From e06e27c9444c8a59e1c1ba0812d57419409fa246 Mon Sep 17 00:00:00 2001
From: Suneil Nyamathi <suneil.nyamathi@yahooinc.com>
Date: Fri, 15 Aug 2025 04:48:32 +0000
Subject: [PATCH 1/2] fix: memory leak from cloneResponse

---
 .../next/src/server/lib/clone-response.ts     | 37 +++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/packages/next/src/server/lib/clone-response.ts b/packages/next/src/server/lib/clone-response.ts
index d87e4558e41e01..b17148fe901770 100644
--- a/packages/next/src/server/lib/clone-response.ts
+++ b/packages/next/src/server/lib/clone-response.ts
@@ -1,3 +1,22 @@
+// @ts-ignore https://nodejs.org/api/stream.html#streamreadableisdisturbedstream
+import { isDisturbed, isErrored } from 'node:stream'
+
+// https://github.com/nodejs/undici/blob/c399fdf4246be4992e89efe42aa83062edd0609f/lib/web/fetch/body.js#L32
+const noop = () => {}
+const registry = new FinalizationRegistry(
+  (weakRef: WeakRef<ReadableStream>) => {
+    const stream = weakRef.deref()
+    if (
+      stream &&
+      !stream.locked &&
+      !isDisturbed(stream) &&
+      !isErrored(stream as any as NodeJS.ReadableStream)
+    ) {
+      stream.cancel('Response object has been garbage collected').then(noop)
+    }
+  }
+)
+
 /**
  * Clones a response by teeing the body so we can return two independent
  * ReadableStreams from it. This avoids the bug in the undici library around
@@ -33,6 +52,24 @@ export function cloneResponse(original: Response): [Response, Response] {
     writable: false,
   })
 
+  // The Fetch Standard allows users to skip consuming the response body by
+  // relying on garbage collection to release connection resources.
+  // https://github.com/nodejs/undici?tab=readme-ov-file#garbage-collection
+  //
+  // To cancel the stream you then need to cancel both resulting branches.
+  // Teeing a stream will generally lock it for the duration, preventing other
+  // readers from locking it.
+  // https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/tee
+
+  // cloned2 is stored in a react cache and cloned for subsequent requests.
+  // It is the original request, and is is garbage collected by a
+  // FinalizationRegistry in Undici, but since we're tee-ing the stream
+  // ourselves, we need to cancel clone1's stream (the response returned from
+  // our dedupe fetch) when clone1 is reclaimed, otherwise we leak memory.
+  if (cloned1.body) {
+    registry.register(cloned1, new WeakRef(cloned1.body))
+  }
+
   const cloned2 = new Response(body2, {
     status: original.status,
     statusText: original.statusText,

From 26140a9e4b2f8801852ab9a8cd2fc7270078dd81 Mon Sep 17 00:00:00 2001
From: Suneil Nyamathi <snyamathi@gmail.com>
Date: Fri, 15 Aug 2025 10:18:22 -0700
Subject: [PATCH 2/2] Update clone-response.ts

---
 .../next/src/server/lib/clone-response.ts     | 24 +++++++------------
 1 file changed, 9 insertions(+), 15 deletions(-)

diff --git a/packages/next/src/server/lib/clone-response.ts b/packages/next/src/server/lib/clone-response.ts
index b17148fe901770..5913d8ee7dd1ad 100644
--- a/packages/next/src/server/lib/clone-response.ts
+++ b/packages/next/src/server/lib/clone-response.ts
@@ -1,21 +1,15 @@
-// @ts-ignore https://nodejs.org/api/stream.html#streamreadableisdisturbedstream
-import { isDisturbed, isErrored } from 'node:stream'
-
-// https://github.com/nodejs/undici/blob/c399fdf4246be4992e89efe42aa83062edd0609f/lib/web/fetch/body.js#L32
 const noop = () => {}
-const registry = new FinalizationRegistry(
-  (weakRef: WeakRef<ReadableStream>) => {
+
+let registry: FinalizationRegistry<WeakRef<ReadableStream>> | undefined
+
+if (globalThis.FinalizationRegistry) {
+  registry = new FinalizationRegistry((weakRef: WeakRef<ReadableStream>) => {
     const stream = weakRef.deref()
-    if (
-      stream &&
-      !stream.locked &&
-      !isDisturbed(stream) &&
-      !isErrored(stream as any as NodeJS.ReadableStream)
-    ) {
+    if (stream && !stream.locked) {
       stream.cancel('Response object has been garbage collected').then(noop)
     }
-  }
-)
+  })
+}
 
 /**
  * Clones a response by teeing the body so we can return two independent
@@ -66,7 +60,7 @@ export function cloneResponse(original: Response): [Response, Response] {
   // FinalizationRegistry in Undici, but since we're tee-ing the stream
   // ourselves, we need to cancel clone1's stream (the response returned from
   // our dedupe fetch) when clone1 is reclaimed, otherwise we leak memory.
-  if (cloned1.body) {
+  if (registry && cloned1.body) {
     registry.register(cloned1, new WeakRef(cloned1.body))
   }