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+import assert from 'node:assert/strict';
+import { afterEach, test, vi } from 'vitest';
+
+// Every claim the stopper makes is about what the host answers, so one double answers both syscalls it
+// reads: `kill(pid, signal)` for the signals and liveness probes, and `ps` for the identity fields.
+// One host state drives both, so no case can be live while its identity says otherwise.
+const { mockRunCmd } = vi.hoisted(() => ({ mockRunCmd: vi.fn() }));
+vi.mock('./exec.ts', async () => ({
+  ...(await vi.importActual<typeof import('./exec.ts')>('./exec.ts')),
+  runCmd: mockRunCmd,
+}));
+
+import {
+  stopProcessAndConfirmExit,
+  type ConfirmedProcessStop,
+  type ProcessStopTarget,
+} from './host-process.ts';
+
+const PID = 4242;
+const BUDGET = { termTimeoutMs: 1_000, killTimeoutMs: 500 };
+type Signal = 'SIGTERM' | 'SIGKILL';
+
+const START = 'Mon Oct 12 00:00:00 2026';
+const OTHER_START = 'Tue Oct 13 09:00:00 2026';
+const COMMAND = '/usr/bin/node /app/dist/src/client/companion-tunnel.js --run-metro-companion';
+
+const STOPPED: ConfirmedProcessStop = { status: 'stopped' };
+/** `ps` found the row and told nothing for that column, which is unknown rather than absent. */
+const EMPTY = { stdout: '', stderr: '', exitCode: 0 };
+const PS_REFUSED = { stdout: '', stderr: 'ps: not permitted', exitCode: 1 };
+const psAnswer = (value: string) => ({ stdout: `${value}\n`, stderr: '', exitCode: 0 });
+const refused = (signal: Signal): ConfirmedProcessStop => ({
+  status: 'retained',
+  reason: 'signal-failed',
+  signal,
+  errno: 'EPERM',
+});
+
+/** What the host will say about the pid, overriding what the recorded process would show. */
+type HostIdentity = Readonly<{ startTime?: string | null; command?: string | null }>;
+
+type HostCase = Readonly<{
+  name: string;
+  /** The identity the caller recorded beside the pid; absent means it tracks only a pid. */
+  recorded?: boolean;
+  /** What the host answers each signal with: a string is the refusing errno, absent accepts. */
+  answers?: Partial<Record<Signal, string>>;
+  /** The signal after which the process has run its course, when it ends because it was asked. */
+  exitsOnSignal?: Signal;
+  /** How many liveness probes it survives before somebody else ends it. */
+  exitsAfterProbes?: number;
+  /**
+   * How a process that has run its course keeps answering to its pid, which is where liveness and
+   * identity disagree: a POSIX zombie is `EPERM` beside a `Z` row, and on Windows a process that
+   * exited while its object still had handles is `EPERM` with no row at all. `'held'` never lets go.
+   */
+  afterExit?: 'zombie' | 'no-row' | 'held';
+  identity?: HostIdentity;
+  /** A `ps` the host refuses outright, so identity cannot be read at all. */
+  psFails?: boolean;
+  /** The signals the stop must spend, in order, and the verdict each host answer earns. */
+  expect: { outcome: ConfirmedProcessStop; signals: Signal[] };
+}>;
+
+type HostModel = Omit<HostCase, 'name' | 'expect'>;
+type Expect = HostCase['expect'];
+const HELD = { afterExit: 'held' } satisfies HostModel;
+const DENIED_ON_TERM = { answers: { SIGTERM: 'EPERM' } } satisfies HostModel;
+const UNREADABLE = { identity: { startTime: null, command: null } } satisfies HostModel;
+const RECORDED = { recorded: true } satisfies HostModel;
+/** The process has run its course: liveness stops answering for it before anything is signaled. */
+const GONE = { exitsAfterProbes: 0 } satisfies HostModel;
+const ZOMBIE = { ...GONE, afterExit: 'zombie' } satisfies HostModel;
+const NO_ROW = { ...GONE, afterExit: 'no-row' } satisfies HostModel;
+const STOPPED_BY_TERM: Expect = { outcome: STOPPED, signals: ['SIGTERM'] };
+const STOPPED_BY_KILL: Expect = { outcome: STOPPED, signals: ['SIGTERM', 'SIGKILL'] };
+const NEVER_SIGNALED: Expect = { outcome: STOPPED, signals: [] };
+const REFUSED_ON_TERM: Expect = { outcome: refused('SIGTERM'), signals: ['SIGTERM'] };
+const REFUSED_ON_KILL: Expect = { outcome: refused('SIGKILL'), signals: STOPPED_BY_KILL.signals };
+const EXIT_TIMED_OUT: Expect = {
+  outcome: { status: 'retained', reason: 'exit-timeout', signal: 'SIGKILL' },
+  signals: STOPPED_BY_KILL.signals,
+};
+const UNVERIFIED: Expect = {
+  outcome: { status: 'retained', reason: 'identity-unverified' },
+  signals: [],
+};
+
+const STOP_CASES: HostCase[] = [
+  // A caller that tracks only a pid — Metro spawns the process itself — can prove the exit only from
+  // the pid being released or from a zombie's row: it holds no identity to compare an answer against.
