test(e2e): route snap tarball downloads through MockServerE2E proxy

ReactNativeBlobUtil.fetch() bypasses shim.js's JS-layer fetch/XHR/expo-fetch
patches because it dispatches directly to OkHttp/NSURLSession. As a result,
the testSpecificMock rules registered for snap tarball downloads never fire,
and tests silently hit the live npm registry.

Rewrite the tarball URL at the source — inside NpmLocation.fetchNpmTarball —
when running in E2E. The request body and headers contract with native
ReactNativeBlobUtil stays untouched; only the target URL changes from
`https://registry.npmjs.org/...` to
`http://localhost:<mockServerPort>/proxy?url=<encoded-target>`.

Changes:
- app/core/Snaps/location/npm.ts: rewrite tarballUrl when isE2E
- app/util/test/utils.js: export FALLBACK_MOCK_SERVER_PORT + getMockServerPortInApp helper, mirroring fixtureServerPort/commandQueueServerPort pattern
- shim.js: store launchArgs.mockServerPort in testConfig so app code can read it
- app/core/Snaps/location/npm.test.ts: 2 new tests covering the rewrite both ways (E2E on / off)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: chrisleewilcox

app/core/Snaps/location/npm.test.ts

@@ -1,25 +1,38 @@
 import { NativeModules } from 'react-native';
+
+// Mock the isE2E flag via a mutable variable so individual tests can toggle it.
+let mockIsE2E = false;
+jest.mock('../../../util/test/utils', () => ({
+  get isE2E() {
+    return mockIsE2E;
+  },
+  getMockServerPortInApp: jest.fn(() => 8000),
+}));
+
+// eslint-disable-next-line import-x/first
 import { NpmLocation } from './npm';
 
 // Mock RNTar native module before anything else
 NativeModules.RNTar = {
   unTar: jest.fn().mockResolvedValue('/document-dir/archive'),
 };
 
+const mockBlobFetch = jest.fn((..._args: unknown[]) =>
+  Promise.resolve({
+    flush: jest.fn(),
+    data: '/document-dir/archive.tgz',
+    respInfo: {
+      status: 200,
+      headers: {
+        'content-length': '2000',
+      },
+    },
+  }),
+);
+
 jest.mock('react-native-blob-util', () => ({
   config: jest.fn(() => ({
-    fetch: jest.fn(() =>
-      Promise.resolve({
-        flush: jest.fn(),
-        data: '/document-dir/archive.tgz',
-        respInfo: {
-          status: 200,
-          headers: {
-            'content-length': '2000',
-          },
-        },
-      }),
-    ),
+    fetch: mockBlobFetch,
   })),
   fs: {
     unlink: jest.fn().mockResolvedValue(undefined),
@@ -46,6 +59,11 @@ jest.mock('react-native-blob-util', () => ({
 }));
 
 describe('NpmLocation', () => {
+  beforeEach(() => {
+    mockBlobFetch.mockClear();
+    mockIsE2E = false;
+  });
+
   // This test is heavily mocked and not necesarily a good indicator that everything works E2E.
   it('fetches and unpacks tarballs', async () => {
     const location = new NpmLocation(new URL('npm:@metamask/example-snap'));
@@ -61,4 +79,32 @@ describe('NpmLocation', () => {
       `module.exports.onRpcRequest = () => null`,
     );
   });
+
+  describe('E2E URL rewriting', () => {
+    it('routes tarball downloads to MockServerE2E /proxy when isE2E is true', async () => {
+      mockIsE2E = true;
+
+      const location = new NpmLocation(new URL('npm:@metamask/example-snap'));
+      await location.fetch('snap.manifest.json');
+
+      expect(mockBlobFetch).toHaveBeenCalledWith(
+        'GET',
+        expect.stringMatching(
+          /^http:\/\/localhost:8000\/proxy\?url=https%3A%2F%2Fregistry\.npmjs\.org%2F/,
+        ),
+      );
+    });
+
+    it('uses the original tarball URL when isE2E is false', async () => {
+      mockIsE2E = false;
+
+      const location = new NpmLocation(new URL('npm:@metamask/example-snap'));
+      await location.fetch('snap.manifest.json');
+
+      expect(mockBlobFetch).toHaveBeenCalledWith(
+        'GET',
+        expect.stringMatching(/^https:\/\/registry\.npmjs\.org\//),
+      );
+    });
+  });
 });

app/core/Snaps/location/npm.ts

@@ -10,6 +10,7 @@ import {
   getNpmCanonicalBasePath,
   TARBALL_SIZE_SAFETY_LIMIT,
 } from '@metamask/snaps-controllers';
+import { isE2E, getMockServerPortInApp } from '../../../util/test/utils';
 
 const { RNTar } = NativeModules;
 
@@ -50,7 +51,17 @@ export class NpmLocation extends BaseNpmLocation {
     let response: FetchBlobResponse | null = null;
     let untarPath: string | null = null;
     try {
-      response = await this.#blobFetch('GET', tarballUrl.toString());
+      // In E2E tests, route the tarball download through MockServerE2E's /proxy
+      // endpoint so test-specific mocks can intercept it. ReactNativeBlobUtil's
+      // native fetch bypasses shim.js's JS-layer fetch/XHR patches, so the
+      // redirect must happen here at the source URL.
+      const fetchUrl = isE2E
+        ? `http://localhost:${getMockServerPortInApp()}/proxy?url=${encodeURIComponent(
+            tarballUrl.toString(),
+          )}`
+        : tarballUrl.toString();
+
+      response = await this.#blobFetch('GET', fetchUrl);
 
       const responseInfo = response.respInfo;
 

app/util/test/utils.js

@@ -5,6 +5,7 @@ export const flushPromises = () => new Promise(setImmediate);
 // iOS: These are overridden by LaunchArgs at runtime
 export const FALLBACK_FIXTURE_SERVER_PORT = 12345;
 export const FALLBACK_COMMAND_QUEUE_SERVER_PORT = 2446;
+export const FALLBACK_MOCK_SERVER_PORT = 8000;
 
 // E2E test configuration required in app
 export const testConfig = {};
@@ -42,5 +43,7 @@ export const getFixturesServerPortInApp = () =>
   testConfig.fixtureServerPort ?? FALLBACK_FIXTURE_SERVER_PORT;
 export const getCommandQueueServerPortInApp = () =>
   testConfig.commandQueueServerPort ?? FALLBACK_COMMAND_QUEUE_SERVER_PORT;
+export const getMockServerPortInApp = () =>
+  testConfig.mockServerPort ?? FALLBACK_MOCK_SERVER_PORT;
 
 export const isRc = process.env.METAMASK_ENVIRONMENT === 'rc';

shim.js

@@ -35,6 +35,7 @@ import { LaunchArguments } from 'react-native-launch-arguments';
 import {
   FALLBACK_FIXTURE_SERVER_PORT,
   FALLBACK_COMMAND_QUEUE_SERVER_PORT,
+  FALLBACK_MOCK_SERVER_PORT,
   isE2E,
   isTest,
   enableApiCallLogs,
@@ -114,6 +115,9 @@ if (isTest) {
   testConfig.commandQueueServerPort = raw?.commandQueueServerPort
     ? raw.commandQueueServerPort
     : FALLBACK_COMMAND_QUEUE_SERVER_PORT;
+  testConfig.mockServerPort = raw?.mockServerPort
+    ? raw.mockServerPort
+    : FALLBACK_MOCK_SERVER_PORT;
 }
 
 // Fix for https://github.com/facebook/react-native/issues/5667