# Agent Note: The subprocess service is its own seam under the bash executors (`dsh-subprocess` / `dsh-subprocess-local`)

Status: implemented

English | [中文](2026-07-26-subprocess-seam.zh.md)

## Problem

`dsh-bash-local` bundled two capabilities that change for different reasons: *running a bash command* (command defaulting, timeout classification, model-friendly terminal environment, the stdout/stderr merge the bash tool renders) and *running and managing a child process* (detached process groups, bounded tail-keep output with spill files, the credential scrub and `DSH_*` merge order, SIGTERM→grace→SIGKILL escalation, kill-and-join disposal). The process half — `run.ts`, roughly half the package — had no seam of its own: a future non-shell runner (a direct-argv executor, a worker supervisor) would have to re-implement or reach into bash internals, and the shared `DSH_*`/`CollectedOutput` vocabulary lived in a package whose name promises shell semantics. The bundling also tied background-process lifetime to the executor's fiber: reloading the bash executor killed every live background process, unlike the sibling [task registry](2026-07-26-task-registry-seam.md), whose registrations deliberately outlive producer fibers.

## Decision

A new `subprocess/` capability family owns "run and manage a process"; the bash family keeps "run a bash command" and consumes it:

- **`@deepseek-ai/dsh-subprocess` (Service Definition)** — the abstract `SubprocessService` owning `ctx.subprocess`: executable lookup, fully explicit ordinary spawns, and the terminal primitive added by the [portable execution-world decision](2026-07-28-portable-execution-world-consumers.md). Each stdio stream independently selects `'pipe'`, `'inherit'`, or bounded collection `{ maxBytes, spill? }`; stdin selects `'ignore'`, `'pipe'`, or `{ data }`. `SubprocessOutcome` carries exit facts with deliberately no timeout/cancel classification, while collected output remains on the handle after settlement. The Service Definition also owns process and terminal handles, the shared scrub, and `DSH_ENV_PREFIX`/`DshEnvironment`/`CollectedOutput`; `argv` is never shell-interpreted.
- **`@deepseek-ai/dsh-subprocess-local` (Service provider)** — `LocalSubprocessService` over the former `run.ts` plumbing (`spawn.ts`) plus `node-pty`: detached groups, bounded collection and private spill files, executable lookup, foreground/session inspection, and disposal that terminates and joins every managed process. `terminate()` owns TERM→grace→KILL for the tree, `waitForExit()` observes tree liveness, and injected `taskkill /T` covers Windows. Ordinary and terminal spawns apply the Service Definition's case-insensitive `KEY`/`PASSWORD`/`SECRET`/`TOKEN` scrub before explicit env. The provider has no config; every limit arrives on the spec, while Bash and PTY presentation environment overrides stay in their Consumers.
- **`dsh-bash-local` (Consumer)** — `inject: ['subprocess']`; maps each resolved `BashExecSpec` onto a `SubprocessSpawnSpec` (`['bash', '-c', command]`), keeps its config, `resolve()` defaulting, fused-deadline `timedOut`/`aborted` classification, the `[stderr]`-marked background read merge with its consuming cursor, and the `onProcessDone` subclass hook. `dsh-bash-sandbox` is unchanged apart from redeclaring the inherited inject; it still wraps at the command-string level and re-enters the inherited spawn path.
- **`dsh-bash` (Service Definition)** — re-exports the moved vocabulary from `dsh-subprocess`, so no bash Consumer changes an import; `BashExecRequest`/`BashExecSpec`/`BashProcess` and the sandbox facts remain bash-owned.

Every composition that loads a bash executor now also loads `@deepseek-ai/dsh-subprocess-local` (CLI, examples, python bundled runtime, create-sdk's bash feature resources, inline test configs).

Background-process lifetime moved from the executor to the subprocess service: the executor no longer retains a live-process set, so an executor reload leaves background work running and readable, and composition teardown (the service's disposal) remains the kill-and-join boundary. One behavioral contract shifted with it: a background spawn failure can no longer be buffered as fake stderr inside the plumbing (the service rejects `done` and buffers nothing for a process that never ran), so the executor injects the `spawn failed: …` note into exactly one `readOutput()` delta.

Observed stream and lifecycle needs then moved the eligible process consumers onto the seam: LSP uses piped protocol streams plus a collected stderr tail; the ACP backend uses piped ndjson, inherited stderr, and a consumer-owned stdin-EOF disposal ladder; PTY uses `spawnTerminal()` while keeping readiness and terminal policy. `dsh-subagent-subprocess` and the private LSP tree helpers were deleted. MCP transport spawning, the SDK package-manager runner, synchronous TUI Git probing, and dependency-light test-support launchers remain outside by ownership or execution shape; their production callers share the scrub where applicable.

## Alternatives considered

**Leave the process plumbing inside `dsh-bash-local` (status quo).** Rejected for the same reason the [task registry split](2026-07-26-task-registry-seam.md) landed: the boundary is stable and already documented in-code (`run.ts`'s module doc said "this layer reacts to an abort signal; the executor owns deadlines and classifies causes"), and keeping it private makes every future non-shell runner either fork the mechanics or depend on a bash-named package for non-bash work. The user-visible driver for this change was exactly this split.

**Keep the original batch-only interface and leave stream consumers bespoke.** Rejected after the observed LSP, ACP, and PTY shapes showed that private process-tree signalling and environment scrubs would otherwise remain duplicated. The Node-shaped dispositions cover those consumers without buffering piped streams.

**Use one `stdio: 'pipe' | 'inherit' | 'collect'` mode for all streams.** Rejected because real consumers mix modes per stream: LSP uses pipe/pipe/collect, ACP uses pipe/pipe/inherit, and Bash uses data/collect/collect.

**Route every process launch through `ctx.subprocess`.** Rejected because the MCP SDK owns its transport spawn, the SDK wizard has no Cordis context and needs inherited redirection, the TUI probe is synchronous, and support launchers deliberately stay independent of product seams. PTY allocation did move behind `spawnTerminal()` because the provider, not the consumer, owns that substrate-specific primitive.

**Put `run_in_background`/task semantics into the subprocess capability seam instead.** Rejected: that boundary already exists — `ctx.tasks` owns ids, ownership, and notices, and the bash tool adapts a `BashProcess` into task hooks. The subprocess seam sits *below* the bash executor, not beside the task registry.

**Move `ENV_OVERRIDES` (TERM=dumb, PAGER=cat …) into the subprocess service.** Rejected: a generic subprocess service must not impose terminal presentation policy on non-terminal consumers; the ambient scrub (credential-shaped and `DSH_*` names) is a security/identity invariant and stays, but terminal friendliness is the bash tool's choice, expressed through the spec's explicit env where a caller's own entry still wins.

## Consequences

Bought: "run and manage a process" is a swappable capability used by Bash, LSP, PTY, and ACP consumers; a containerized or remote process backend slots in without changing their domain semantics; tree signalling, escalation, bounded collection, terminal mechanics, and credential scrubbing each have one implementation; and background processes survive executor reloads, matching the task registry's lifetime model. Process and terminal plumbing is tested through `dsh-subprocess-local`; consumer suites pin only their owned behavior against the real service.

Cost: one more package pair and one more composition row wherever a consumer loads; a missing subprocess provider leaves the consumer pending by standard service-injection behavior. Every backend implements executable lookup, three stdio modes, tree lifecycle, and one terminal primitive. The moved-vocabulary re-exports keep `dsh-bash` imports working but mean two packages name the same types; the subprocess seam is the owner. The spawn-failure note became single-delivery through Bash's consuming read cursor instead of repeatable stderr-buffer content.
