fix: awaitable isMaster timeout must respect connectTimeoutMS

lib/core/sdam/monitor.js

@@ -200,16 +200,19 @@ function checkServer(monitor, callback) {
     const topologyVersion = monitor[kServer].description.topologyVersion;
     const isAwaitable = topologyVersion != null;
 
-    const cmd = isAwaitable
-      ? { ismaster: true, maxAwaitTimeMS, topologyVersion: makeTopologyVersion(topologyVersion) }
-      : { ismaster: true };
-
-    const options = isAwaitable
-      ? { socketTimeout: connectTimeoutMS + maxAwaitTimeMS, exhaustAllowed: true }
-      : { socketTimeout: connectTimeoutMS };
-
-    if (isAwaitable && monitor[kRTTPinger] == null) {
-      monitor[kRTTPinger] = new RTTPinger(monitor[kCancellationToken], monitor.connectOptions);
+    const cmd = { ismaster: true };
+    const options = { socketTimeout: connectTimeoutMS };
+
+    if (isAwaitable) {
+      cmd.maxAwaitTimeMS = maxAwaitTimeMS;
+      cmd.topologyVersion = makeTopologyVersion(topologyVersion);
+      if (connectTimeoutMS) {
+        options.socketTimeout = connectTimeoutMS + maxAwaitTimeMS;
+      }
+      options.exhaustAllowed = true;
+      if (monitor[kRTTPinger] == null) {
+        monitor[kRTTPinger] = new RTTPinger(monitor[kCancellationToken], monitor.connectOptions);
+      }
     }
 
     monitor[kConnection].command('admin.$cmd', cmd, options, (err, result) => {

test/functional/mongo_client_options.test.js

@@ -110,6 +110,28 @@ describe('MongoClient Options', function() {
     });
   });
 
+  it('NODE-2874: connectTimeoutMS=0 causes monitoring to time out', function(done) {
+    const heartbeatFrequencyMS = 500;
+    const client = this.configuration.newClient({
+      connectTimeoutMS: 0, // no connect timeout
+      heartbeatFrequencyMS // fast 500ms heartbeat
+    });
+    client.connect(() => {
+      // success after 5 heartbeats
+      const success = setTimeout(() => client.close(done), heartbeatFrequencyMS * 5);
+
+      // fail on first error
+      const listener = ev => {
+        if (ev.newDescription.error) {
+          clearTimeout(success);
+          client.removeListener('serverDescriptionChanged', listener);
+          client.close(() => done(ev.newDescription.error));
+        }
+      };
+      client.on('serverDescriptionChanged', listener);
+    });
+  });
+
   /**
    * @ignore
    */