fix(NODE-4608): prevent parallel monitor checks (#3404)

Author: baileympearson

src/sdam/monitor.ts

@@ -328,6 +328,10 @@ function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
 
 function monitorServer(monitor: Monitor) {
   return (callback: Callback) => {
+    if (monitor.s.state === STATE_MONITORING) {
+      process.nextTick(callback);
+      return;
+    }
     stateTransition(monitor, STATE_MONITORING);
     function done() {
       if (!isInCloseState(monitor)) {

test/integration/server-discovery-and-monitoring/server_discovery_and_monitoring.spec.test.ts

@@ -1,50 +1,8 @@
-/* eslint-disable @typescript-eslint/no-non-null-assertion */
-import { Socket } from 'net';
 import * as path from 'path';
 
 import { loadSpecTests } from '../../spec';
 import { runUnifiedSuite } from '../../tools/unified-spec-runner/runner';
-import { TestFilter } from '../../tools/unified-spec-runner/schema';
-import { sleep } from '../../tools/utils';
-
-const filter: TestFilter = ({ description }) => {
-  switch (description) {
-    case 'Network error on Monitor check':
-    case 'Network timeout on Monitor check':
-      return 'TODO(NODE-4608): Disallow parallel monitor checks';
-    default:
-      return false;
-  }
-};
 
 describe('SDAM Unified Tests', function () {
-  afterEach(async function () {
-    if (this.currentTest!.pending) {
-      return;
-    }
-    // TODO(NODE-4573): fix socket leaks
-    const LEAKY_TESTS = [
-      'Command error on Monitor handshake',
-      'Network error on Monitor check',
-      'Network timeout on Monitor check',
-      'Network error on Monitor handshake',
-      'Network timeout on Monitor handshake'
-    ];
-
-    await sleep(250);
-    const sockArray = (process as any)._getActiveHandles().filter(handle => {
-      // Stdio are instanceof Socket so look for fd to be null
-      return handle.fd == null && handle instanceof Socket && handle.destroyed !== true;
-    });
-    if (!sockArray.length) {
-      return;
-    }
-    for (const sock of sockArray) {
-      sock.destroy();
-    }
-    if (!LEAKY_TESTS.some(test => test === this.currentTest!.title)) {
-      this.test!.error(new Error(`Test failed to clean up ${sockArray.length} socket(s)`));
-    }
-  });
-  runUnifiedSuite(loadSpecTests(path.join('server-discovery-and-monitoring', 'unified')), filter);
+  runUnifiedSuite(loadSpecTests(path.join('server-discovery-and-monitoring', 'unified')));
 });