Ephemeral Mount Modes (`tmpfs`, `ephemeral`)

Status: Open — per-mount Git isolation (shared | worktree | clone) is shipped; storage-kind modes (tmpfs, per-instance ephemeral volumes) remain an unimplemented design proposal

Shipped baseline

The MountIsolation enum currently has three variants — Shared, Worktree, Clone — and MountConfig requires a host src on every mount. These cover git semantics for the mounted path (bind mount, per-container worktree, full directory copy) but have no way to express storage kind: a path that should be per-instance and hidden from the host, or a path that should be tmpfs-backed for speed and guaranteed teardown. Today an operator can only get either behavior by hand-writing a Docker --mount/--tmpfs flag outside jackin.role.toml or the workspace config. Docker's read-only-root security profile (docker_profile.rs tmpfs_paths) mounts a fixed preset of tmpfs paths for hardened containers, but that's an internal security mechanism, not the declarative per-mount tmpfs/ephemeral modes described below — it doesn't cover arbitrary operator-declared paths like ~/.gradle/daemon.

Remaining work

1. Extend MountIsolation to shared | worktree | clone | tmpfs | ephemeral.
2. Extend the workspace mount TOML schema so tmpfs/ephemeral mounts can omit src (host source), and reject readonly on those two modes.
3. Add an optional size field on tmpfs, reusing the parsing helper from declarative resource limits.
4. Docker translator: tmpfs → --tmpfs <dst>:size=<n>,mode=1777; ephemeral → --mount type=volume,source=jackin-<container>-<slug>,target=<dst>.
5. Per-instance Docker volume label scheme so purge can enumerate and delete ephemeral volumes (no such labeling exists in cleanup.rs today).
6. Role-level mount defaults via [[runtime.mounts]] in jackin.role.toml, with workspace mounts overriding by dst.
7. Docs updates for mount isolation and the Mounts guide once shipped.

Out of scope

• tmpfs size enforcement beyond Docker's own default behavior.
• ephemeral mode with operator-managed, non-purge-cleaned persistence (would need a distinct volume = "<name>" mode).
• File ownership/mode flags on tmpfs.
• Cross-instance shared ephemeral volumes (handled instead by the existing shared-cache mount pattern).

Related work

• Research: Ephemeral mount modes — design proposal (multicode comparison, schema/backend design, open questions)
• Agent Orchestration Program
• Selectable sandbox backends — cross-backend translation home for a future Apple-container implementation
• Declarative resource limits — shares the size-parsing helper