[devtools] Add an endpoint to poll for server status

Author: bgw

packages/next/src/client/components/react-dev-overlay/server/restart-dev-server-middleware.ts

@@ -15,14 +15,23 @@ export function getRestartDevServerMiddleware({
   telemetry,
   turbopackProject,
 }: RestartDevServerMiddlewareConfig) {
-  return async function (
+  /**
+   * Some random value between 1 and Number.MAX_SAFE_INTEGER (inclusive). The same value is returned
+   * on every call to `__nextjs_server_status` until the server is restarted.
+   *
+   * Can be used to determine if two server status responses are from the same process or a
+   * different (restarted) process.
+   */
+  const executionId: number =
+    Math.floor(Math.random() * Number.MAX_SAFE_INTEGER) + 1
+
+  async function handleRestartRequest(
     req: IncomingMessage,
     res: ServerResponse,
-    next: () => void
-  ): Promise<void> {
-    const { pathname, searchParams } = new URL(`http://n${req.url}`)
-    if (pathname !== '/__nextjs_restart_dev' || req.method !== 'POST') {
-      return next()
+    searchParams: URLSearchParams
+  ) {
+    if (req.method !== 'POST') {
+      return middlewareResponse.methodNotAllowed(res)
     }
 
     const invalidatePersistentCache = searchParams.has(
@@ -48,4 +57,31 @@ export function getRestartDevServerMiddleware({
 
     return middlewareResponse.noContent(res)
   }
+
+  async function handleServerStatus(req: IncomingMessage, res: ServerResponse) {
+    if (req.method !== 'GET') {
+      return middlewareResponse.methodNotAllowed(res)
+    }
+
+    return middlewareResponse.json(res, {
+      executionId,
+    })
+  }
+
+  return async function (
+    req: IncomingMessage,
+    res: ServerResponse,
+    next: () => void
+  ): Promise<void> {
+    const { pathname, searchParams } = new URL(`http://n${req.url}`)
+
+    switch (pathname) {
+      case '/__nextjs_restart_dev':
+        return await handleRestartRequest(req, res, searchParams)
+      case '/__nextjs_server_status':
+        return await handleServerStatus(req, res)
+      default:
+        return next()
+    }
+  }
 }