# Subprocess

English | [中文](subprocess.zh.md)

The subprocess seam is split across interface ([dsh-subprocess](../../packages/subprocess/subprocess), `ctx.subprocess`) and implementation ([dsh-subprocess-local](../../packages/subprocess/subprocess-local)); its consumers are other capability seams and out-of-process backends — the [bash executor family](bash.md) (collect-mode batch output), the LSP host (piped protocol streams + a collected stderr tail), and the ACP subagent backend (piped protocol streams + inherited stderr). This seam owns the managed `DSH_*` environment namespace, the shared credential scrub (`scrubbedParentEnv`), and the `CollectedOutput` shape; [dsh-bash](../../packages/bash/bash) re-exports the vocabulary so bash consumers keep one import root.

Source: [`packages/subprocess/subprocess/src/types.ts`](../../packages/subprocess/subprocess/src/types.ts)

## Managed environment namespace and captured output

`DSH_*` variables are Harness-owned child-process facts; implementations discard ambient `DSH_*` names before the caller's explicit `env` merges, so a current fact arrives only as a deliberate entry, and each collected stream reports its truncation and spill-recovery state through `CollectedOutput`.

```ts type-equiv
/** One environment key inside the managed {@link DSH_ENV_PREFIX} namespace. */
type DshEnvironmentKey = `${typeof DSH_ENV_PREFIX}${string}`
```

```ts type-equiv
/** Trusted DeepSeek Harness variables for one child-process execution. */
type DshEnvironment = Readonly<Record<DshEnvironmentKey, string>>
```

```ts type-equiv
/** One captured stream: the (possibly truncated) text plus recovery info. */
interface CollectedOutput {
  /** Collected text — the TAIL of the stream when truncated. */
  text: string
  /** True when bytes were dropped from `text`. */
  truncated: boolean
  /** Path to a file holding the COMPLETE stream, when truncated and available. */
  spillPath?: string
}
```

## Node-shaped stdio dispositions

Each stream's disposition is explicit, chosen per consumer: raw pipes for protocol framing (LSP JSON-RPC, ACP ndjson), inherit for pass-through diagnostics, and collect mode for bounded batch output — with the spill file optional, so a diagnostic tail (a language server's stderr) buffers without leaving files behind.

```ts type-equiv
/**
 * stdin disposition. `'ignore'` leaves fd 0 on `/dev/null`; `'pipe'` exposes
 * {@link SubprocessHandle.stdin} for the caller's ongoing protocol writes;
 * `{ data }` writes the bytes and closes (the batch shape).
 */
type SubprocessStdinMode = 'ignore' | 'pipe' | { readonly data: string }
```

```ts type-equiv
/**
 * Bounded in-memory collection for one output stream, with an optional
 * full-stream spill file. Omitting `spill` keeps only the in-memory tail —
 * the diagnostic-tail shape (a language server's stderr); including it makes
 * the complete stream recoverable up to its cap (the bash tool shape).
 */
interface SubprocessCollect {
  /** In-memory cap in bytes; overflow keeps the TAIL. */
  maxBytes: number
  /** Full-stream spill file; absent disables spilling entirely. */
  spill?: {
    /** Whole-stream byte cap; a larger stream discards its now-incomplete spill. */
    maxBytes: number
  }
}
```

```ts type-equiv
/**
 * stdout/stderr disposition. `'pipe'` exposes the raw `Readable` for the
 * caller's protocol decoding; `'inherit'` passes the parent's descriptor
 * through (child diagnostics land on the harness's own stream); a
 * {@link SubprocessCollect} object buffers boundedly with offset-based reads.
 */
type SubprocessOutputMode = 'pipe' | 'inherit' | SubprocessCollect
```

```ts type-equiv
/** Per-stream stdio dispositions, all explicit — this seam applies no defaults. */
interface SubprocessStdio {
  stdin: SubprocessStdinMode
  stdout: SubprocessOutputMode
  stderr: SubprocessOutputMode
}
```

## The fully-explicit spawn spec

The seam applies no defaults: every disposition, limit, and directory is explicit on the spec, so the caller's own config — not a hidden subprocess-service default — decides them. `argv` is never shell-interpreted.

```ts type-equiv
/**
 * A fully-specified spawn request. This seam applies no defaults: every
 * disposition, limit, and directory is explicit, so the caller's own config —
 * not a hidden subprocess-service default — decides them (the `dsh-bash`
 * request/spec split is the owning template).
 */
interface SubprocessSpawnSpec {
  /** Executable and arguments; `argv[0]` is the program. Never shell-interpreted here. */
  argv: readonly string[]
  /** Working directory for the child. */
  cwd: string
  /** Per-stream stdio dispositions. */
  stdio: SubprocessStdio
  /**
   * Grace period in milliseconds for the {@link SubprocessHandle.terminate}
   * escalation and for draining still-open collected pipes after the process
   * exits (an inherited descriptor held by a surviving descendant cannot hold
   * the outcome open indefinitely).
   */
  graceMs: number
  /**
   * Abort signal — starts the terminate escalation on the process tree when
   * it fires. The caller owns deadlines and cause classification; this seam
   * only reacts to the abort.
   */
  signal?: AbortSignal | undefined
  /**
   * Explicit environment entries merged onto the implementation's scrubbed
   * parent base (see `scrubbedParentEnv`), with no namespace validation:
   * every entry is a deliberate caller opt-in, so a forwarded
   * credential-shaped entry or a current `DSH_*` fact survives precisely
   * because this layer merges after the scrub that drops its ambient
   * namesake.
   */
  env?: Record<string, string> | undefined
}
```

