feat(driver-podman): add userns config

[giuseppe]

Summary

Add user namespace support to the Podman compute driver. The userns config option maps to Podman's namespace modes with parameterized value support (auto:size=65536, keep-id:uid=1000,gid=1000). When userns is configured, the driver extracts the supervisor binary to a host-side cache and bind-mounts it instead of using an OCI image volume, because the kernel does not support idmapped mounts on overlay.

A preparatory refactor commit extracts shared supervisor binary helpers (extraction, caching, ELF validation) from the Docker driver into openshell-core::driver_utils so both drivers reuse the same code with hardened integrity checks.

Related Issue

Fixes: #2554

Changes

Commit 1: refactor(driver): extract shared supervisor binary helpers into openshell-core

• Move extract_first_tar_entry, write_cache_binary_atomic, supervisor_cache_path,
  temp_extract_container_name, and validate_linux_elf_binary from Docker driver to
  openshell-core::driver_utils
• Add entry-type and empty-payload checks to extract_first_tar_entry
• Parameterize supervisor_cache_path by driver name (docker-supervisor vs podman-supervisor)

Commit 2: feat(driver-podman): add userns config with supervisor bind-mount fallback

• Add userns field to PodmanComputeConfig (TOML userns, CLI --userns, env
  OPENSHELL_PODMAN_USERNS)
• Split mode string on first : into Podman API nsmode + value fields
• Set idmappings.AutoUserNs = true when base mode is auto
• Validate mode at startup: auto/keep-id accept params, host/private/nomap reject them
• Extract supervisor binary to host cache and bind-mount when userns is active (all modes except host)
• Add copy_from_container API method to PodmanClient
• Update architecture docs, gateway config reference, and driver README

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[mrunalp]

Potential Concerns

1. Resource leak on extraction failure — driver.rs:647-655
   Every other error path in this block calls cleanup_created() first. This one uses ?:
   let supervisor_bin_path = if self.config.userns.is_some() {
   Some(extract_supervisor_bin(&self.client, &self.config).await
   .map_err(ComputeDriverError::from)?,) // <- skips cleanup_created()
   } else { None };
   A failed extraction orphans the workspace volume and both per-sandbox secrets.

2. Introduces two clippy warnings → mise run lint fails. tasks/rust.toml:14 runs cargo clippy --workspace --all-targets -- -D warnings. Locally:

• clippy::too_many_arguments — build_container_spec_for_image is now 8/7 (container.rs:930)
• clippy::useless_conversion — the redundant .map_err(ComputeDriverError::from) at driver.rs:651

3. Duplicates the Docker driver's extraction code, minus its integrity check. openshell-driver-docker/src/lib.rs:3149-3415 already has extract_supervisor_bin_from_image, extract_supervisor_binary_bytes, extract_first_tar_entry, write_cache_binary_atomic, supervisor_cache_path, and a byte-identical temp_extract_container_name. The Docker version calls validate_linux_elf_binary both after write and on cache hit; the Podman copy trusts cache_path.is_file() blindly. Since this file becomes the sandbox's security-enforcement supervisor, a truncated, corrupt, or symlinked cache entry executes unchecked. These belong in openshell-core::driver_utils shared by both drivers.

4. userns values other than bare auto are likely broken. Podman's Namespace splits mode and value, so auto:size=65536 or keep-id:uid=1000 would be sent as an invalid nsmode and also skip AutoUserNs. userns = "host" silently disables userns isolation and ns:/path joins an arbitrary namespace — worth an allowlist plus a config-load error instead of an obscure Podman API failure at create time.

5. No tests for the new extraction path. The four added tests cover spec shape only. extract_first_tar_entry (which doesn't check entry type or non-empty payload — a symlinked /openshell-sandbox yields a 0-byte "binary"), supervisor_cache_path, and write_cache_binary_atomic are untested, though the Docker equivalents have tests at driver-docker/src/tests.rs:2199-2223 that could be lifted.

