fix(topology): correct logic for checking for sessions support

The logic for checking for sessions support was flawed, in that if
it was connect to a single server it would check the conditions for
single server session support and, failing that, would back off to
the generalized support check. These cases have been split up, and
unit tests were added.

Additionally, a typeo was fixed when determining if a topology had
known servers.

NODE-2342

lib/core/sdam/topology.js

@@ -448,10 +448,11 @@ class Topology extends EventEmitter {
    * @return Whether the topology should initiate selection to determine session support
    */
   shouldCheckForSessionSupport() {
-    return (
-      (this.description.type === TopologyType.Single && !this.description.hasKnownServers) ||
-      !this.description.hasDataBearingServers
-    );
+    if (this.description.type === TopologyType.Single) {
+      return !this.description.hasKnownServers;
+    }
+
+    return !this.description.hasDataBearingServers;
   }
 
   /**

lib/core/sdam/topology_description.js

@@ -254,7 +254,7 @@ class TopologyDescription {
    * Determines if the topology description has any known servers
    */
   get hasKnownServers() {
-    return Array.from(this.servers.values()).some(sd => sd.type !== ServerDescription.Unknown);
+    return Array.from(this.servers.values()).some(sd => sd.type !== ServerType.Unknown);
   }
 
   /**

test/unit/sdam/topology_tests.js

@@ -0,0 +1,73 @@
+'use strict';
+const Topology = require('../../../lib/core/sdam/topology').Topology;
+const Server = require('../../../lib/core/sdam/server').Server;
+const ServerDescription = require('../../../lib/core/sdam/server_description').ServerDescription;
+const expect = require('chai').expect;
+const sinon = require('sinon');
+
+describe('Topology (unit)', function() {
+  describe('shouldCheckForSessionSupport', function() {
+    beforeEach(function() {
+      this.sinon = sinon.sandbox.create();
+
+      // these are mocks we want across all tests
+      this.sinon.stub(Server.prototype, 'monitor');
+      this.sinon
+        .stub(Topology.prototype, 'selectServer')
+        .callsFake(function(selector, options, callback) {
+          const server = Array.from(this.s.servers.values())[0];
+          callback(null, server);
+        });
+    });
+
+    afterEach(function() {
+      this.sinon.restore();
+    });
+
+    it('should check for sessions if connected to a single server and has no known servers', function(done) {
+      const topology = new Topology('someserver:27019');
+      this.sinon.stub(Server.prototype, 'connect').callsFake(function() {
+        this.emit('connect');
+      });
+
+      topology.connect(() => {
+        expect(topology.shouldCheckForSessionSupport()).to.be.true;
+        topology.close(done);
+      });
+    });
+
+    it('should not check for sessions if connected to a single server', function(done) {
+      const topology = new Topology('someserver:27019');
+      this.sinon.stub(Server.prototype, 'connect').callsFake(function() {
+        this.emit(
+          'descriptionReceived',
+          new ServerDescription('someserver:27019', { ok: 1, maxWireVersion: 5 })
+        );
+
+        this.emit('connect');
+      });
+
+      topology.connect(() => {
+        expect(topology.shouldCheckForSessionSupport()).to.be.false;
+        topology.close(done);
+      });
+    });
+
+    it('should check for sessions if there are no data-bearing nodes', function(done) {
+      const topology = new Topology('mongos:27019,mongos:27018,mongos:27017');
+      this.sinon.stub(Server.prototype, 'connect').callsFake(function() {
+        this.emit(
+          'descriptionReceived',
+          new ServerDescription(this.name, { ok: 1, msg: 'isdbgrid', maxWireVersion: 5 })
+        );
+
+        this.emit('connect');
+      });
+
+      topology.connect(() => {
+        expect(topology.shouldCheckForSessionSupport()).to.be.false;
+        topology.close(done);
+      });
+    });
+  });
+});