fix: do not attempt to auth against an arbiter

Fixes NODE-1909

lib/connection/connect.js

@@ -155,7 +155,7 @@ function performInitialHandshake(conn, options, callback) {
     conn.lastIsMasterMS = new Date().getTime() - start;
 
     const credentials = options.credentials;
-    if (credentials) {
+    if (!ismaster.arbiterOnly && credentials) {
       credentials.resolveAuthMechanism(ismaster);
       authenticate(conn, credentials, callback);
       return;

test/tests/unit/connect_tests.js

@@ -0,0 +1,93 @@
+'use strict';
+
+const BSON = require('bson');
+const mock = require('mongodb-mock-server');
+const expect = require('chai').expect;
+
+const connect = require('../../../lib/connection/connect');
+const MongoCredentials = require('../../../lib/auth/mongo_credentials').MongoCredentials;
+const genClusterTime = require('./common').genClusterTime;
+
+describe('Connect Tests', function() {
+  const test = {};
+  beforeEach(() => {
+    return mock.createServer().then(mockServer => {
+      test.server = mockServer;
+      test.connectOptions = {
+        host: test.server.host,
+        port: test.server.port,
+        bson: new BSON(),
+        credentials: new MongoCredentials({
+          username: 'testUser',
+          password: 'pencil',
+          source: 'admin',
+          mechanism: 'plain'
+        })
+      };
+    });
+  });
+
+  afterEach(() => mock.cleanup());
+  it('should auth against a non-arbiter', function(done) {
+    const whatHappened = {};
+
+    test.server.setMessageHandler(request => {
+      const doc = request.document;
+      const $clusterTime = genClusterTime(Date.now());
+
+      if (doc.ismaster) {
+        whatHappened.ismaster = true;
+        request.reply(
+          Object.assign({}, mock.DEFAULT_ISMASTER, {
+            $clusterTime
+          })
+        );
+      } else if (doc.saslStart) {
+        whatHappened.saslStart = true;
+        request.reply({ ok: 1 });
+      }
+    });
+
+    connect(test.connectOptions, err => {
+      try {
+        expect(whatHappened).to.have.property('ismaster', true);
+        expect(whatHappened).to.have.property('saslStart', true);
+      } catch (_err) {
+        err = _err;
+      }
+
+      done(err);
+    });
+  });
+
+  it('should not auth against an arbiter', function(done) {
+    const whatHappened = {};
+    test.server.setMessageHandler(request => {
+      const doc = request.document;
+      const $clusterTime = genClusterTime(Date.now());
+      if (doc.ismaster) {
+        whatHappened.ismaster = true;
+        request.reply(
+          Object.assign({}, mock.DEFAULT_ISMASTER, {
+            $clusterTime,
+            arbiterOnly: true
+          })
+        );
+      } else if (doc.saslStart) {
+        whatHappened.saslStart = true;
+        request.reply({ ok: 0 });
+      }
+    });
+
+    connect(test.connectOptions, err => {
+      try {
+        expect(whatHappened).to.have.property('ismaster', true);
+        expect(whatHappened).to.not.have.property('saslStart');
+      } catch (_err) {
+        err = _err;
+      }
+
+      done(err);
+    });
+  });
+});