add better logs around worker restarts (#57004)

We currently log when a worker is restarted but not when we send the kill signal, which can create a delta in logs of cryptic errors while the worker is exiting. This explicitly logs when we're terminating the static worker prior to a restart, and also adds an optional logger fn so that we pretty-print the messages. 

[slack x-ref](https://vercel.slack.com/archives/C061DJBG8PN/p1697491350970269)

packages/next/src/build/index.ts

@@ -1207,6 +1207,7 @@ export default async function build(
 
         return new Worker(staticWorkerPath, {
           timeout: timeout * 1000,
+          logger: Log,
           onRestart: (method, [arg], attempts) => {
             if (method === 'exportPage') {
               const pagePath = arg.path

packages/next/src/lib/worker.ts

@@ -21,11 +21,12 @@ export class Worker {
     options: FarmOptions & {
       timeout?: number
       onRestart?: (method: string, args: any[], attempts: number) => void
+      logger?: Pick<typeof console, 'error' | 'info' | 'warn'>
       exposedMethods: ReadonlyArray<string>
       enableWorkerThreads?: boolean
     }
   ) {
-    let { timeout, onRestart, ...farmOptions } = options
+    let { timeout, onRestart, logger = console, ...farmOptions } = options
 
     let restartPromise: Promise<typeof RESTARTED>
     let resolveRestartPromise: (arg: typeof RESTARTED) => void
@@ -68,10 +69,9 @@ export class Worker {
           _child?: ChildProcess
         }[]) {
           worker._child?.on('exit', (code, signal) => {
-            // log unexpected exit if .end() wasn't called
             if ((code || (signal && signal !== 'SIGINT')) && this._worker) {
-              console.error(
-                `Static worker unexpectedly exited with code: ${code} and signal: ${signal}`
+              logger.error(
+                `Static worker exited with code: ${code} and signal: ${signal}`
               )
             }
           })
@@ -88,6 +88,11 @@ export class Worker {
       if (!worker) return
       const resolve = resolveRestartPromise
       createWorker()
+      logger.warn(
+        `Sending SIGTERM signal to static worker due to timeout${
+          timeout ? ` of ${timeout / 1000} seconds` : ''
+        }. Subsequent errors may be a result of the worker exiting.`
+      )
       worker.end().then(() => {
         resolve(RESTARTED)
       })

test/production/app-dir/worker-restart/worker-restart.test.ts

@@ -8,6 +8,12 @@ describe('worker-restart', () => {
     })
 
     const output = stdout + stderr
+    expect(output).toContain(
+      'Sending SIGTERM signal to static worker due to timeout of 10 seconds. Subsequent errors may be a result of the worker exiting.'
+    )
+    expect(output).toContain(
+      'Static worker exited with code: null and signal: SIGTERM'
+    )
     expect(output).toContain(
       'Restarted static page generation for /bad-page because it took more than 10 seconds'
     )