Clean-up fetch metrics tracking (#64746)

This ensures we only track fetch metrics in development mode as that's
the only time we report them currently, this also adds an upper limit on
how many metrics we store per-request as previously this was unbounded,
on top of that this ensures we don't keep tracking fetch metrics after
the request has ended as we only report on request end, and finally this
adds a clean-up to the reference we attach to the request object
containing the fetch metrics once we have used them.

Closes: #64212

Closes NEXT-3159

packages/next/src/client/components/static-generation-async-storage.external.ts

@@ -51,6 +51,8 @@ export interface StaticGenerationStore {
 
   isDraftMode?: boolean
   isUnstableNoStore?: boolean
+
+  requestEndedState?: { ended?: boolean }
 }
 
 export type StaticGenerationAsyncStorage =

packages/next/src/server/app-render/app-render.tsx

@@ -590,7 +590,8 @@ async function renderToHTMLOrFlightImpl(
   pagePath: string,
   query: NextParsedUrlQuery,
   renderOpts: RenderOpts,
-  baseCtx: AppRenderBaseContext
+  baseCtx: AppRenderBaseContext,
+  requestEndedState: { ended?: boolean }
 ) {
   const isNotFoundPath = pagePath === '/404'
 
@@ -631,6 +632,8 @@ async function renderToHTMLOrFlightImpl(
     isNodeNextRequest(req)
   ) {
     req.originalRequest.on('end', () => {
+      requestEndedState.ended = true
+
       if ('performance' in globalThis) {
         const metrics = getClientComponentLoaderMetrics({ reset: true })
         if (metrics) {
@@ -1448,14 +1451,23 @@ export const renderToHTMLOrFlight: AppPageRender = (
         {
           urlPathname: pathname,
           renderOpts,
+          requestEndedState: { ended: false },
         },
         (staticGenerationStore) =>
-          renderToHTMLOrFlightImpl(req, res, pagePath, query, renderOpts, {
-            requestStore,
-            staticGenerationStore,
-            componentMod: renderOpts.ComponentMod,
+          renderToHTMLOrFlightImpl(
+            req,
+            res,
+            pagePath,
+            query,
             renderOpts,
-          })
+            {
+              requestStore,
+              staticGenerationStore,
+              componentMod: renderOpts.ComponentMod,
+              renderOpts,
+            },
+            staticGenerationStore.requestEndedState || {}
+          )
       )
   )
 }

packages/next/src/server/async-storage/static-generation-async-storage-wrapper.ts

@@ -9,6 +9,7 @@ import type { FetchMetric } from '../base-http'
 
 export type StaticGenerationContext = {
   urlPathname: string
+  requestEndedState?: { ended?: boolean }
   renderOpts: {
     incrementalCache?: IncrementalCache
     isOnDemandRevalidate?: boolean
@@ -50,7 +51,7 @@ export const StaticGenerationAsyncStorageWrapper: AsyncStorageWrapper<
 > = {
   wrap<Result>(
     storage: AsyncLocalStorage<StaticGenerationStore>,
-    { urlPathname, renderOpts }: StaticGenerationContext,
+    { urlPathname, renderOpts, requestEndedState }: StaticGenerationContext,
     callback: (store: StaticGenerationStore) => Result
   ): Result {
     /**
@@ -96,6 +97,7 @@ export const StaticGenerationAsyncStorageWrapper: AsyncStorageWrapper<
       isDraftMode: renderOpts.isDraftMode,
 
       prerenderState,
+      requestEndedState,
     }
 
     // TODO: remove this when we resolve accessing the store outside the execution context

packages/next/src/server/lib/patch-fetch.ts

@@ -163,7 +163,13 @@ function trackFetchMetric(
   staticGenerationStore: StaticGenerationStore,
   ctx: Omit<FetchMetric, 'end' | 'idx'>
 ) {
-  if (!staticGenerationStore) return
+  if (
+    !staticGenerationStore ||
+    staticGenerationStore.requestEndedState?.ended ||
+    process.env.NODE_ENV !== 'development'
+  ) {
+    return
+  }
   staticGenerationStore.fetchMetrics ??= []
 
   const dedupeFields = ['url', 'status', 'method'] as const
@@ -181,6 +187,25 @@ function trackFetchMetric(
     end: Date.now(),
     idx: staticGenerationStore.nextFetchId || 0,
   })
+
+  // only store top 10 metrics to avoid storing too many
+  if (staticGenerationStore.fetchMetrics.length > 10) {
+    // sort slowest first as these should be highlighted
+    staticGenerationStore.fetchMetrics.sort((a, b) => {
+      const aDur = a.end - a.start
+      const bDur = b.end - b.start
+
+      if (aDur < bDur) {
+        return 1
+      } else if (aDur > bDur) {
+        return -1
+      }
+      return 0
+    })
+    // now grab top 10
+    staticGenerationStore.fetchMetrics =
+      staticGenerationStore.fetchMetrics.slice(0, 10)
+  }
 }
 
 interface PatchableModule {

packages/next/src/server/next-server.ts

@@ -1232,6 +1232,7 @@ export default class NextNodeServer extends BaseServer<
               }
             }
           }
+          delete normalizedReq.fetchMetrics
           originalResponse.off('close', reqCallback)
         }
         originalResponse.on('close', reqCallback)