Remove listener when mongos destroyed

lib/topologies/mongos.js

@@ -299,10 +299,15 @@ Mongos.prototype.connect = function(options) {
     );
   });
 
+  const serverDescriptionChangedCallback = event => {
+    self.emit('serverDescriptionChanged', event);
+  };
+
   servers.forEach(function(server) {
-    server.on('serverDescriptionChanged', function(event) {
-      self.emit('serverDescriptionChanged', event);
-    });
+    server.on('serverDescriptionChanged', serverDescriptionChangedCallback);
+    server.on('destroy', () =>
+      server.removeListener('serverDescriptionChanged', serverDescriptionChangedCallback)
+    );
   });
 
   // Emit the topology opening event
@@ -851,7 +856,6 @@ Mongos.prototype.destroy = function(options) {
     // Move to list of disconnectedProxies
     moveServerFrom(self.connectedProxies, self.disconnectedProxies, x);
   });
-
   // Emit the final topology change
   emitTopologyDescriptionChanged(self);
   // Emit toplogy closing event

test/tests/unit/mongos/events_tests.js

@@ -0,0 +1,54 @@
+'use strict';
+
+const expect = require('chai').expect;
+const Mongos = require('../../../../lib/topologies/mongos');
+const mock = require('../../../mock');
+const MongosFixture = require('../common').MongosFixture;
+
+const test = new MongosFixture();
+
+describe('EventEmitters (Mongos)', function() {
+  afterEach(() => mock.cleanup());
+  beforeEach(() => {
+    return mock.createServer().then(mockServer => {
+      test.server = mockServer;
+    });
+  });
+
+  it('should remove `serverDescriptionChanged` listeners when server is closed', {
+    metadata: { requires: { topology: ['single'] } },
+    test: function(done) {
+      test.server.setMessageHandler(req => {
+        const doc = req.document;
+        if (doc.ismaster) {
+          req.reply(Object.assign({}, test.defaultFields));
+        }
+      });
+
+      const mongos = new Mongos([test.server.address()], {
+        connectionTimeout: 30000,
+        socketTimeout: 30000,
+        haInterval: 500,
+        size: 1
+      });
+
+      mongos.on('error', done);
+      mongos.once('connect', () => {
+        expect(mongos.disconnectedProxies).to.have.length(1);
+        expect(mongos.disconnectedProxies[0].listenerCount('serverDescriptionChanged')).to.equal(1);
+        // After we connect, destroy/close the server
+        mongos.destroy();
+        mongos.on('topologyClosed', () => {
+          expect(mongos.disconnectedProxies).to.have.length(1);
+          expect(mongos.disconnectedProxies[0].listenerCount('serverDescriptionChanged')).to.equal(
+            0
+          );
+        });
+
+        done();
+      });
+
+      mongos.connect();
+    }
+  });
+});