`urunc exec` subcommand not implemented

[shivansh-source]

Summary

urunc does not implement the exec subcommand required by the OCI Runtime Specification.
When orchestrators call urunc exec on a running container, urunc returns:

Impact

Any tool relying on exec for sidecar coordination or container introspection breaks silently.
This was discovered while investigating Argo Workflows integration (#573), where the absence
of exec causes workflow pods to hang indefinitely after the unikernel exits.

Expected behavior

urunc exec <container-id> -- <cmd> should execute a process in the container's context per
the OCI Runtime Spec.
Since unikernels don't support process injection, running the command as a host-side process
in the container's namespaces (similar to Kata Containers) seems a reasonable approach.

related to -#573

[shivansh-source]

this blocks integration of tools like argo workflows

[shivansh-source]

/cc @cmainas PTAL

[cmainas]

Hello @shivansh-source ,

thank you for opening this issue. As far as I concern, the OCI spec does not mention exec as a requirement (see https://github.com/opencontainers/runtime-spec/blob/6999a89a76a0329f440d5740497bedb9dd431297/runtime.md#operations). Please let me know if I am wrong.

Furthermore, the support of the exec operation in urunc is tricky, since unikernels do not support multiple processes. We plan to have partial support only for supported guests (e.g. LInux) though.