fix: recognize occupied ports returning HTTP errors

packages/cli-tools/src/__tests__/isPackagerRunning.test.ts

@@ -0,0 +1,115 @@
+import {createServer} from 'http';
+import {AddressInfo} from 'net';
+import {runInThisContext} from 'vm';
+import isPackagerRunning from '../isPackagerRunning';
+import findDevServerPort from '../findDevServerPort';
+import {askForPortChange} from '../port';
+
+jest.mock('../port', () => ({
+  askForPortChange: jest.fn(),
+  logAlreadyRunningBundler: jest.fn(),
+  logChangePortInstructions: jest.fn(),
+}));
+
+const withServer = async (
+  status: number,
+  body: string,
+  run: (port: number) => Promise<void>,
+  headers: Record<string, string> = {},
+) => {
+  const server = createServer((_request, response) => {
+    response.writeHead(status, headers);
+    response.end(body);
+  });
+  await new Promise<void>((resolve) => server.listen(0, resolve));
+  try {
+    await run((server.address() as AddressInfo).port);
+  } finally {
+    await new Promise<void>((resolve, reject) =>
+      server.close((error) => (error ? reject(error) : resolve())),
+    );
+  }
+};
+
+// Jest 26 does not expose Node's built-in fetch in its test environment.
+const originalFetch = global.fetch;
+beforeAll(() => {
+  global.fetch = runInThisContext('fetch');
+});
+afterAll(() => {
+  global.fetch = originalFetch;
+});
+
+beforeEach(() => {
+  jest.clearAllMocks();
+  (askForPortChange as jest.Mock).mockResolvedValue({change: true});
+});
+
+test.each([400, 404, 500])(
+  'recognizes a port occupied by an HTTP %i server',
+  async (status) => {
+    await withServer(status, 'Not Metro', async (port) => {
+      expect(await isPackagerRunning(port)).toBe('unrecognized');
+    });
+  },
+);
+
+test.each([400, 404, 500])(
+  'offers another port when the current server responds with HTTP %i',
+  async (status) => {
+    await withServer(status, 'Not Metro', async (port) => {
+      const result = await findDevServerPort(port, '/test/project');
+      expect(askForPortChange).toHaveBeenCalledWith(port, result.port);
+      expect(result.port).toBeGreaterThan(port);
+      expect(result.startPackager).toBe(true);
+    });
+  },
+);
+
+test('recognizes a successful response from another server', async () => {
+  await withServer(200, 'Not Metro', async (port) => {
+    expect(await isPackagerRunning(port)).toBe('unrecognized');
+  });
+});
+
+test('recognizes Metro and its project root', async () => {
+  await withServer(
+    200,
+    'packager-status:running',
+    async (port) => {
+      expect(await isPackagerRunning(port)).toEqual({
+        status: 'running',
+        root: '/test/project',
+      });
+    },
+    {'X-React-Native-Project-Root': '/test/project'},
+  );
+});
+
+test('does not start another Metro instance for the same project', async () => {
+  await withServer(
+    200,
+    'packager-status:running',
+    async (port) => {
+      expect(await findDevServerPort(port, '/test/project')).toEqual({
+        port,
+        startPackager: false,
+      });
+      expect(askForPortChange).not.toHaveBeenCalled();
+    },
+    {'X-React-Native-Project-Root': '/test/project'},
+  );
+});
+
+test('returns not_running when no server is listening', async () => {
+  let port = 0;
+  await withServer(200, '', async (listeningPort) => {
+    port = listeningPort;
+  });
+  expect(await isPackagerRunning(port)).toBe('not_running');
+  expect(await findDevServerPort(port, '/test/project')).toEqual({
+    port,
+    startPackager: true,
+  });
+  expect(askForPortChange).not.toHaveBeenCalled();
+});

packages/cli-tools/src/isPackagerRunning.ts

@@ -7,6 +7,7 @@
  */
 
 import {fetch} from './fetch';
+import {CLIError} from './errors';
 
 /**
  * Indicates whether or not the packager is running. It returns a promise that
@@ -42,8 +43,9 @@ async function isPackagerRunning(
     }
 
     return 'unrecognized';
-  } catch (_error) {
-    return 'not_running';
+  } catch (error) {
+    // An HTTP error response still means a process is listening on this port.
+    return error instanceof CLIError ? 'unrecognized' : 'not_running';
   }
 }