fix(subprocess): defer native handshake allocation

packages/subprocess/subprocess-local/src/runner-launch.ts

@@ -16,7 +16,7 @@ import {
 import type { RunnerEvent, RunnerFiles, RunnerRequest } from './runner-protocol.ts'
 import { childEnv } from './spawn.ts'
 
-const handshakeWait = new Int32Array(new SharedArrayBuffer(4))
+let handshakeWait: Int32Array | undefined
 const RUNNER_HANDSHAKE_TIMEOUT_MS = 10_000
 const RUNNER_EVENT_POLL_MS = 100
 const PACKAGED_RUNNER_ARG = '--dsh-internal-subprocess-runner'
@@ -102,6 +102,7 @@ function waitForRunnerHandshake(child: ChildProcess, files: RunnerFiles): Runner
     if (terminal?.type === 'spawn-error' || terminal?.type === 'runner-error') return { pid: -1, events }
     if (child.pid === undefined) throw new Error('native subprocess runner failed to start')
     if (runnerExited(child, child.pid)) throw new Error('native subprocess runner exited before reporting target start')
+    handshakeWait ??= new Int32Array(new SharedArrayBuffer(4))
     Atomics.wait(handshakeWait, 0, 0, 5)
   }
   throw new Error(`native subprocess runner did not report target start within ${String(RUNNER_HANDSHAKE_TIMEOUT_MS)}ms`)

packages/subprocess/subprocess-local/tests/spawn-runner.spec.ts

@@ -63,6 +63,20 @@ describe('spawn runner transport', () => {
     expect(spawnRunnerInvocation()).toEqual(sourceInvocation)
   })
 
+  it('does not require SharedArrayBuffer until a native handshake runs', async () => {
+    const descriptor = Object.getOwnPropertyDescriptor(globalThis, 'SharedArrayBuffer')
+    Object.defineProperty(globalThis, 'SharedArrayBuffer', { configurable: true, value: undefined })
+    vi.resetModules()
+    try {
+      const isolated = await import('../src/runner-launch.ts')
+      expect(isolated.runnerStdio(spec())).toEqual(['ignore', 'pipe', 'pipe'])
+    } finally {
+      if (descriptor === undefined) Reflect.deleteProperty(globalThis, 'SharedArrayBuffer')
+      else Object.defineProperty(globalThis, 'SharedArrayBuffer', descriptor)
+      vi.resetModules()
+    }
+  })
+
   it('re-enters a packaged executable through its private runner dispatch', () => {
     const packagedProcess = process as NodeJS.Process & { pkg?: unknown }
     const original = Object.getOwnPropertyDescriptor(packagedProcess, 'pkg')