmtime configuration file is on the node.

That's why I’d like to add a new API that passes the OCI spec information to the shim, so that shim can create a properly configured compiler.

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Please let me know if I’m misunderstanding something.
Is there a way for the shim (containerd-shim-spin) to access the OCI spec during precompilation, or to locate the file created from the ConfigMap on the node, without making any changes to runwasi ?

[jsturtevant]

Thanks, its been awhile but what you've described sounds right.

Is there a way for the shim (containerd-shim-spin) to access the OCI spec during precompilation, or to locate the file created from the ConfigMap on the node, without making any changes to runwasi ?

I think using the higher level trait in containerd-shim-spin means you don't have access to this. In a prior version the shims had a bit more information.

There might be other way's we might be able to make this more generic but would need some concentrated effort. So I think I am ok with this as an option now.

[jprendes]

LGTM