fix: move session support check to operation layer

Monitor checks run on a timer and network errors cause a reset
of the topology that would be corrected in the next cycle of the
monitor, we were checking a property that gets updated by this async
work. By moving the check to the operations layer we will allow users
to obtain a session regardless of support and then emit an error if
there is not support when the session is used in an operation.

NODE-3100

lib/mongo_client.js

@@ -457,10 +457,6 @@ MongoClient.prototype.startSession = function(options) {
     throw new MongoError('Must connect to a server before calling this method');
   }
 
-  if (!this.topology.hasSessionSupport()) {
-    throw new MongoError('Current topology does not support sessions');
-  }
-
   return this.topology.startSession(options, this.s.options);
 };
 

lib/operations/execute_operation.js

@@ -47,6 +47,8 @@ function executeOperation(topology, operation, callback) {
     } else if (operation.session.hasEnded) {
       throw new MongoError('Use of expired sessions is not permitted');
     }
+  } else if (operation.session && operation.session.explicit) {
+    throw new MongoError('Current topology does not support sessions');
   }
 
   let result;

test/unit/sessions/client.test.js

@@ -16,7 +16,7 @@ describe('Sessions', function() {
 
     it('should throw an exception if sessions are not supported', {
       metadata: { requires: { topology: 'single' } },
-      test: function(done) {
+      test() {
         test.server.setMessageHandler(request => {
           var doc = request.document;
           if (doc.ismaster) {
@@ -27,21 +27,32 @@ describe('Sessions', function() {
         });
 
         const client = this.configuration.newClient(`mongodb://${test.server.uri()}/test`);
-        client.connect(function(err, client) {
-          expect(err).to.not.exist;
-          expect(() => {
-            client.startSession();
-          }).to.throw(/Current topology does not support sessions/);
-
-          client.close(done);
-        });
+        return client
+          .connect()
+          .then(function(client) {
+            const session = client.startSession();
+            return client
+              .db()
+              .collection('t')
+              .insertOne({ a: 1 }, { session });
+          })
+          .then(() => {
+            expect.fail('Expected an error to be thrown about not supporting sessions');
+          })
+          .catch(error => {
+            expect(error.message).to.equal('Current topology does not support sessions');
+          })
+          .then(() => {
+            return client.close();
+          });
       }
     });
 
     it('should throw an exception if sessions are not supported on some servers', {
       metadata: { requires: { topology: 'single' } },
       test() {
         const replicaSetMock = new ReplSetFixture();
+        let client;
         return replicaSetMock
           .setup({ doNotInitHandlers: true })
           .then(() => {
@@ -92,14 +103,23 @@ describe('Sessions', function() {
             return replicaSetMock.uri();
           })
           .then(uri => {
-            const client = this.configuration.newClient(uri);
+            client = this.configuration.newClient(uri);
             return client.connect();
           })
           .then(client => {
-            expect(client.topology.s.description.logicalSessionTimeoutMinutes).to.not.exist;
-            expect(() => {
-              client.startSession();
-            }).to.throw(/Current topology does not support sessions/);
+            const session = client.startSession();
+            return client
+              .db()
+              .collection('t')
+              .insertOne({ a: 1 }, { session });
+          })
+          .then(() => {
+            expect.fail('Expected an error to be thrown about not supporting sessions');
+          })
+          .catch(error => {
+            expect(error.message).to.equal('Current topology does not support sessions');
+          })
+          .then(() => {
             return client.close();
           });
       }