6. Stale architecture doc. architecture/compute-runtimes.md:173 still says Podman delivers the supervisor via "Read-only OCI image volume" — now conditional. Line 120 mentions image volumes too. AGENTS.md requires this in the same branch.

7. Process gaps. DCO check is failing — the contributor-assistant bot needs the sign-off comment on the PR, not just the trailer. The PR body is the unfilled template (no Summary, Related Issue, Testing, or checked boxes), and a feature needs a linked accepted issue per AGENTS.md. No pre-flight for --userns=auto, which needs a larger subuid range than the existing check_subuid_range warning covers.

Minor: extraction runs on the sandbox-create path rather than driver startup (first create per digest pays inspect + create + download + remove); the mid-file use openshell_core::driver_utils::SUPERVISOR_IMAGE_BINARY_PATH; at driver.rs:964 breaks the file's import convention; #[serde(rename = "AutoUserNs")] would be cleaner than #[allow(non_snake_case)].

[giuseppe]

I have read the DCO document and I hereby sign the DCO.

[giuseppe]

thanks! Comments addressed

[mrunalp]

• ## PR review: Request changes

Findings

1. [P2] Canonicalize namespace modes before serialization.
   Validation is case-insensitive and accepts nomap, but the container spec forwards the original value. Podman expects no-map, so nomap always fails; uppercase values such as AUTO also pass startup validation but fail creation. Normalize to Podman’s canonical values. PR code
   (

   OpenShell/crates/openshell-driver-podman/src/container.rs

   Lines 1219 to 1226 in 444fe35

   	userns: config.userns.as_deref().map(|raw| {
   	let (base, params) = raw
   	.split_once(':')
   	.map_or((raw, None), |(b, p)| (b, Some(p)));
   	UserNS {
   	nsmode: base.to_string(),
   	value: params.map(ToString::to_string),
   	}

   ), Podman contract
   (https://github.com/containers/podman/blob/f3829809246caaa533c1af29ecf27f613d3d2fd7/pkg/specgen/namespaces.go#L52-L65).

2. [P2] Reject or fully implement private.
   The config accepts private, but only auto receives idmappings. Podman requires at least one UID/GID mapping for a private user namespace, so every sandbox creation using this advertised mode fails. Either remove it from the allowlist or add mapping configuration. PR validation
   (

   OpenShell/crates/openshell-driver-podman/src/config.rs

   Lines 334 to 362 in 444fe35

   	/// Validate the optional `userns` mode against the supported allowlist.
   	///
   	/// Supported modes: `auto` (with optional params, e.g. `auto:size=65536`),
   	/// `host`, `keep-id` (with optional params), `private`, and `nomap`.
   	/// Modes that don't accept parameters (`host`, `private`, `nomap`) are
   	/// rejected when a colon-separated suffix is present.
   	pub fn validate_userns(&self) -> Result<(), crate::client::PodmanApiError> {
   	let Some(mode) = self.userns.as_deref() else {
   	return Ok(());
   	};
   	let (base, has_params) = mode
   	.split_once(':')
   	.map_or((mode, false), |(b, _)| (b, true));
   	match base.to_ascii_lowercase().as_str() {
   	"auto" | "keep-id" => Ok(()),
   	"host" | "private" | "nomap" => {
   	if has_params {
   	Err(crate::client::PodmanApiError::InvalidInput(format!(
   	"userns mode '{base}' does not accept parameters",
   	)))
   	} else {
   	Ok(())
   	}
   	}
   	_ => Err(crate::client::PodmanApiError::InvalidInput(format!(
   	"unsupported userns mode '{mode}'; \
   	supported modes: auto, host, keep-id, nomap, private",
   	))),
   	}

   ), Podman requirement
   (https://github.com/containers/podman/blob/f3829809246caaa533c1af29ecf27f613d3d2fd7/pkg/specgen/namespaces.go#L530-L543).

Required follow-ups

• Add a real Podman userns=auto sandbox E2E; unit tests only verify generated JSON and currently encode the incorrect nomap value.
• Update debug-openshell-cluster with userns/cache/bind-mount troubleshooting.