Ensure process exits after build completes (#23207)

This ensures we exit the build process after the build completes even if timers/connections are left open since this can cause the process to hang indefinitely. This was the previous behavior although got changed un-intentionally in https://github.com/vercel/next.js/commit/e27b7e996d6e7e6772272f3d189ce93dac25fc3d

This updates our production test with an open timer to ensure it doesn't block the process from completing. 

packages/next/bin/next.ts

@@ -106,7 +106,15 @@ if (typeof React.Suspense === 'undefined') {
 process.on('SIGTERM', () => process.exit(0))
 process.on('SIGINT', () => process.exit(0))
 
-commands[command]().then((exec) => exec(forwardedArgs))
+commands[command]()
+  .then((exec) => exec(forwardedArgs))
+  .then(() => {
+    if (command === 'build') {
+      // ensure process exits after build completes so open handles/connections
+      // don't cause process to hang
+      process.exit(0)
+    }
+  })
 
 if (command === 'dev') {
   const { CONFIG_FILE } = require('../next-server/lib/constants')

test/integration/production/next.config.js

+// intervals/open connections shouldn't block build from exiting
+setInterval(() => {}, 250)
+
 module.exports = {
   onDemandEntries: {
     // Make sure entries are not getting disposed.