From 340bb1b922dfe3d192412140dbc699641de21e18 Mon Sep 17 00:00:00 2001
From: Moti Zilberman <moti@meta.com>
Date: Mon, 11 May 2026 03:56:59 -0700
Subject: [PATCH] Properly reject `loadBundleFromServer` on non-2xx HTTP status

Summary:
Changelog:

[General][Fixed] Handle HTTP error responses when loading lazy bundles from Metro

Differential Revision: D104423888
---
 .../__tests__/loadBundleFromServer-test.js    | 26 +++++++++++-
 .../Core/Devtools/loadBundleFromServer.js     | 40 +++++++++++++++----
 2 files changed, 58 insertions(+), 8 deletions(-)

diff --git a/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js b/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js
index 7657099fd2e1..473c876862b5 100644
--- a/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js
+++ b/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js
@@ -65,7 +65,9 @@ const addListener = jest.fn((name, fn) => {
   } else if (name === 'didCompleteNetworkResponse') {
     void Promise.resolve().then(() => fn([REQUEST_ID, mockRequestError]));
   } else if (name === 'didReceiveNetworkResponse') {
-    void Promise.resolve().then(() => fn([REQUEST_ID, null, mockHeaders]));
+    void Promise.resolve().then(() =>
+      fn([REQUEST_ID, mockStatus, mockHeaders]),
+    );
   }
   return {remove: removeListener};
 });
@@ -83,10 +85,12 @@ let loadBundleFromServer: (bundlePathAndQuery: string) => Promise<void>;
 let mockHeaders: {'Content-Type': string};
 let mockDataResponse;
 let mockRequestError = null;
+let mockStatus: number = 200;
 
 beforeEach(() => {
   jest.resetModules();
   jest.clearAllMocks();
+  mockStatus = 200;
   loadBundleFromServer = require('../loadBundleFromServer').default;
 });
 
@@ -222,6 +226,26 @@ test('shows and hides the loading view around concurrent requests', async () =>
   expect(loadingViewMock.hide).toHaveBeenCalledTimes(1);
 });
 
+test.each([404, 500])(
+  'loadBundleFromServer will throw for HTTP %d status',
+  async (status: number) => {
+    mockHeaders = {'Content-Type': 'application/javascript'};
+    mockDataResponse = 'Error response body';
+    mockRequestError = null;
+    mockStatus = status;
+
+    const error: unknown = await loadBundleFromServer(
+      '/Fail.bundle?platform=ios',
+    ).catch(e => e);
+
+    invariant(
+      error instanceof LoadBundleFromServerRequestError,
+      'Expected a LoadBundleFromServerRequestError',
+    );
+    expect(error.message).toMatch(new RegExp(`HTTP ${status}`));
+  },
+);
+
 test('loadBundleFromServer does not cache errors', async () => {
   mockHeaders = {'Content-Type': 'application/json'};
   mockDataResponse = JSON.stringify({message: 'Error thrown from Metro'});
diff --git a/packages/react-native/Libraries/Core/Devtools/loadBundleFromServer.js b/packages/react-native/Libraries/Core/Devtools/loadBundleFromServer.js
index eb5555d60f7a..c45a783883d3 100644
--- a/packages/react-native/Libraries/Core/Devtools/loadBundleFromServer.js
+++ b/packages/react-native/Libraries/Core/Devtools/loadBundleFromServer.js
@@ -60,7 +60,10 @@ function asyncRequest(
 ): Promise<{body: string, headers: {[string]: string}}> {
   let id = null;
   let responseText = null;
-  let headers = null;
+  let head: ?{
+    status: number,
+    headers: ?{[string]: string},
+  } = null;
   let dataListener;
   let completeListener;
   let responseListener;
@@ -91,7 +94,7 @@ function asyncRequest(
         'didReceiveNetworkResponse',
         ([requestId, status, responseHeaders]) => {
           if (requestId === id) {
-            headers = responseHeaders;
+            head = {status, headers: responseHeaders};
           }
         },
       );
@@ -99,20 +102,43 @@ function asyncRequest(
         'didCompleteNetworkResponse',
         ([requestId, errorMessage, isTimeout]) => {
           if (requestId === id) {
-            if (errorMessage) {
+            if (
+              errorMessage != null ||
+              isTimeout ||
+              responseText == null ||
+              head == null ||
+              head.status < 200 ||
+              head.status >= 300
+            ) {
+              let summary = null,
+                cause: ?string = null;
+              if (errorMessage != null || isTimeout) {
+                summary = isTimeout
+                  ? 'Request timed out'
+                  : 'Could not load bundle';
+                cause = errorMessage;
+              } else if (responseText == null || head == null) {
+                summary = 'Request completed with missing response data';
+              }
+              if (head && (head.status < 200 || head.status >= 300)) {
+                const {status} = head;
+                summary = [`HTTP ${status} when loading bundle ${url}`, summary]
+                  .filter(Boolean)
+                  .join('; ');
+                cause = cause ?? responseText;
+              }
               reject(
                 new LoadBundleFromServerRequestError(
-                  'Could not load bundle',
+                  summary ?? `Unknown error when loading bundle ${url}`,
                   url,
                   isTimeout,
                   {
-                    cause: errorMessage,
+                    cause,
                   },
                 ),
               );
             } else {
-              //$FlowFixMe[incompatible-type]
-              resolve({body: responseText, headers});
+              resolve({body: responseText, headers: head.headers ?? {}});
             }
           }
         },