test(NODE-5724): heartbeat started is emitted before socket is connected

test/unit/sdam/server_discovery_and_monitoring.prose.test.ts

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+import { expect } from 'chai';
+import { once } from 'events';
+import { createServer, type Server } from 'net';
+
+import { MongoClient, SERVER_HEARTBEAT_FAILED, SERVER_HEARTBEAT_STARTED } from '../../mongodb';
+
+describe('Heartbeat tests', function () {
+  let client: MongoClient;
+  // Mock server
+  let server: Server;
+  // Shared array
+  const events: string[] = [];
+  const PORT = 9999;
+  const CONN_STRING = `mongodb://localhost:${PORT}`;
+
+  beforeEach(async function () {
+    // Create TCP server that responds to hellos by closing the connection
+    // and pushing "client connection created" to shared array
+    server = createServer(clientSocket => {
+      events.push('client connected');
+
+      clientSocket.once('data', () => {
+        events.push('client hello received');
+        clientSocket.destroy();
+      });
+    });
+    server.listen(PORT);
+
+    await once(server, 'listening');
+
+    // set up client to connect to mock server with the following configuration
+    // {
+    //    serverSelectionTimeoutMS: 500,
+    // }
+    client = new MongoClient(CONN_STRING, {
+      serverSelectionTimeoutMS: 500
+    });
+
+    // Listen to `ServerHeartbeatStartedEvent` and `ServerHeartbeatSucceededEvent`, pushing the
+    // event name to the shared array when event is emitted
+    for (const e of [SERVER_HEARTBEAT_STARTED, SERVER_HEARTBEAT_FAILED]) {
+      client.on(e, () => {
+        events.push(e);
+      });
+    }
+  });
+
+  afterEach(async function () {
+    if (server.listening) server.close();
+  });
+
+  it('emits the first HeartbeatStartedEvent before the monitoring socket was created', async function () {
+    // Attempt to connect to mock server
+    const maybeError = await client.connect().catch(e => e);
+    // Catch error
+    expect(maybeError).to.be.instanceOf(Error);
+
+    expect(events).to.deep.equal([
+      SERVER_HEARTBEAT_STARTED,
+      'client connected',
+      'client hello received',
+      SERVER_HEARTBEAT_FAILED
+    ]);
+  });
+});