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

Design proposal for two new per-mount isolation modes — tmpfs and ephemeral — that cover storage kind rather than git semantics. Tracked as an open roadmap item: Ephemeral mount modes.

Problem

jackin❯ per-mount isolation vocabulary is shared | worktree | clone. These modes describe git semantics for the mounted path. They don't cover a different axis the operator also cares about: storage kind.

Specifically, two storage shapes have no declarative way to be expressed today:

A path the agent can read and write, but whose contents should NOT appear on the host filesystem and SHOULD reset between launches. (Examples: ~/.gradle/daemon, an agent's /var/tmp, an OpenCode session-state directory that should be per-instance.)
A path that lives in tmpfs (memory-backed) for performance or for guaranteed teardown when the container exits. (Examples: /tmp, a temporary ~/.ssh shadow during a rebase script.)

Today an operator can only achieve the first by writing a Docker volume into a --mount arg and managing its lifecycle by hand, and the second by passing --tmpfs ad-hoc. Neither lives in jackin.role.toml or the workspace config.

multicode confirms this is a real, frequently-used distinction. Their example config.toml declares 7 paths under isolated and 1 under tmpfs — both essential to keeping concurrent agents from corrupting each other's caches and to keeping host state clean.

Why It Matters

Parallel agents share host caches today. Two agents both running gradle build race on ~/.gradle/daemon because jackin❯ has no way to declare "this directory is per-instance, not host-shared".
Toolchain hygiene matters at scale. With the autonomous task queue in place, an agent fleet runs hundreds of builds a day. Each one should start with a clean per-instance scratch space; today, they don't.
/tmp should default to tmpfs. Many roles bake configuration assuming /tmp is fast and ephemeral. jackin❯ default /tmp is the container's own filesystem — fine for correctness, but larger and slower than necessary.
This is the only mount-mode gap found in the verification pass comparing jackin❯ against multicode's [isolation] block. Closing it brings the mount surface to parity.

Inspiration in multicode

Sources:

README — Isolation (mounting policy and Apple-container subsections describe per-workspace isolation rules)
Config — config.toml [isolation] block — the isolated = [...] and tmpfs = [...] lists in the example

[isolation]
isolated = [
  "~/.local/share/opencode",
  "/var/tmp",
  "~/.gradle/daemon",
  "~/.gradle/.tmp",
  "~/.gradle/kotlin-profile",
  "~/.local/share/kotlin",
  "~/.local/state/opencode",
]
tmpfs = [
  "/tmp",
]

Semantics:

isolated: the path inside the workspace is mapped to a per-workspace dedicated mount (or tmpfs overlay). It's writable while the workspace runs but not visible on the host and wiped per workspace start. Implementation: bwrap --tmpfs <path> overlays or a dedicated mount; the path doesn't share with host.
tmpfs: explicit tmpfs mount. Memory-backed, ephemeral, gone on exit. Implementation: bwrap --tmpfs <path>.

The two are subtly different: isolated may be backed by disk for larger working sets (build artifacts), while tmpfs is always memory. multicode lets the operator pick.

Recommended Shape

Extend the per-mount isolation enum vocabulary by two more modes, plus extend the workspace mount schema so they don't all need a host src.

Schema extension

The existing per-mount isolation schema requires a host src:

[[workspaces.X.mounts]]
src = "~/projects/x"
dst = "/workspace/x"
isolation = "worktree"

Add two modes that don't need src:

[[workspaces.X.mounts]]
dst = "/tmp"
isolation = "tmpfs"
size = "512MiB"          # optional; Docker default 64MiB is small

[[workspaces.X.mounts]]
dst = "/home/agent/.gradle/daemon"
isolation = "ephemeral"
# 'ephemeral' = per-instance Docker volume, persists across container
# restarts of the same instance, deleted on `purge`. Not bind-mounted
# to host.

V1 enum vocabulary becomes: shared | worktree | tmpfs | ephemeral (plus clone, already shipped). Validation:

tmpfs and ephemeral reject src (no host source).
tmpfs and ephemeral reject readonly (writability is their whole point; if you want read-only, you want shared with readonly = true).
tmpfs accepts an optional size field with the same parsing rules as declarative resource limits.

Backend translation

Docker (today): tmpfs → --tmpfs <dst>:size=<n>,mode=1777. ephemeral → --mount type=volume,source=jackin-<container>-<slug>,target=<dst>, with the volume created on first load and deleted by purge (the per-instance cleanup helper already enumerates Docker volumes by label; add a label and reuse it).
Apple container (when selectable backends ships): use the equivalent allocation primitives.
bwrap (if it ever lands): --tmpfs for both modes; the ephemeral case is bwrap's natural shape.

Role declarations

Roles can declare ephemeral defaults in jackin.role.toml:

[[runtime.mounts]]
dst = "/home/agent/.gradle/daemon"
isolation = "ephemeral"

[[runtime.mounts]]
dst = "/tmp"
isolation = "tmpfs"
size = "1GiB"

These are role-level defaults that workspaces inherit unless overridden by a workspace mount on the same dst. Useful so a Java role can declare its Gradle hygiene once and every workspace gets the right behavior.

Defer

tmpfs size enforcement beyond Docker's default behavior. Use Docker's, document the limit.
ephemeral mode with persistent behavior across purge (operator-managed volume name). Out of V1; if needed, add a fifth mode volume = "<name>" that doesn't auto-clean.
File ownership / mode flags on tmpfs (mode = "0700"). Defer until a use case surfaces.
Cross-instance shared ephemeral volumes ("all my Java agents share one Gradle daemon dir"). Out of V1; the use case is handled by shared-cache mounts under shared cache pattern (a shared ~/.gradle mount overlaid on an isolated source tree).

Open Questions

Naming. ephemeral is descriptive but not great. Alternatives: instance (it's per-instance), volume (it's a Docker volume), private (it's not host-visible). Recommended: ephemeral — matches the operator's mental model ("this is gone when I purge").
Default tmpfs for /tmp. Should every role get /tmp as tmpfs by default, or stay opt-in? Recommended: opt-in for V1 — defaults can change downstream after seeing real usage.
Interaction with worktree isolation. A workspace with a worktree-isolated source mount and a sibling ephemeral mount should compose cleanly (parent-mount-before-child rule). Confirm the existing collapse/ordering logic handles a no-src child mount correctly.
Is this its own leaf or an extension to mount isolation? This is treated as a standalone item because it ships independently and the design doesn't require any per-mount- isolation refactor. The Workspaces and Mounts guides get notes pointing here once this is implemented.

Ephemeral mount modes (roadmap item)
Agent Orchestration Program
Per-mount isolation — the existing modes; this proposal extends the enum vocabulary
Selectable sandbox backends — cross-backend translation home
Declarative resource limits — shares the size-parsing helper

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

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