Ensure next commands respect termination signals (vercel#19433)

Fixes vercel#16173

## What

Restores handling of termination signals, `SIGTERM` and `SIGINT`, to allow graceful termination of next commands. Seems to have been removed during a child process refactor vercel#6450, was this intentional?

## Why 

Currently the command processes have to be forcefully killed. This would help those using Next.js with custom servers and tools like Docker and Kubernetes that rely on termination signals to shutdown instances.

---

Where would be a good location to add some tests? [test/integration/cli/test/index.test.js](https://github.com/vercel/next.js/blob/fc98c13a2e6bc545073de0eedbe045352b387f6d/test/integration/cli/test/index.test.js)?

packages/next/bin/next.ts

@@ -104,6 +104,10 @@ if (typeof React.Suspense === 'undefined') {
   )
 }
 
+// Make sure commands gracefully respect termination signals (e.g. from Docker)
+process.on('SIGTERM', () => process.exit(0))
+process.on('SIGINT', () => process.exit(0))
+
 commands[command]().then((exec) => exec(forwardedArgs))
 
 if (command === 'dev') {

test/integration/cli/test/index.test.js

@@ -4,6 +4,7 @@ import {
   findPort,
   killApp,
   launchApp,
+  nextBuild,
   runNextCommand,
   runNextCommandDev,
 } from 'next-test-utils'
@@ -85,6 +86,35 @@ describe('CLI Usage', () => {
       })
       expect(stderr).not.toContain('UnhandledPromiseRejectionWarning')
     })
+
+    test('should exit when SIGINT is signalled', async () => {
+      const killSigint = (instance) =>
+        setTimeout(() => instance.kill('SIGINT'), 500)
+      const { code, signal } = await runNextCommand(['build', dir], {
+        ignoreFail: true,
+        instance: killSigint,
+      })
+      // Node can only partially emulate signals on Windows. Our signal handlers won't affect the exit code.
+      // See: https://nodejs.org/api/process.html#process_signal_events
+      const expectedExitCode = process.platform === `win32` ? null : 0
+      const expectedExitSignal = process.platform === `win32` ? 'SIGINT' : null
+      expect(code).toBe(expectedExitCode)
+      expect(signal).toBe(expectedExitSignal)
+    })
+    test('should exit when SIGTERM is signalled', async () => {
+      const killSigterm = (instance) =>
+        setTimeout(() => instance.kill('SIGTERM'), 500)
+      const { code, signal } = await runNextCommand(['build', dir], {
+        ignoreFail: true,
+        instance: killSigterm,
+      })
+      // Node can only partially emulate signals on Windows. Our signal handlers won't affect the exit code.
+      // See: https://nodejs.org/api/process.html#process_signal_events
+      const expectedExitCode = process.platform === `win32` ? null : 0
+      const expectedExitSignal = process.platform === `win32` ? 'SIGTERM' : null
+      expect(code).toBe(expectedExitCode)
+      expect(signal).toBe(expectedExitSignal)
+    })
   })
 
   describe('dev', () => {
@@ -193,6 +223,37 @@ describe('CLI Usage', () => {
       })
       expect(stderr).not.toContain('UnhandledPromiseRejectionWarning')
     })
+
+    test('should exit when SIGINT is signalled', async () => {
+      const killSigint = (instance) =>
+        setTimeout(() => instance.kill('SIGINT'), 500)
+      const port = await findPort()
+      const { code, signal } = await runNextCommand(['dev', dir, '-p', port], {
+        ignoreFail: true,
+        instance: killSigint,
+      })
+      // Node can only partially emulate signals on Windows. Our signal handlers won't affect the exit code.
+      // See: https://nodejs.org/api/process.html#process_signal_events
+      const expectedExitCode = process.platform === `win32` ? null : 0
+      const expectedExitSignal = process.platform === `win32` ? 'SIGINT' : null
+      expect(code).toBe(expectedExitCode)
+      expect(signal).toBe(expectedExitSignal)
+    })
+    test('should exit when SIGTERM is signalled', async () => {
+      const killSigterm = (instance) =>
+        setTimeout(() => instance.kill('SIGTERM'), 500)
+      const port = await findPort()
+      const { code, signal } = await runNextCommand(['dev', dir, '-p', port], {
+        ignoreFail: true,
+        instance: killSigterm,
+      })
+      // Node can only partially emulate signals on Windows. Our signal handlers won't affect the exit code.
+      // See: https://nodejs.org/api/process.html#process_signal_events
+      const expectedExitCode = process.platform === `win32` ? null : 0
+      const expectedExitSignal = process.platform === `win32` ? 'SIGTERM' : null
+      expect(code).toBe(expectedExitCode)
+      expect(signal).toBe(expectedExitSignal)
+    })
   })
 
   describe('start', () => {
@@ -312,6 +373,45 @@ describe('CLI Usage', () => {
 
       await killApp(instance)
     })
+
+    test('should exit when SIGINT is signalled', async () => {
+      const killSigint = (instance) =>
+        setTimeout(() => instance.kill('SIGINT'), 500)
+      await nextBuild(dir)
+      const port = await findPort()
+      const { code, signal } = await runNextCommand(
+        ['start', dir, '-p', port],
+        {
+          ignoreFail: true,
+          instance: killSigint,
+        }
+      )
+      // Node can only partially emulate signals on Windows. Our signal handlers won't affect the exit code.
+      // See: https://nodejs.org/api/process.html#process_signal_events
+      const expectedExitCode = process.platform === `win32` ? null : 0
+      const expectedExitSignal = process.platform === `win32` ? 'SIGINT' : null
+      expect(code).toBe(expectedExitCode)
+      expect(signal).toBe(expectedExitSignal)
+    })
+    test('should exit when SIGTERM is signalled', async () => {
+      const killSigterm = (instance) =>
+        setTimeout(() => instance.kill('SIGTERM'), 500)
+      await nextBuild(dir)
+      const port = await findPort()
+      const { code, signal } = await runNextCommand(
+        ['start', dir, '-p', port],
+        {
+          ignoreFail: true,
+          instance: killSigterm,
+        }
+      )
+      // Node can only partially emulate signals on Windows. Our signal handlers won't affect the exit code.
+      // See: https://nodejs.org/api/process.html#process_signal_events
+      const expectedExitCode = process.platform === `win32` ? null : 0
+      const expectedExitSignal = process.platform === `win32` ? 'SIGTERM' : null
+      expect(code).toBe(expectedExitCode)
+      expect(signal).toBe(expectedExitSignal)
+    })
   })
 
   describe('export', () => {

test/lib/next-test-utils.js

@@ -130,7 +130,7 @@ export function runNextCommand(argv, options = {}) {
       })
     }
 
-    instance.on('close', (code) => {
+    instance.on('close', (code, signal) => {
       if (
         !options.stderr &&
         !options.stdout &&
@@ -142,6 +142,7 @@ export function runNextCommand(argv, options = {}) {
 
       resolve({
         code,
+        signal,
         stdout: stdoutOutput,
         stderr: stderrOutput,
       })