+  { name: 'leaves on SIGTERM', exitsOnSignal: 'SIGTERM', expect: STOPPED_BY_TERM },
+  { name: 'ignores SIGTERM and takes SIGKILL', exitsOnSignal: 'SIGKILL', expect: STOPPED_BY_KILL },
+  { name: 'was gone before anything was signaled', ...GONE, expect: NEVER_SIGNALED },
+  { name: 'is held by another owner', ...DENIED_ON_TERM, ...HELD, expect: REFUSED_ON_TERM },
+  { name: 'refuses SIGKILL', answers: { SIGKILL: 'EPERM' }, ...HELD, expect: REFUSED_ON_KILL },
+  {
+    name: 'is released inside the budget',
+    ...DENIED_ON_TERM,
+    exitsAfterProbes: 3,
+    expect: STOPPED_BY_TERM,
+  },
+  { name: 'leaves a zombie the budget cannot wait out', ...ZOMBIE, expect: STOPPED_BY_TERM },
+  {
+    // An absent row is not proof of absence: a hidden or protected process is absent from `ps` too,
+    // so the pid that still answers `kill(pid, 0)` is retained rather than forgotten.
+    name: 'leaves no row while its pid still answers',
+    ...NO_ROW,
+    ...DENIED_ON_TERM,
+    expect: REFUSED_ON_TERM,
+  },
+  { name: 'outlives both budgets', ...HELD, expect: EXIT_TIMED_OUT },
+
+  // A caller that recorded an identity — the companion's state file — additionally proves a stale pid,
+  // and an unreadable identity stops the stopper before it signals a process nobody can name.
+  { name: 'recorded process was gone first', ...RECORDED, ...GONE, expect: NEVER_SIGNALED },
+  { name: 'recorded process left a zombie', ...RECORDED, ...ZOMBIE, expect: NEVER_SIGNALED },
+  {
+    name: 'recorded process refuses SIGTERM',
+    ...RECORDED,
+    ...DENIED_ON_TERM,
+    ...HELD,
+    expect: REFUSED_ON_TERM,
+  },
+  {
+    // Windows hides only the command line of an elevated or other-user process, and that is the #3540
+    // companion: it must be signaled and retained, never reported stopped.
+    name: 'recorded process hides its command line, as an elevated one does',
+    ...RECORDED,
+    identity: { command: null },
+    ...DENIED_ON_TERM,
+    ...HELD,
+    expect: REFUSED_ON_TERM,
+  },
+  {
+    name: 'recorded process hides its start time',
+    ...RECORDED,
+    identity: { startTime: null },
+    ...DENIED_ON_TERM,
+    ...HELD,
+    expect: REFUSED_ON_TERM,
+  },
+  {
+    // POSIX `hidepid`, or a `ps` that times out: the pid answers `kill(pid, 0)` and nothing else.
+    name: 'live recorded pid whose whole identity is unreadable',
+    ...RECORDED,
+    ...UNREADABLE,
+    ...HELD,
+    expect: UNVERIFIED,
+  },
+  {
+    name: 'live recorded pid whose process tool the host refuses',
+    ...RECORDED,
+    psFails: true,
+    ...HELD,
+    expect: UNVERIFIED,
+  },
+  {
+    name: 'pid the host has handed to another lifetime',
+    ...RECORDED,
+    identity: { startTime: OTHER_START },
+    expect: NEVER_SIGNALED,
+  },
+  {
+    name: 'pid whose command line names a different program',
+    ...RECORDED,
+    identity: { command: '/usr/bin/other --serve' },
+    expect: NEVER_SIGNALED,
+  },
+];
+
+function driveHost(model: HostCase): { signals: Signal[] } {
+  const signals: Signal[] = [];
+  let ended = false;
+  let probes = 0;
+  const hasEnded = () =>
+    ended ||
+    (model.afterExit !== 'held' && probes >= (model.exitsAfterProbes ?? Number.POSITIVE_INFINITY));
+  vi.spyOn(process, 'kill').mockImplementation(((_pid: number, signal: string | number) => {
+    if (signal === 0) {
+      probes += 1;
+      // A released pid answers `ESRCH`. A zombie, and on Windows a process whose object still has
+      // handles, both keep answering `EPERM`, and only the process table tells those apart.
