Clarify non-fatal React Native DevTools preparation errors

packages/debugger-shell/src/node/__tests__/prepareDebuggerShell-test.js

@@ -0,0 +1,30 @@
+/**
+ * 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
+ */
+
+jest.mock('../private/LaunchUtils', () => ({
+  prepareDebuggerShellFromDotSlashFile: jest.fn(() =>
+    Promise.reject(new Error('spawn failed')),
+  ),
+}));
+
+const {unstable_prepareDebuggerShell} = require('../../');
+
+describe('unstable_prepareDebuggerShell', () => {
+  test('returns a human readable message for unexpected errors', async () => {
+    const result = await unstable_prepareDebuggerShell({flavor: 'prebuilt'});
+
+    expect(result.code).toBe('unexpected_error');
+    expect(result.verboseInfo).toBe('spawn failed');
+    expect(result.humanReadableMessage).toContain(
+      'Using a fallback version instead',
+    );
+  });
+});

packages/debugger-shell/src/node/index.flow.js

@@ -175,6 +175,9 @@ async function unstable_prepareDebuggerShell({
   } catch (e) {
     return {
       code: 'unexpected_error',
+      humanReadableMessage:
+        'An unexpected error occurred while preparing React Native DevTools. ' +
+        'Using a fallback version instead.',
       verboseInfo: e.message,
     };
   }

packages/dev-middleware/src/__tests__/StandaloneFuseboxShell-test.js

@@ -232,4 +232,38 @@ describe('enableStandaloneFuseboxShell experiment', () => {
       });
     });
   });
+
+  describe('preparation failure logging', () => {
+    const logger = {
+      error: jest.fn(),
+      info: jest.fn(),
+      warn: jest.fn(),
+    };
+    const {ToolLauncherWithFuseboxShell} = setupToolLauncherWithFuseboxShell(
+      () =>
+        Promise.resolve({
+          code: 'unexpected_error',
+          verboseInfo: 'LaunchProcess: failed to execvp',
+        }),
+    );
+    const server = withServerForEachTest({
+      logger,
+      unstable_toolLauncher: ToolLauncherWithFuseboxShell,
+      unstable_experiments: {
+        enableStandaloneFuseboxShell: true,
+      },
+    });
+
+    test('logs an unexpected error as a non-fatal warning', async () => {
+      // Access the server to ensure the dev middleware has been created
+      expect(server.serverBaseUrl).toBeDefined();
+      await jest.runOnlyPendingTimersAsync();
+
+      expect(logger.error).not.toHaveBeenCalled();
+      expect(logger.warn).toHaveBeenCalledTimes(1);
+      const [message] = logger.warn.mock.calls[0];
+      expect(message).toContain('Using a fallback version instead');
+      expect(message).toContain('LaunchProcess: failed to execvp');
+    });
+  });
 });

packages/dev-middleware/src/createDevMiddleware.js

@@ -177,15 +177,16 @@ function createWrappedEventReporter(
             case 'not_implemented':
               break;
             case 'unexpected_error': {
+              // Preparation failures are not fatal: the debugger falls back to
+              // opening in a browser window, so log a warning, not an error.
               let message =
                 event.result.humanReadableMessage ??
-                'An unknown error occurred while installing React Native DevTools.';
+                'An unknown error occurred while installing React Native DevTools. ' +
+                  'Using a fallback version instead.';
               if (event.result.verboseInfo != null) {
                 message += ` Details:\n\n${event.result.verboseInfo}`;
-              } else {
-                message += '.';
               }
-              logger?.error(message);
+              logger?.warn(message);
               break;
             }
             case 'possible_corruption':