NODE-2147: Proper server topology management.

lib/sdam/server.js

@@ -126,10 +126,11 @@ class Server extends EventEmitter {
     this.s.pool.on('error', errorEventHandler(this));
     this.s.pool.on('parseError', parseErrorEventHandler(this));
 
-    // it is unclear whether consumers should even know about these events
-    // this.s.pool.on('timeout', timeoutEventHandler(this));
-    // this.s.pool.on('reconnect', reconnectEventHandler(this));
-    // this.s.pool.on('reconnectFailed', errorEventHandler(this));
+    // relay events to ensure topology reconnection/connection status consistency.
+    // for more details, check: NODE-2147: Unexpected behaviours using useUnifiedTopology: true
+    this.s.pool.on('timeout', timeoutEventHandler(this));
+    this.s.pool.on('reconnect', reconnectEventHandler(this));
+    this.s.pool.on('reconnectFailed', errorEventHandler(this));
 
     // relay all command monitoring events
     relayEvents(this.s.pool, this, ['commandStarted', 'commandSucceeded', 'commandFailed']);
@@ -389,4 +390,19 @@ function parseErrorEventHandler(server) {
   };
 }
 
+function timeoutEventHandler(server) {
+  return function() {
+    server.s.state = STATE_DISCONNECTED;
+    server.emit('error', new MongoNetworkError('Pool timeout event triggered.'));
+    server.emit('close');
+  };
+}
+
+function reconnectEventHandler(server) {
+  return function() {
+    server.s.state = STATE_CONNECTING;
+    server.emit('reconnect');
+  };
+}
+
 module.exports = Server;

lib/sdam/topology.js

@@ -54,7 +54,8 @@ const LOCAL_SERVER_EVENTS = SERVER_RELAY_EVENTS.concat([
   'connect',
   'descriptionReceived',
   'close',
-  'ended'
+  'ended',
+  'reconnectFailed'
 ]);
 
 /**
@@ -843,6 +844,11 @@ function updateServers(topology, incomingServerDescription) {
 
 function serverConnectEventHandler(server, topology) {
   return function(/* isMaster, err */) {
+    // close topology on reconnectFailed.
+    server.s.pool.on('reconnectFailed', () => {
+      topology.close();
+    });
+
     server.monitor({
       initial: true,
       heartbeatFrequencyMS: topology.description.heartbeatFrequencyMS

lib/topologies/server.js

@@ -382,7 +382,7 @@ var eventHandler = function(self, event) {
       event === 'timeout' ||
       event === 'reconnect' ||
       event === 'attemptReconnect' ||
-      'reconnectFailed'
+      event === 'reconnectFailed'
     ) {
       // Remove server instance from accounting
       if (