Propagate Bundler transformer initialization errors

packages/metro/src/Bundler.js

@@ -46,7 +46,12 @@ export default class Bundler {
           type: 'transformer_load_failed',
           error,
         });
+        throw error;
       });
+
+    // Observe initialization failures immediately so callers can await the
+    // original promise later without triggering an unhandled rejection.
+    this._initializedPromise.catch(() => {});
   }
 
   getWatcher(): EventEmitter {

packages/metro/src/__tests__/Bundler-test.js

@@ -0,0 +1,99 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+import type {ConfigT} from 'metro-config';
+
+import Bundler from '../Bundler';
+import Transformer from '../DeltaBundler/Transformer';
+import DependencyGraph from '../node-haste/DependencyGraph';
+import {getDefaultConfig} from 'metro-config';
+
+jest.mock('../DeltaBundler/Transformer');
+jest.mock('../node-haste/DependencyGraph');
+
+type ClassMock = JestMockFn<ReadonlyArray<unknown>, unknown>;
+
+// $FlowFixMe[incompatible-type] Jest automocks the default export
+const MockTransformer: ClassMock = Transformer;
+// $FlowFixMe[incompatible-type] Jest automocks the default export
+const MockDependencyGraph: ClassMock = DependencyGraph;
+
+describe('Bundler', () => {
+  let config: ConfigT;
+  let reporter;
+
+  beforeEach(() => {
+    reporter = {update: jest.fn()};
+    config = {...getDefaultConfig.getDefaultValues('/'), reporter};
+
+    MockDependencyGraph.mockImplementation(() => ({
+      ready: jest.fn().mockResolvedValue(),
+    }));
+
+    jest.spyOn(console, 'error').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    jest.restoreAllMocks();
+  });
+
+  test.each([
+    ['ready', (bundler: Bundler) => bundler.ready()],
+    [
+      'transformFile',
+      // $FlowFixMe[incompatible-type] Transform options are unused before initialization fails
+      (bundler: Bundler) => bundler.transformFile('/entry.js', {}),
+    ],
+  ])(
+    'propagates Transformer initialization errors from %s',
+    async (_method, invoke) => {
+      const error = new Error('Transformer initialization failed');
+      MockTransformer.mockImplementation(() => {
+        throw error;
+      });
+
+      const bundler = new Bundler(config);
+
+      await expect(invoke(bundler)).rejects.toBe(error);
+      expect(reporter.update).toHaveBeenCalledWith({
+        type: 'transformer_load_failed',
+        error,
+      });
+    },
+  );
+
+  test('does not emit an unhandled rejection before ready is called', async () => {
+    jest.useRealTimers();
+
+    const error = new Error('Transformer initialization failed');
+    const unhandledRejections: Array<unknown> = [];
+    const onUnhandledRejection = (reason: unknown) => {
+      unhandledRejections.push(reason);
+    };
+
+    MockTransformer.mockImplementation(() => {
+      throw error;
+    });
+    process.on('unhandledRejection', onUnhandledRejection);
+
+    try {
+      const bundler = new Bundler(config);
+
+      await new Promise(resolve => setImmediate(resolve));
+
+      await expect(bundler.ready()).rejects.toBe(error);
+      expect(unhandledRejections).toEqual([]);
+    } finally {
+      process.off('unhandledRejection', onUnhandledRejection);
+      jest.useFakeTimers();
+    }
+  });
+});