fix(process): keep Windows tree termination at normal priority (#32559)

src/packages/process-host/src/windows-tree-kill.test.ts

@@ -1,5 +1,17 @@
 import { ChildProcess } from 'node:child_process'
+import type * as Os from 'node:os'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+const { getPriorityMock, setPriorityMock } = vi.hoisted(() => ({
+  getPriorityMock: vi.fn(),
+  setPriorityMock: vi.fn()
+}))
+
+vi.mock('node:os', async (importOriginal) => ({
+  ...(await importOriginal<typeof Os>()),
+  getPriority: getPriorityMock,
+  setPriority: setPriorityMock
+}))
 import {
   _setWindowsTaskkillLauncherForTests,
   inFlightWindowsTreeKill,
@@ -154,4 +166,75 @@ describe('windows tree kill tracker', () => {
 
     expect(launch).toHaveBeenCalledTimes(2)
   })
+
+  describe('termination helper scheduling', () => {
+    const originalPlatform = process.platform
+
+    beforeEach(() => {
+      Object.defineProperty(process, 'platform', { configurable: true, value: 'win32' })
+      getPriorityMock.mockReset().mockReturnValue(10)
+      setPriorityMock.mockReset()
+      launch.mockImplementationOnce(() => {
+        const killer = new FakeProcess()
+        Object.defineProperty(killer, 'pid', { value: 5678 })
+        launched.push(killer)
+        return killer
+      })
+    })
+
+    afterEach(() => {
+      Object.defineProperty(process, 'platform', {
+        configurable: true,
+        value: originalPlatform
+      })
+    })
+
+    it.each([10, 19])('restores normal priority for a background helper at %i', (priority) => {
+      getPriorityMock.mockReturnValue(priority)
+      const killRoot = vi.fn()
+      const kill = startWindowsTreeKill(new FakeProcess(), 1234, killRoot)
+
+      expect(setPriorityMock).toHaveBeenCalledWith(5678, 0)
+      expect(kill.verdict()).toBe('live')
+      expect(killRoot).not.toHaveBeenCalled()
+      launched[0].emit('close', 0)
+      expect(kill.verdict()).toBe('exited')
+    })
+
+    it.each([0, -7, -14])('preserves a helper already at priority %i', (priority) => {
+      getPriorityMock.mockReturnValue(priority)
+      startWindowsTreeKill(new FakeProcess(), 1234, vi.fn())
+
+      expect(setPriorityMock).not.toHaveBeenCalled()
+      launched[0].emit('close', 0)
+    })
+
+    it.each(['read', 'write'])('keeps tracking when the priority %s fails', async (operation) => {
+      const fail = (): never => {
+        throw new Error('priority access denied or helper already exited')
+      }
+      if (operation === 'read') {
+        getPriorityMock.mockImplementation(fail)
+      } else {
+        setPriorityMock.mockImplementation(fail)
+      }
+      const killRoot = vi.fn()
+      const kill = startWindowsTreeKill(new FakeProcess(), 1234, killRoot)
+
+      await vi.advanceTimersByTimeAsync(WINDOWS_TASKKILL_WAIT_MS)
+      expect(kill.verdict()).toBe('live')
+      expect(killRoot).not.toHaveBeenCalled()
+      launched[0].emit('close', 0)
+      await expect(kill.settled).resolves.toBe('exited')
+    })
+
+    it('leaves POSIX scheduling unchanged', () => {
+      Object.defineProperty(process, 'platform', { configurable: true, value: 'linux' })
+      startWindowsTreeKill(new FakeProcess(), 1234, vi.fn())
+
+      expect(getPriorityMock).not.toHaveBeenCalled()
+      expect(setPriorityMock).not.toHaveBeenCalled()
+      launched[0].emit('close', 0)
+    })
+  })
 })

src/packages/process-host/src/windows-tree-kill.ts

@@ -1,4 +1,5 @@
 import { spawn as nodeSpawn, type ChildProcess } from 'node:child_process'
+import { constants, getPriority, setPriority } from 'node:os'
 import { windowsSystem32Binary } from './windows-system-binary'
 
 /**
@@ -118,6 +119,16 @@ export function startWindowsTreeKill(
     finish('unverifiable')
     return kill
   }
+  if (process.platform === 'win32' && killer.pid !== undefined) {
+    try {
+      // Tree termination must not inherit a background caller's scheduling penalty.
+      if (getPriority(killer.pid) > constants.priority.PRIORITY_NORMAL) {
+        setPriority(killer.pid, constants.priority.PRIORITY_NORMAL)
+      }
+    } catch {
+      /* Priority changes are best effort; keep tracking the original termination. */
+    }
+  }
   killer.once('error', () => finish('unverifiable'))
   killer.once('close', (code) => finish(code === 0 ? 'exited' : 'unverifiable'))
   deadline = setTimeout(abandon, WINDOWS_TASKKILL_DEADLINE_MS)