## Handles: streams, readers, and tree-scoped termination

A spawn returns a live handle immediately. Collect-mode readers take whole-stream byte offsets and never consume, so independent readers cannot steal one another's deltas; piped streams belong to the caller. Termination is tree-scoped on every platform: `terminate()` — the only termination verb — escalates SIGTERM→grace→SIGKILL, and `waitForExit()` observes the whole tree — enough for a consumer to build its own teardown ladder (the ACP backend's stdin-EOF-first `disposeAcpChild` is the template).

```ts type-equiv
/**
 * A live child process rooted in its own process tree. Collected output
 * remains readable after exit; piped streams belong to the caller.
 *
 * Termination is tree-scoped everywhere: POSIX signals the detached process
 * group (falling back to the direct child when the group is gone), Windows
 * terminates the tree via `taskkill /T`, so helper processes cannot outlive
 * the handle unnoticed.
 */
interface SubprocessHandle {
  /** Process id (tree root); -1 when the spawn itself failed. */
  readonly pid: number
  /** The child's stdin, present iff spawned with `stdin: 'pipe'`. */
  readonly stdin: Writable | undefined
  /** The child's raw stdout, present iff spawned with `stdout: 'pipe'`. */
  readonly stdout: Readable | undefined
  /** The child's raw stderr, present iff spawned with `stderr: 'pipe'`. */
  readonly stderr: Readable | undefined
  /** Offset-based readers for collect-mode streams (also readable after exit). */
  readonly collected: SubprocessCollectedOutputs
  /** Resolves at process close with exit facts; rejects only for spawn-level failures. */
  readonly done: Promise<SubprocessOutcome>
  /**
   * Begin the SIGTERM → `graceMs` → SIGKILL escalation on the process tree
   * (Windows force-terminates immediately) — the seam's only termination
   * verb. Idempotent, a no-op once the tree is gone (the pid may be reused),
   * and also triggered by the spec's abort signal.
   */
  terminate(): void
  /**
   * Wait until the process tree has exited — the tree, not just the direct
   * child, so a still-running helper is observable before teardown returns.
   * @param signal - optional bound for the wait.
   * @returns `true` when the tree exited, `false` when the signal aborted first.
   */
  waitForExit(signal?: AbortSignal): Promise<boolean>
}
```

```ts type-equiv
/**
 * Cursor-free incremental access to one collected output stream. Offsets are
 * whole-stream byte coordinates owned by the caller, so independent readers
 * cannot consume one another's output; `readFrom(0)` after settlement is the
 * batch result (`lossy` then means the in-memory tail lost its head — the
 * {@link CollectedOutput.truncated} fact).
 */
interface SubprocessOutputReader {
  /**
   * Read everything captured since `fromByte`. When that offset has slid out
   * of the in-memory tail window the read is `lossy` — it returns the whole
   * retained tail and the gap is only recoverable from the spill file.
   * @param fromByte - whole-stream offset to resume from (a prior read's `nextOffset`; 0 for the first read).
   * @returns the delta text, the next offset, the `lossy` flag, and the spill path when one exists.
   */
  readFrom(fromByte: number): SubprocessOutputRead
}
```

```ts type-equiv
/** One incremental {@link SubprocessOutputReader.readFrom} read. */
interface SubprocessOutputRead {
  /** Stream text from the requested offset (the whole retained tail when lossy). */
  text: string
  /** Whole-stream offset to resume from on the next read. */
  nextOffset: number
  /** True when the requested offset slid out of the in-memory tail window. */
  lossy: boolean
  /** Path to the full-stream spill file, when one was created and remains intact. */
  spillPath?: string
}
```

```ts type-equiv
/** Offset-based readers for the streams spawned in collect mode. */
interface SubprocessCollectedOutputs {
  /** Present iff stdout is a {@link SubprocessCollect}. */
  readonly stdout?: SubprocessOutputReader
  /** Present iff stderr is a {@link SubprocessCollect}. */
  readonly stderr?: SubprocessOutputReader
}
```


## Outcomes carry exit facts only

`done` reports Node's close-event vocabulary and no cause classification — the service kills on abort but never decides why (the caller reads the deadline signal it owns, e.g. the bash executor's `timedOut`/`aborted` split). Collected output stays readable through `handle.collected` after settlement, so batch and streaming callers share one access path.

```ts type-equiv
/**
 * Exit facts of one closed process — Node's `close`-event vocabulary.
 * Deliberately carries NO timeout or cancellation classification (the caller
 * reads the signal it owns to classify causes) and NO output: collected
 * streams stay readable through {@link SubprocessHandle.collected} after
 * settlement, so batch and streaming callers share one access path.
 */
interface SubprocessOutcome {
  /** Exit code; null when the process died from a signal. */
  exitCode: number | null
  /** Terminating signal (e.g. 'SIGTERM'); null on normal exit. */
  signal: NodeJS.Signals | null
}
```

## Service behavior

The abstract [`SubprocessService`](../../packages/subprocess/subprocess/src/index.ts) seam defines `spawn` only; [`LocalSubprocessService`](../../packages/subprocess/subprocess-local/src/index.ts) is the local implementation (detached trees, per-disposition wiring, credential scrub, terminate-and-join disposal). See [`dsh-subprocess`](../../packages/subprocess/subprocess/README.md) for the seam contract and [`dsh-subprocess-local`](../../packages/subprocess/subprocess-local/README.md) for the mechanics.
