refactor: don't require a final callback for withConnection

Author: mbroadst

lib/cmap/connection_pool.js

@@ -299,10 +299,12 @@ class ConnectionPool extends EventEmitter {
       // don't callback with `err` here, we might want to act upon it inside `fn`
 
       fn(err, conn, (fnErr, result) => {
-        if (fnErr) {
-          callback(fnErr);
-        } else {
-          callback(undefined, result);
+        if (typeof callback === 'function') {
+          if (fnErr) {
+            callback(fnErr);
+          } else {
+            callback(undefined, result);
+          }
         }
 
         if (conn) {

test/unit/cmap/connection_pool.test.js

@@ -6,6 +6,7 @@ const ConnectionPool = require('../../../lib/cmap/connection_pool').ConnectionPo
 const EventEmitter = require('events').EventEmitter;
 const mock = require('mongodb-mock-server');
 const BSON = require('bson');
+const cmapEvents = require('../../../lib/cmap/events');
 
 const chai = require('chai');
 chai.use(require('../../functional/spec-runner/matcher').default);
@@ -115,6 +116,35 @@ describe('Connection Pool', function() {
         cb(new Error('my great error'));
       }, callback);
     });
+
+    it('should still manage a connection if no callback is provided', function(done) {
+      server.setMessageHandler(request => {
+        const doc = request.document;
+        if (doc.ismaster) {
+          request.reply(mock.DEFAULT_ISMASTER_36);
+        }
+      });
+
+      const pool = new ConnectionPool(
+        Object.assign({ bson: new BSON(), maxPoolSize: 1 }, server.address())
+      );
+
+      const events = [];
+      pool.on('connectionCheckedOut', event => events.push(event));
+      pool.on('connectionCheckedIn', event => {
+        events.push(event);
+
+        expect(events).to.have.length(2);
+        expect(events[0]).to.be.instanceOf(cmapEvents.ConnectionCheckedOutEvent);
+        expect(events[1]).to.be.instanceOf(cmapEvents.ConnectionCheckedInEvent);
+        pool.close(done);
+      });
+
+      pool.withConnection((err, conn, cb) => {
+        expect(err).to.not.exist;
+        cb();
+      });
+    });
   });
 
   describe('spec tests', function() {