test(subprocess): publish exit fixture state atomically

packages/subprocess/subprocess-local/tests/fixtures/managed-tree.ts

@@ -1,5 +1,5 @@
 import { spawn } from 'node:child_process'
-import { writeFile } from 'node:fs/promises'
+import { rename, writeFile } from 'node:fs/promises'
 
 const [statePath] = process.argv.slice(2)
 if (statePath === undefined) throw new Error('usage: managed-tree.ts <state-path>')
@@ -12,5 +12,7 @@ const descendant = spawn(process.execPath, [
 ], { stdio: 'ignore' })
 if (descendant.pid === undefined) throw new Error('managed descendant did not publish a pid')
 
-await writeFile(statePath, JSON.stringify({ root: process.pid, descendant: descendant.pid }))
+const pendingStatePath = `${statePath}.pending-${process.pid}`
+await writeFile(pendingStatePath, JSON.stringify({ root: process.pid, descendant: descendant.pid }))
+await rename(pendingStatePath, statePath)
 setInterval(() => {}, 60_000)