Respond with HTTP 404 if bundle entry point cannot be resolved

Summary:
Changelog: **[Fix]** Respond with HTTP 404 when a bundle entry point cannot be resolved.

Currently, Metro uses HTTP code 500 (Internal Server Error) for all errors. In the case of "bundle not found" errors, it can be useful to respond with 404 (Not Found) instead. This seemingly was Metro's behaviour at one point (D9772264) but has since regressed.

Reviewed By: huntie

Differential Revision: D51856058

fbshipit-source-id: 63ea7a7cbab5b0347def962867dcc720ac84946b

packages/metro/src/Server.js

@@ -60,6 +60,9 @@ const getGraphId = require('./lib/getGraphId');
 const parseOptionsFromUrl = require('./lib/parseOptionsFromUrl');
 const splitBundleOptions = require('./lib/splitBundleOptions');
 const transformHelpers = require('./lib/transformHelpers');
+const {
+  UnableToResolveError,
+} = require('./node-haste/DependencyGraph/ModuleResolution');
 const parsePlatformFilePath = require('./node-haste/lib/parsePlatformFilePath');
 const MultipartResponse = require('./Server/MultipartResponse');
 const symbolicate = require('./Server/symbolicate');
@@ -610,11 +613,23 @@ class Server {
        * `entryFile` is relative to projectRoot, we need to use resolution function
        * to find the appropriate file with supported extensions.
        */
-      const resolvedEntryFilePath = await this._resolveRelativePath(entryFile, {
-        relativeTo: 'server',
-        resolverOptions,
-        transformOptions,
-      });
+      let resolvedEntryFilePath;
+      try {
+        resolvedEntryFilePath = await this._resolveRelativePath(entryFile, {
+          relativeTo: 'server',
+          resolverOptions,
+          transformOptions,
+        });
+      } catch (error) {
+        const formattedError = formatBundlingError(error);
+
+        const status = error instanceof UnableToResolveError ? 404 : 500;
+        res.writeHead(status, {
+          'Content-Type': 'application/json; charset=UTF-8',
+        });
+        res.end(JSON.stringify(formattedError));
+        return;
+      }
       const graphId = getGraphId(resolvedEntryFilePath, transformOptions, {
         unstable_allowRequireContext:
           this._config.transformer.unstable_allowRequireContext,

packages/metro/src/integration_tests/__tests__/server-test.js

@@ -101,4 +101,20 @@ describe('Metro development server serves bundles via HTTP', () => {
       ]),
     );
   });
+
+  test('responds with 404 when the bundle cannot be resolved', async () => {
+    const response = await fetch(
+      'http://localhost:' + config.server.port + '/doesnotexist.bundle',
+    );
+    expect(response.status).toBe(404);
+  });
+
+  test('responds with 500 when an import inside the bundle cannot be resolved', async () => {
+    const response = await fetch(
+      'http://localhost:' +
+        config.server.port +
+        '/build-errors/inline-requires-cannot-resolve-import.bundle',
+    );
+    expect(response.status).toBe(500);
+  });
 });