+      if (hasEnded() && model.afterExit === undefined)
+        throw Object.assign(new Error('no such process'), { code: 'ESRCH' });
+      if (hasEnded()) throw Object.assign(new Error('operation not permitted'), { code: 'EPERM' });
+      return true;
+    }
+    const name = signal as Signal;
+    signals.push(name);
+    const errno = model.answers?.[name];
+    if (errno) throw Object.assign(new Error(`kill refused: ${errno}`), { code: errno });
+    if (model.exitsOnSignal === name) ended = true;
+    return true;
+  }) as typeof process.kill);
+
+  // Absent means the recorded process, `null` means the host withholds the column, and a string is
+  // what it says instead.
+  const identity = {
+    startTime: model.identity?.startTime === undefined ? START : model.identity.startTime,
+    command: model.identity?.command === undefined ? COMMAND : model.identity.command,
+  };
+  mockRunCmd.mockImplementation(async (_command: string, args: readonly string[]) => {
+    if (model.psFails) return PS_REFUSED;
+    // A process the host has fully released, and one that left no row, both answer no fields.
+    if (hasEnded() && model.afterExit !== 'zombie') return EMPTY;
+    const field = String(args[3]);
+    if (field === 'state=') return psAnswer(hasEnded() && model.afterExit === 'zombie' ? 'Z' : 'S');
+    // Withheld is its own answer: `ps` finds the row and reports nothing for that column, which is
+    // how Windows answers `CommandLine` for another user's process.
+    const column = field === 'lstart=' ? identity.startTime : identity.command;
+    return column === null ? EMPTY : psAnswer(column);
+  });
+  return { signals };
+}
+
+function targetOf(model: HostCase): ProcessStopTarget {
+  if (!model.recorded) return { pid: PID };
+  return { pid: PID, startTime: START, matches: (command) => command === COMMAND };
+}
+
+afterEach(() => {
+  vi.useRealTimers();
+  vi.restoreAllMocks();
+  mockRunCmd.mockReset();
+});
+
+// The host model above answers with POSIX `ps` fields; a Windows host answers the same questions from
+// CIM, which never reports a zombie state. win32 branches live in host-process-windows.test.ts.
+test.skipIf(process.platform === 'win32').each(STOP_CASES)(
+  'stopProcessAndConfirmExit $name',
+  async (testCase) => {
+    const host = driveHost(testCase);
+    // Both stage budgets, advanced on a fake clock: a retained outcome must not cost real time.
+    vi.useFakeTimers();
+    const stop = stopProcessAndConfirmExit(targetOf(testCase), BUDGET);
+    await vi.advanceTimersByTimeAsync(BUDGET.termTimeoutMs + BUDGET.killTimeoutMs);
+
+    assert.deepEqual(await stop, testCase.expect.outcome);
+    assert.deepEqual(host.signals, testCase.expect.signals);
+  },
+);
+
+// `kill(0, SIGTERM)` addresses this very process group, so an unvalidated pid must not reach it.
+test.each([0, -1, 1.5, Number.NaN])(
+  'never signals a pid that is not a process: %s',
+  async (pid) => {
+    const signals: string[] = [];
+    vi.spyOn(process, 'kill').mockImplementation(((_pid: number, signal: string) => {
+      signals.push(signal);
+      throw Object.assign(new Error('no such process'), { code: 'ESRCH' });
+    }) as typeof process.kill);
+    assert.deepEqual(await stopProcessAndConfirmExit({ pid }, BUDGET), STOPPED);
+    assert.deepEqual(signals, []);
+  },
+);
+
+// Stale or malformed process state can name the caller's own pid, and SIGTERM would end the very
+// process trying to stop it.
+test('refuses to stop the current process', async () => {
+  const signals: string[] = [];
+  vi.spyOn(process, 'kill').mockImplementation(((_pid: number, signal: string) => {
+    signals.push(signal);
+    return true;
+  }) as typeof process.kill);
+  await assert.rejects(() => stopProcessAndConfirmExit({ pid: process.pid }, BUDGET), RangeError);
+  assert.deepEqual(signals, []);
+});
+
+// Node's own argument faults carry a `code` that is not an errno, and reading one as a refusal would
+// send the caller off to stop a process that never existed. The fault itself has to reach it.
+test('propagates an argument fault instead of reporting it as a host refusal', async () => {
+  const failure = Object.assign(new TypeError('bad signal'), { code: 'ERR_INVALID_ARG_TYPE' });
+  vi.spyOn(process, 'kill').mockImplementation(() => {
+    throw failure;
+  });
+  const settled = await stopProcessAndConfirmExit({ pid: PID }, BUDGET).catch(
+    (error: unknown) => error,
+  );
+  assert.equal(settled, failure);
+});